Radio Frequency Engineering Information
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Radio System Calculator - Version 7-14-2010
This file will assist radio system designers by calculating many common parameters when used with a Microsoft Excel spreadsheet program. This file MAY work with other spreadsheet programs, but it is not guaranteed to do so. The following functions are included in this version:
General Purpose Section
1. Calculate Free Space Loss in miles.
2. Convert watts to dBm and dBw
3. Convert dBm to watts
4. Convert uV to dBm
5. Convert dBm to uV
6. Convert dBw to uV
7. Convert dBm and Freq to field strength; dBu and uV/meter
8. Convert dBu and Freq to dBm
9. Calculate Return Loss and VSWR
10. Calculate Vertical spaced antenna - antenna isolation
11. Calculate Horizontal spaced antenna - antenna isolation
12. Calculate distance to horizon for a given antenna height
13. Calculate radio horizon and - 3dB horizons using antenna height, beamwidth and downtilt.
14. Calculate signal levels for Item 12 (above) using frequency and ERP.
15, 16, 17. Convert Coordinate formats to other Coordinate formats.
18. Calculate distance & azimuth between two sites. Distance calculation uses FCC's formula.
Signal Booster Section
A. Calculate Free Space Loss in Feet.
B. Calculate Composte power of multiple carriers.
C. Calculate broadband amp input noise threshold and noise output levels using NF and BW.
D. Calculate 3rd Order IM in Class A Amplifiers.
E. Calculate 2nd & 3rd Order IM using 3rd OIP and Output Level.
F. Calculate maximum output power per carrier for multiple input carriers.
NEW: Calculate propagation delays for coax, free space and fiber cables. NEW: Metric Conversion Section
DISCLAIMER: This file is posted for the general use of visitors to this site. The accuracy or suitability of the formulas and terminology used is NOT guaranteed in any way or for any reason, PERIOD! No warranty. Not for sale. Use at your own risk. Copyrighted. All rights reserved.
Click here to Download the RF Calculator now
Introduction to Signal Boosters and In-building RF distribution.
GENERAL DESCRIPTION:
A Signal Booster is a device which can often be used to improve radio communications in an area where normally good radio signal levels are degraded due to obstacles in the radio path.
The most common misconception in applying Signal Boosters lies in expecting a Signal Booster to operate in the same fashion (and signal levels) as a common repeater station.
Signal Boosters are especially useful in many situations where conventional, trunking or paging radio signals are too weak to provide reliable communications. Specifically:
Signal Boosters should be considered especially when;
TX RX Systems has the broadest choice of models and the most experience in manufacturing these types of Signal Boosters and continues to improve and expand their products to meet any requirement, including channelized versions for locations with high unwanted signal levels. It is advisable to always contact TX RX Systems or their local representative for the latest models and for specific application assistance as well as system engineering support when you have a requirement.
FCC REQUIREMENTS:
Signal Boosters must be FCC type accepted and labeled as such when operating on any frequency regulated by the FCC.
The FCC has released specific rule for the use of signal boosters, effective July 19, 1996.
These rules allow the use of Signal Boosters within most FCC authorized radio systems without any additional licensing requirement.
The FCC has designated two classes of signal boosters:
THIS DOCUMENT ONLY DEALS WITH FCC "CLASS B" (Broadband) TYPE DEVICES!
SIGNAL BOOSTERS VS RADIO REPEATERS:
Although Signal Boosters are sometimes confused with regular repeater stations, there are important differences in the application and design of these very different devices.
Signal Boosters have the following characteristics:
1. Signal Boosters are designed to provide additional signal strength in areas where the
non-boosted signal would be adequate if there were no obstacles in the radio path.
2. Signal Boosters are not designed nor intended to be used to increase a radio systems
outer edge range.
In fact, the FCC rules for Signal Boosters are based on this condition.
3. Signal Boosters do not change the radio frequencies when the signal is boosted,
therefore the users do not have to switch channels on their radios to use the boosted
signals. This eliminates operational confusion, special user training, and additional
channel selection capabilities in the radios.
4. Class B Signal Boosters can boost many radio channels at the same time which can be more
economical than regular single channel repeater stations or Class A Channelized Signal Boosters.
5. Signal Boosters do not demodulate the radio signals and therefore do not require
modulation adjustments or audio signal processing of any kind. Signal Boosters do not
alter modulation characteristics.
6. Signal Boosters have lower output power levels than conventional repeaters.
7. In general, Signal Boosters are more compact, use less power and require less
maintenance than a normal repeater.
SIGNAL BOOSTER CONFIGURATIONS:
Signal Boosters are available in several design types;
a. Class B: Fixed Gain (Non-OLC), non-channelized types, where the gain and power levels are set to meet very specific or unusual requirements.
b. Class B: Automatic gain adjusting, or OLC (Output Level Control), non-channelized types, where the Signal Booster automatically sets the gain over a wide range of input signal levels.
c. Class A: Channelized versions that only pass specific radio channels on a channel by channel basis.
These are more expensive than non-channelized types and used primarily at the over-
the-air (OTA) interface in areas where many high level unwanted channels exist. In complex
system, these may also be required to meet FCC out-of-band emission limit requirements.
Again, this document only deals with Class B, Non- channelized, OLC type Signal Booster designs as this type is more commonly used to date. Non-OLC versions, Channelized types and customized Signal Boosters are also available from TX RX Systems.
THE COMPONENTS OF A CLASS B SIGNAL BOOSTER
The specific organization of a Signal
Booster can vary greatly according to the specific application and installation
requirements. In general, a Signal Booster with OLC contains:
- A gain block (a wide band, linear amplitude amplifier).
- Frequency selective filters/cavities placed at the amplifiers input and output ports.
- A special output level control (OLC) circuit.
- A power supply/regulator.
Signal Boosters may be constructed for
either:
A. One-way (such as paging coverage improvement or portable talk-back only), or
B. Bi-directional Amplification (two way) duplex signal amplification, the most popular of all designs at this time.
Signal Boosters may be applied in a manner similar to a coaxial cable RF signal distribution system. Non-radiating (common) coaxial cables and/or radiating coaxial cables (sometimes called "leaky coax") can be used to route the RF signal between Signal Boosters and radio coverage areas. Multiple Signal Boosters may be connected in series to provide radio coverage for multi-story buildings, long tunnels or separated areas of communications.
TX RX Systems has power splitters, antenna decouplers and antennas which permit complex RF communications signal distribution and signal branching applications.
AC and/or DC power operation is possible, as well as battery back-up operation. In some cases, power for the Signal Booster may also be applied at the Signal Booster from a distant location using the coaxial cable as a DC power cable.
TX RX Signal Boosters are available in weatherproof stainless steel NEMA cabinets suitable for outdoor or unprotected locations, lower cost painted steel cabinets or rack mounted systems.
BASIC ONE WAY CLASS B SIGNAL BOOSTERS:
The components and operation of a one-way Signal Booster shares many aspects that are common to a two-way Signal Booster, therefore a good understanding of the one-way Signal Booster is important to understand two-way Signal Boosters.
TX RX Systems "OLC" type designs are used in this presentation as they are the most common in use today.
A one way OLC type Signal Booster is organized as follows;
The operating frequency band and bandwidth of the Signal Booster is determined by the characteristics of the bandpass filters (cavities) used. These characteristics are engineered to meet the specific applications requirement. Bandwidth values generally range from 250 KHz to 20 MHz and typically 2 to 10 MHz.
The amplifiers will amplify all signals within the bandpass, therefore it is important to reduce the bandwidth to the minimum practical bandwidth for optimum operation. Note that 'practical' also relates to size and cost.
In all TX RX Signal Boosters the amplifiers are special ultra-linear amplifiers with a preset gain value. Common OLC Signal Booster models are available with gains ranging up to about 80 dB. The maximum output power of the output amplifier is set below the amplifiers 1 dB compression point ( +35 dBm) to prevent damage to the amplifier and minimize undesirable RF output signal components. This results in a typical operational output of about + 25 dBm for one or two channels input, depending on the specific model used.
The output power level is monitored by the OLC circuit and a gain adjusting control voltage is fed back to an electronic attenuator at the preamplifier input. The dynamic (output level control) range of the OLC circuit is typically 30 to 40 dB depending on the specific model used.
Since the amplifier gain and OLC control dynamic range are fixed values, coaxial pads should be inserted before the electronic attenuator when the input signal levels are high enough to constantly cause attenuation at the electronic attenuator. The addition of coaxial pads, if required, restores the full dynamic operating range of the Signal Booster and makes the output signal levels more consistent and coverage more predictable. Fixed value coaxial attenuator pads are preferred over variable attenuators due to the infrequent need to alter the settings, the higher cost of variable attenuators and the lesser possibility of noise being generated within the attenuator itself.
The use of coaxial pads instead of amplifier gain adjustments allows the amplifiers to operate within the best possible operational range, which is often compromised in circuits using current or voltage adjustments to control the gain. The amount of relative attenuation is easily observed by measuring the DC voltage applied to the electronic attenuator control input. The factory provides a verified chart of control voltages with each Signal Booster shipped.
Decoupled RF test points are also provided in standard versions of TX RX Systems Signal Boosters to permit performance testing or alignment while the Signal Booster is in operation. Non-disruptive testing is a common feature in most TX RX Systems products.
As no input-to-output frequency conversion occurs in a Signal Booster, the RF output must not be close coupled to the RF input to prevent feedback saturation of the Signal Booster. The isolation within the Signal Booster amplifier itself is well over any possible gain of the Signal Booster and does not present a concern so long as the factory connections and shielding is maintained. The amount of external output to input isolation requirement is usually not a problem since the input and output are normally located on different sides of the signal blocking obstacle (wall, ground, etc.).
If insufficient isolation exists, antenna placements, input pads and/or amplifier stage gain reductions must be implemented.
When a radiating cable, or "leaky coax", is used at the input and output ports of a Signal Booster there is little potential of feedback due to the high coupling losses of such cables.
A good rule of thumb is a minimum output-to-input path loss equal to the maximum operating gain of the Signal Booster plus 10 dB. For example, a Signal booster with 55 dB overall maximum gain should have minimum of -65 dB output-to-input isolation.
OUTPUT SIGNAL LEVEL:
The non-channelized Signal Booster will amplify all signals within the input filters bandpass therefore the effective output power per single channel will change as more channels are amplified.
In other words, the maximum output power level is shared by each input channel. The exact proportion of 'sharing' is determined by the number of channels AND the power level of each channel in relation to all the other input channels. A chart illustrating the effect of multiple channel amplification is included in the Addendum.
When using a non-channelized Signal Booster where many undesirable channels may be present at the input also, the effect of those channels may be reduced by several means;
- Use a directional antenna to direct the signals to the desired repeater site. In some cases, a corner reflector may be used to increase rejection of unwanted signals from the sides of the path. This increases the desired signal level and greatly reduces the power loss effects on the desired channel at the output of the Signal Booster caused by undesired signals.
- Use more selective bandpass filters in the signal path coming from the antenna. Some filters may have to be placed outside of the Signal Booster or in a larger cabinet because of their physical size.
- If the above steps are inadequate, then channelized, Class A, signal boosters should be considered.
Note that, in many cases, any additional filters only have to protect the INPUT from the 'outside world' because only 'desirable' signals normally exist inside the area to be covered.
INPUT SIGNAL LEVEL:
The input signal level is perhaps the greatest item of confusion between conventional repeater stations and Signal Boosters.
A conventional repeater is designed to provide radio coverage for great distances and requires only a few microvolts of signal input.
A Signal Booster is designed to "fill-in" an area that would normally have a very good signal level and therefore operates best at higher input signal levels.
A nominal input signal level of -60 dBm can be used as a guideline input signal level for most standard Signal Booster configurations, although Signal Boosters will operate at much lower levels with the acceptance of reduced output signal levels.
As examples:
A -60 dBm input level applied to a Signal Booster with 75 dB gain would have a maximum output level of +15 dBm.
A -85 dBm input level to the same Signal Booster would provide a -10 dBm output signal level.
Sensitivity and quietening level specifications are not applicable to Class B Signal Boosters due to the relatively high level of the input signals and the lack of any frequency conversion or demodulation circuits.
NOISE:
The noise figures of the amplifiers are normally of no concern except when many high gain Signal Boosters are placed in series. This is minimized by reducing the gains of the Signal Boosters and losses between the Signal Boosters. The first amplifier is the dominant contributor to the noise figure any design using amplifiers in series. The factory will provide assistance in determining specific noise figures for complex systems using several Signal Boosters in series, as the resultant noise figure will be much less than the simple addition of the individual Signal Boosters noise figures.
Note that the "system noise figure" specified by TX RX Systems is the worse possible overall value, with the maximum gain of all stages applied.
POWER SOURCES:
Signal Boosters are normally configured to use a 120 or 240 AC power source.
Optionally, they may be operated from a DC source, which requires a DC voltage converter if the DC source is less than 24 volts.
Non-disruptive automatic transfer battery backup options are available.
If power is not available at the Signal Boosters physical location, the Signal Booster may be powered by inserting a DC supply voltage on the coaxial cable at a point where the cable is near a power source. This requires the addition of optional coupling devices that are available from TX RX Systems and an external DC power supply.
ACCESSORIES:
Additional supporting devices, such as power splitters, antenna decouplers, indoor antennas, terminating loads for radiating cables, etc. are also available from TX RX Systems to complete almost any possible application of Signal Boosters.
TWO WAY (Bi-Directional, Boadband) CLASS B SIGNAL BOOSTERS:
Two-way Signal Boosters are basically two, one-way signal booster sections in the same cabinet with one section operating in one frequency band and direction and the other section operating on another frequency band in the opposite direction.
All the previous one-way Signal Booster criteria applies to two-way Signal Boosters.
Two way Signal Boosters use the bandpass filters in a manner similar to a duplexer to maintain separation and isolation between the two frequency bands while using a single coaxial port for the RF signals on each side of the amplifier sections.
In frequency bands below 450 MHz, the variable and small amount of separation between transmit and receive frequencies frequently requires larger filters and cabinets. These are engineered, quoted and specified on a case by case basis by the factory.
Naturally, Signal Boosters cannot amplify a simplex frequency signal in both directions.
TX RX Systems has FCC type accepted Signal Booster models in the VHF , UHF and
800 - 960 MHz bands.
Antennas, Radiating Cables and other Radiators:
The 'external' antenna used for the path between the Signal Booster and the distant base/repeater station should be a directional, gain antenna which will improve the signal path and reduce undesired signal that may occur in other directions. Parabolic and corner reflector type antennas often have better rejection of unwanted signals that appear to the side of the antenna.
Non-radiating cables should be low loss, well shielded coaxial cables fpr best results.
The amount of radiation and signal levels inside the area served by Signal Boosters can be controlled by selection of radiating coaxial cables (i.e. "leaky coax") or antennas as radiator elements. In some cases both may be used.
The choices of radiators is based on several factors;
- The topography of the coverage area. (Long and narrow areas versus large open areas).
- Costs of radiators and installation.
- Maintaining input levels to the Signal Boosters within the OLC control range.
Radiating cables provide coverage that can be easily controlled and is especially applicable to tunnels, stairwells, passageways, etc. The disadvantages of using radiating cables is the cost of the cable, its additional installation concerns and the high amount of coupling losses. The primary advantages of using radiating cables inside passageways and tunnels is due to the fact that the distance between the cable and portable radios does not vary greatly and the high coupling losses are usually acceptable because the portable is always near the cable.
Antennas provide the minimum coupling loss and often lower installed cost. Generally, unity gain omni-directional antennas are preferable.
More than one antenna may be coupled into a non-radiating cable to distribute the signal over a large area. Multiple antennas should be placed such, or decoupled, so that the signal levels coming into the Signal Booster are as uniform as practical.
Decouplers, 'taps' and 'splitters' are used to connect antennas and cables to the main feed lines. These devices are avaiable in many types and values, including multi-port and multi-band models with coupling ranges from 3 to 40 dB per port. These, along with simple coaxial RF pads, allow the system designer to distribute the signals equally throughout an area.
Internal gain antennas can be a disadvantage when the radio users roam from side to the center of the antennas beam, causing great signal level variations.
Isolation between the Signal Boosters output-to-input ports may become more difficult to achieve when gain antennas are placed near outside windows, doorways, etc.
An example of a system using both types of radiators: Radiating cable is used inside a stairwell that leads to a large open underground parking garage. The end of the radiating cable is in the garage and connected to a antenna placed high on one side of the garage. The high antenna placement prevents a radio from being close to the antenna to minimize signal level variations to the Signal Booster.
Multipath effects:
Some system engineers initially express concern over the possibility of conflicting signals coming from both the direct path and the Signal Booster output. In practice however, the multipath effects when using a Signal Booster has been found to be less than that normally experienced in any mobile radio system.
The output of the Signal Booster is usually much greater than that found in the direct path, allowing inherent FM capture effects to minimize any interaction at the receivers when the radio is operated inside the coverage area provided by the Signal Booster.
There is always the possibility that the differential between the two signals will not be adequate for FM capture at some location at the outer edge of the Signal Boosters coverage. The most likely place for this to occur is when the radio user is near a doorway or an outside facing window.
Other factors also aid in reducing multipath resulting from Signal Boosters because of the nature of the users and the application itself; (1) Signal Boosters are usually used to improve signals in areas where portable radios (as opposed to mobiles) are primarily used. (2) Portable radio users instinctively make minor adjustments in their radio positions to get the clearest reception. This is also the location of lowest multipath.
Signal Booster multipath is similar to that inherent in any mobile/portable radio system used in an urban area and often with a much smaller area of potential equal level (non-capturing) multipath signals.
SIGNAL BOOSTERS IN SERIES:
In large facilities and long tunnels, Signal Boosters may be connected in series.
The path between Signal Boosters may be either coaxial cables (both non-radiating or radiating) or antennas.
It is general practice to set up one Signal Booster to 'talk to the outside world' and this is where the levels are adjusted so that the 'inside' portion has balanced levels of signal between the input and output paths.
Balanced, in this case, usually means the following Signal Boosters will all have the same configuration of pads and gains. The greatest differential in signals are those over the air to and from the external repeater station.
This technique becomes more apparent as the system design is developed.
MULTIPLE FREQUENCY BANDS:
It is sometimes desirable to operate the Signal Boosters on more than one RF frequency band in the same direction. This cases usually use one coaxial cable to reduce cable costs or due to other limitations at the installation site.
In these cases, there are several approaches;
1. Bandpass filters are paralleled to pass each band through the same amplifier sections.
Four bandpass filters would be required for a two band two way Signal Booster.
2. Two or more single band Signal Boosters are operated in parallel using crossband coupler devices. This arrangement allows maximum signal level control for each frequency band and is the preferred method, as each band can be controlled independently.
There are some systems that operate with more than two bands through the same Signal Booster amplifiers. Specially engineered combinations are available.
Some two band combinations (UHF + 800, UHF + 900, 800 + 900) are becoming more common.
Some VHF or UHF combinations (152 MHz + 173 MHz or 450 MHz + 470 MHz for example) may also use parallel filters to reduce the number of undesired channels between the desired bands.
Another case would be to pass a paging channel one way while passing two way communications in another frequency band.
All of these possibilities require factory engineering assistance due to the unique operation of each combination. TX RX Systems will provide a prompt and free preliminary technical and cost evaluation of any requirement.
DOING YOUR OWN PRELIMINARY SYSTEM DESIGN:
Worksheets and examples are available to assist you in the preliminary design of a Signal Booster system in the Appendix.
These worksheets allow you to organize and analyze the various system design factors and to get a better understanding of the principals used in Signal Booster system designs.
Perhaps as important, the worksheet information will provide the basic information required by the TX RX Systems engineers to assist you in your requirement and establish common terminology.
DETERMINING SIGNAL LEVELS:
The primary objective of a signal level survey is to determine where the radios cannot communicate or the signal levels are too marginal to be reliable.
Remember: If only one critical location in a building cannot get the signal, you will probably need a Signal Booster, regardless of any other higher signal levels in that same structure.
Test the most probable low signal areas first. Once it has been determined a Signal Booster is required, the layout can often be arranged to accommodate other areas of lesser priority.
System engineers can minimize initial measurements to those areas where Signal Boosters would probably be required in any case. Typical examples are;
- Second level or lower basements.
- Tunnels over 2000' or without end to end visibility.
- Windowless, thick walled structures. Especially stairwells and elevator areas.
It is not difficult to estimate the approximate signal strength that would be normally be present in the general vicinity surrounding an obstructed area using common 'over the air' propagation engineering methods, but it is more difficult to determine the signal strength inside an obstructed area by theoretical engineering models alone.
There are many variables in the nature of the materials causing the blockage and other obstacles within the area to be covered. For example, wall and floor construction attenuations can range from about 3 dB to over 90 dB. Even the type of glass used in a building window can have attenuation factors from near 0 dB to over 20 dB.
A Signal Booster system that is designed with little reserve and marginal signal levels limits the future flexibility of the system. The area to be covered can also be a dynamic and changing situation. What may be adequate today may not stay that way as station sites are changed, structures are altered and departments relocate.
Signal level measurements are more accurate when the test signal levels are relatively similar to those expected when the Signal Boosters are installed. Excessively low level testing measurements can be influenced by undesired signals and noise, especially when a high gain, wide bandwidth spectrum analyzer is used.
Measurements made with a high quality receiver (not a broadband scanner), which has had its RF signal level test point (i.e. RSSI) calibrated, is usually accurate enough for the tolerances required for a system design.
Sometimes, potential Signal Booster requirements for a proposed system can be based on simple 'walk-throughs' of areas using portables in the same band that communicate with an existing or similar radio system base station in the same frequency band located close to a proposed radio systems station site.
In critical "must talk" coverage areas, there is no good substitute for on-site signal measurements using close simulations of the anticipated distant base station facilities.
It is strongly recommended that the system design include considerable safety margin to accomodate future changes in the system. A 15 dB 'use factor' derating is commonly used.
Introduction to RF Signal Distribution using Fiber Optics
Introduction
In recent years, the use of fiber optic (FO) cable in place of more conventional coaxial cables has become viable in many applications due to advances in analog fiber optic technology . This discussion is intended to make the typical wireless system designer familiar with fiber optic terms, specifications and devices as they would apply in a radio frequency (RF) system design.
For the purposes of introduction to FO technology some generalizations are used. When specific specifications are used, they are based on manufacturers published specifications available at the time this text was written, such as the current Foxcom Wireless specification sheets.
Please note that this presentation does not cover digital data or LAN type products or applications which use digital fiber interfaces instead of analog.
Why use Fiber Optic system for RF applications:
RF system designers are familiar with the two major limiting characteristics of coaxial cables: the RF loss increases with frequency and length. Coaxial cables are limited in the length that they may be used without additional amplification. Long lengths of coaxial cables, such as might be required in tunnels or building-to-building, rapidly become cost prohibitive, especially if in-line RF amplifiers are required.
Fiber optic cables have very low losses compared to coaxial cables. With RF over fiber distances of 10 miles or more being practical, the system designer has a new tool to help solve difficult RF distribution challenges that would normally be impractical using coaxial cables.
The fact that FO cables do not 'leak' or couple RF signals makes them ideal when routing through noisy RF environments or running long lengths parallel to other FO or RF cables. This same feature makes FO cables ideal when used in an application that cannot tolerate EME emissions, such as TEMPEST sites, etc.
The lack of electrical conductivity may also be attractive in some applications, such as electric utilities.
Fiber optic cables can be much smaller and lighter weight than corresponding RF cables and are non-metallic, making installation and routing much simpler.
Fiber Optic System Conventions and Components:
The basic components of a RF - FO system consists of (1) fiber cable (including associated connectors), (2) FO Transmitters and (2) FO Receivers. The transmitter and receiver components may be small stand-alone packages or multiple, rack mounted plug-in units. Additional FO devices, such as splitters and duplexers, that expand the system design options will be discussed later.
The Fiber Optic Cable:
Fiber optic cable comes in many different types, configurations and specifications. Rather than go into great detail of the many variations, we will concentrate on the most important basics relative to RF applications.
'Single mode'.
In general, multimode has been in use longer than single mode and meets the needs for shorter data communications applications where the data rate is comparatively low (i.e. <100 Mbps) when compared to RF signals (100 to 2200 MHz).
While multimode cable can be used for very short cable runs (less than 2000 ft, typically), the distance restriction can limit the usefulness and future expansion of a system.
Single mode is the preferred type of cable for RF applications, especially for longer cable runs where higher output laser type transmitters can be used and where very wide bandwidth and high RF frequencies are required. Single mode losses are less than multimode. It is practical to design a single mode fiber system that has very wide bandwidth with zero net loss, end to end, up to about 12 miles without any in-line amplification.
The physical installation considerations for FO cable is usually simpler than installing 1/2" corrugated coaxial cable, with comparable bending radius requirements. However, FO cable works best with minimum bends and turns of the cable, so installation in conduit or raceways is preferred to give the optimum support and maximum practical bending radius. These considerations are determined by the number of fibers in the same cable sheath and the mechanical construction of the cable. Fire resistant and Plenum rated cables are common. The costs of FO cable are dependent upon the number of fibers and the mechanical construction of the cable. It is common practice to include several spare fibers (called "dark fibers") for growth due to the small incremental cost increase.
Connectors for high performance single mode cables are unique and are generally less expensive than coaxial cable connectors. Most fiber connectors require special equipment to install the connectors or splices for minimum optical loss.
APC (Angle Polished Connectors) are used on single mode fiber to reduce the reflections back to the laser transmitter which could reduce efficiency or cause damage in extreme cases. In APC connectors the fiber end is slightly angled (8 degrees) so that any reflected light is highly attenuated instead of propagating freely within the fiber. Some training and special tools are required to properly attach and test APC connectors.
Unlike coax connectors, many popular versions, such as FC/APC types, are all the same gender and use a "sleeve" (the equivalent of a coax 'splice' or dual female connector) to complete each connection. Sleeves are purely mechanical with the optical coupling occurring dierctly between the butted ends of the two connectors.
For loss approximations, use 0.38 dB/Km or 0.515 dB/Mile for single mode fiber cable plus 0.25 dB loss per FC/APC connector. Note: A splice or in-line connection would use two connectors and one sleeve for a total loss of approximately 0.5 dB per splice.
FO Transmitters:
Most FO transmitters that are designed for data transmission are modulated in a 'digital' (off-on) two state manner. RF FO 'analog' transmitters however are linear in operation, the light source being amplitude modulated at the RF frequency. Analog FO transmitters suitable for wireless RF applications are less common and more expensive than simpler digital FO transmitters, mostly due to the additional circuits needed maintain high linearity and stability over temperature.
Analog FO transmitters may use LED emitters for moderate level output (shorter range/low fiber loss) applications and solid state laser emitters for high power output (longer distance/higher fiber loss) applications.
In older analog designs, the RF input frequency was down converted to a lower "IF" frequency because the linear bandwidth did not extend to the input frequency range. This approach inherently increased the probability of intermodulation, RF distortion and, in some cases, unacceptable RF envelope delays. Fortunately there is no longer a need to convert frequencies below 2200 MHz as improved modulators and emitters have become available.
Modern linear transmitters modulate the optical emitter at the RF frequency directly. RF system frequency bandwidths of 100 to 2200 MHz, or more, using one transmitter and one receiver over one fiber are commonly available at moderate cost.
FO Receivers:
Analog FO receivers are required to convert the FO signals back to RF frequencies. In more advanced designs, the gain of the receiver may be adjustable to prevent overdrive. Overdrive can distort the recovered RF signals. Receivers also include linear compensation circuits and are commonly available.
A basic FO - RF System:
In figure 1, the most basic one way system is shown.
In this example, RF from a RF transmitter is injected into the FOT (Fiber Optic Transmitter) at one end of the fiber optic cable. At the other end of the cable, the Fiber Optic Receiver (FOR) converts the signal back to RF. The input level is O dBm or less and the utput level is equal to the input level. Note there is a net zero loss of RF signal end-to-end.
In Figure 2, we have refined the system shown in figure 1 into a Downlink Only, one way system:
1. The RF source is a decoupled signal from a RF transmitter (RF TX). The full output power of the RF transmitter would damage the FOT. Assuming the RF transmitter is also connected to an outside antenna, the decoupler used is a directional type to further reduce any unwanted signals that may be coupled via the antenna to the FOT. The -50 dB decoupler adds approximately 0.1 dB loss to the power going to the outside antenna. With the RF TX output level of + 52 dBm, we have a +2 dBm level in to the band pass filter, below.
2. A Band Pass Filter (BPF), with 2 dB insertion loss in the example, is used between the RF transmitter and FOT to further reduce any unwanted RF signals such as signals out of the filter passband coming from the antenna, spurious emissions and RF transmitter sideband noise. This approach reduces the possibility of intermodulation in the FO system and conserves optical power to the desired frequency band. Adding the 2 dB loss of the BPF gives a 0 dBm level to the input of the FOT.
NOTE ON WIDEBAND CIRCUIT POWER: The use of bandpass filters improves the output power per carrier by reducing unwanted energy within the very wide passband of the fiber optic system. Like all broadband systems, the maximum power allowable is based on the 'composite', or total, power of ALL the signals passing through the system. Any undesired energy that is removed by filters increases the power available to desired channels. Therefore, bandpass filters that closely match the desired passband bandwidth(s) are highly recommended.
This approach is particularly applicable to the combined output of several transmitters (i.e. SMR, AMPS, etc.) when a combline or other wide bandwidth filter is used. The decoupler is placed between the output of the transmitter combiner and the antenna coaxial cable. The amount of decoupling would be set so that the maximum possible output power (i.e. maximum composite power) of all the transmitters does not exceed the maximum allowable input to the FOT.
Most FO transmitters (FOT) are designed for a maximum RF input level of 0 dBm, which is easy to obtain from most base or repeater stations.
Over the air inputs to the FOT usually require additional filters and a low noise amplifier (LNA), such as those in TX RX Systems broadband or channelized signal boosters.
The 9/125 single mode fiber optic cable has an approximate loss of 0.515 dB/mile in the example.
It is important to know the connector losses if the FO cable because connectors and splices can account for the majority of optic loss in 'real' installations. In the example, an estimate of 0.5 dB was used for each paired connection (actually 0.25 dB per connector times two connectors per connection).
The FO receiver (FOR) in the example is designed for a 'zero net loss' system design with up to 9 dB cable loss. This method of stating the system performance is typical of fiber optic system specifications, which use a 'zero loss' basis to determine the optical loss budget in a system. In other words, they design towards a 'zero loss' fiber optic link overall. Naturally, the system may still operate adequately with less or more loss, dependent upon the dynamic range of the system.
The maximum FO receiver optical input level (i.e. that which generates 0 dBm RF output) is usually the highest level where linear reception is assured. If that level is exceeded, RF performance will deteriorate and could even damage the receiver if exceeded greatly. The system designer must avoid overdriving the FO receiver by reducing the gain of the FO receiver (assuming it has an adjustment) or increasing the optical circuit loss. Optical pads are available if needed and are used at the penalty of increasing the system noise floor slightly.
The system designer must also establish the lowest input level of each signal as that will be critical when determining signal to noise performance later. While the signal may remain within the dynamic range of the receiver, it may become too low relative to the noise floor of the system and degrade performance due to too reduced signal-to-noise ration (S/SN).
Another band pass filter is added to the receiver output and it may be a minimal filter or even deleted in some cases. The purpose of this filter is to simply reduce unwanted signals and noise that may effect later RF amplifiers or nearby RF receivers.
Since the FOR output should near 0 dBm in the example;
(1) It may be too much signal if it is fed directly to a RF receiver. A high value RF pad (i.e. 90 dB) would reduce the RF signal to -92 dBm (including the 2 dB loss of the second BPF) which is still a very strong RF receiver input level AND reduce the output noise floor at the same time.
(2) If the FO receiver output is to be sent to RF Receivers over the air, a RF amplifier (i.e. 'signal booster') will probably be required to overcome the free space losses and other signal perturbations that will occur over the air. A free space loss of -115 dB and a +25 dB signal booster amplifier is used in the example to deliver a -92 signal level to the RF receiver over the air.
In figure 3, an Uplink Only, one-way example is shown. It is very similar to the Transmit Only (figure 3) except in the over-the-air path, on the right side of the drawing, an OLC* (Output Level Control) circuit is added and the gain of the amplifier is much greater.
The OLC* circuit samples the output and reduces the gain to maintain a more stabilized output level when the input signal levels vary widely. Since the input from the RF TX is over the air, the signal levels into the OLC-AMP combination will vary widely. Broadband OLC* circuits normally provide up to 40 dB of dynamic range. The more stable output of the OLC-AMP will allow the FOT to operate near its optimum input level even though the RF signals are fluctuating. This improves the SNR of the whole receive path.
Note that a high value RF attenuator, or Pad., is applied to the FOR output.
This serves several purposes;
1. It lowers the noise from the FOR into the RF receiver input to prevent
masking of signals coming via the antenna.
2. It lowers the desired RF signal and minimizes any receiver intermodulation
.
3. It reduces any outdoor re-radiation of the FOT signals via the outside
antenna.
TWO-WAY RF/FO System:
A two-way system (figure 4) is basically combining the Transmit only (figure 3) and Receive Only (figure 4) as one system.
At the RF end of the duplex system, the decouplers are normally located at either the two radio antenna ports (transmitter and receiver) or the duplexer radio ports (not antenna port) to take advantage of any additional TX - RX isolation existing in the radio system.
If the decoupler is to be placed in a duplexed antenna line, only one directional decoupler is used but the filters should be equal or superior than the duplexer filters to maintain the original RX-TX isolation of the radio equipment. Additionally, the decoupling value must be the smaller of the transmit or receiver decoupler requirement with pads used between the filters and FO modules to adjust levels accordingly.
HOW MANY FIBERS TO USE:
The optical signals used in the fiber optic cable have similar interference considerations as RF signals inside a coaxial cable. Since the FO cable is bi-directional two signals on the same optical frequency can't be separated from one another and cause interference to each other.
In wide bandwidth analog FO applications, 1310 nm is the most common and inexpensive optical signal frequency. 1310 nm is near the minimum loss frequency of single mode fibers, making longer distances possible. When one optical frequency is used, it simplifies testing and minimizes spare equipment models. That means two fibers can be used in a two-way system design, one for the 'downlink' (repeater transmit frequency) and one for the 'uplink' (the portable transmit frequency).
Since most fiber optic cable installation contain many 'pairs' (i.e. 6, 24, 50, etc.) in anticipation of future growth, the addition of a few more fibers has minimal effect on overall cable installation costs. Unused single mode fibers may already exist in a previously installed fiber cable.
However, if the optic fiber availability is limited, there is the equivalent of duplexing two optical signals onto one fiber, using a different frequency in each direction. The second frequency is usually 1550 nm. This frequency is also near the lowest loss frequency of single mode fibers, but slightly more expensive to implement in fiber optic transmitters, another reason for favoring 1310 nm if possible.
In fiber optic systems, a 1310/1550 nm duplexer is called a "WDM" which is the abbreviation for "Wavelength-Division Multiplexer". It has three ports: (1) 1310 nm, (2) 1550 nm and (3) duplexed (FO cable) port. It is connected using the standard FO connectors, discussed earlier. These are passive devices not requiring external power.
Unlike RF duplexers, WDM devices can have directional characteristics and come in different configurations. A "unidirectional' type has the two signals going the same direction, which is NOT normally applicable to our requirement. A "Bi-directional" type is most like a RF duplexer as seen below:
WDM devices add loss to the system and therefore reduce maximum operating
cable lengths. Losses include internal coupling losses as well as connector
losses. While specifications from brand to brand may vary slightly, the general
specifications are:
- 1310 to 1550 port isolation (Directivity): >55 dB
- 1310 or 1550 port to cable insertion loss: <1 dB
- Return loss using APC connectors: <-55 dB
USING FIBER TO CONNECT MULTIPLE LOCATIONS:
In some situations, the fiber optic circuit may have to connect several remote sites to one radio site. In cellular and PCS systems, the remote sites are sometimes called microsites or microcells.
A frequent application is a 'campus' like situation where the repeaters are located atop one major building and several other buildings (with RF obstructed areas, basements, etc.) are located within a few miles of the repeater building.
Fiber optic cables often already connect the buildings for data communications and, if it is single mode type fiber, spare fibers can be used to enhance radio signals with the outer buildings.
Obviously, we could use a dedicated FO transmitter and FO receiver for each
remote site. However if the fiber losses are low, it is possible to reduce the
quantity of transmitters by using optical splitters. Optical splitters are
available in several versions, usually with binary multiples of outputs; 2, 4, 8
etc. The most common is equally divided 2 and 4 way splits. Like RF splitters,
the losses are directly related to the number of splits:
- 2 way = 3.5 dB with APC connectors, typical.
- 4 way = 6.5 dB with APC connectors, typical.
Just like RF again, the fibers feeding the FO receivers must remain separated, so there will be one FO receiver required for each remote site.
In Figure 6 note that each FO receiver output at the repeater site has individual pads to reduce the composite noise floor and provide FOR-to-FOR isolation and minimize receive combiner intermodulation. For example, if 40 dB pads are used, an additional 80 dB of combiner port-to-port isolation occurs.
In real applications, it is also good practice to include decoupled taps as test points to read the RF levels going into the combiner for test and maintenance. Be sure to include the RF combiner losses in signal level calculations and pad estimates.
A similar system design is possible when WDM's are used. If WDMs are used, the number of fibers is reduced by 50% but a WDM must be added at each remote site and another WDM for each fiber added at the repeater site. In the 4 remote site example, it would take 8 WDM's to operate all the fibers full duplex and 4 FO transmitters and 4 FO receivers would have to be 1550 nm models.
MORE COMPLEX SYSTEMS:
With the building blocks described here, much more complex systems can be developed. Many other devices are available to amplify, multiplex and split optical signals, however many are not suitable for linear operation.
System designers using proven products have the choice of;
- Several versions of stand alone, one way 'boxes'.
- Integrated duplex models with integral WDM's in one case.
- Rack mounted systems for more complex applications.
- In-building distribution systems with 'headend' units capable of interfacing
with multiple remote fiber
- RF 'remote hub units'.
With the many devices discussed above, it is possible to design RF distribution systems that were not possible or practical a few years ago.
One example are new FO devices that are designed to integrate the remote end components into one compact and inexpensive unit that has moderate output RF levels that compare to a distributed antenna system radiator. The big difference is no coaxial cable is required, only the fiber and a power source at the far end.
Introduction to Duplexers
Why are duplexers used?
Radio receivers can be damaged if high level
RF signals, like those directly from a transmitter output, is applied to the
receiver antenna.
Additionally, receivers may become ‘desensitized’ (or ‘de-sensed’) and not receive weak signals when high noise levels or another signal near the receive frequency is present at the receivers antenna input.
Obviously, radio receivers and transmitters cannot be directly connected to the
same antenna without some device being used to:
(1) switch the antenna
between the transmitter and receiver so that they are never connected to the
same antenna at the same time.
(2) When the transmit and receive frequencies
are different, filters may he used to reduce the transmit signal levels to an
acceptable low level at the receivers antenna input. Naturally, you cannot
filter out the transmitter signal when it is the same as the receiver frequency.
Definition of a duplexer:
A device which allows a transmitter operating on one frequency and a receiver
operating on a different frequency to share one common antenna with a minimum of
interaction and degradation of the different RF signals.
Duplex Operation
Duplexers
are often the key component that allows two way radios to operate in a full
duplex manner. Full duplex means the transmitter and receiver can operate
simultaneously as opposed to the ‘push-to-talk’ manner used in non-duplex (or
‘simplex’) operating modes. Remember, The radio system must use two frequencies per ‘channel’ to use the
kinds of duplexers we are discussing.
Recently, some very specialized digital radio systems that are under development are emulating duplex operation by switching the transmitter and receiver off and on extremely rapidly. This is not real full duplex operation but appears similar to the radio users. This discussion does not address this approach, but instead deals with more common accepted land mobile practices. Duplexers are the devices that allow a mobile telephone to operate like a wired telephone, with either or both people speaking at any time without using a microphone switch to enable the radio transmitters.
Repeaters
Most radio systems today use repeaters located on top of buildings, towers or on
hill tops. These repeaters use two frequencies in a duplex fashion to extend the
range of the radio system and make signals much stronger. In most cases, a
duplexer is used as part of the repeater station.
The duplexers at repeaters may serve several objectives:
Why not use two antennas?
Two antennas may be used instead of
a duplexer, provided the antennas are placed far enough apart that the
transmitter signals do not interfere with the receiver. Two transmission lines
will also be required. The isolation required between the transmitter and receiver is a
complex issue and influenced greatly by the specific transmitter and frequencies
used, the bandwidth of the channel, the difference in frequencies of the two
frequencies to be used and the minimum amount of receiver degradation that is
acceptable to the user. It is not unusual to have a radio system require as much as 80
to 100 dB isolation between the transmitter output and the receiver input.
When two antennas are used, the type of antennas, the physical spacing and the orientation of the antennas to one another are also major concerns. The antenna to antenna isolation can also be influenced by the presence of other antennas on the same tower as well as other nearby transmitters and mechanical structures. These factors may change over time and be out of the control of the repeater operator. Antenna separation designs should also consider any additional receiver protection that may be required for other transmitters that may be present on the same tower.
In some extreme cases, duplex filters AND antenna separations may both be required to obtain satisfactory operation. This generally only occurs at lower frequencies with small differences between the transmit and receive frequencies or when closely spaced channels are combined. Typical Antenna Spacing Isolation Values: (In dB) (Based on vertically polarized half-wave dipoles. Actual experience will vary due to local conditions, antenna variations, etc.)
TYPES OF FILTERS USED IN DUPLEXERS
There are several ways to implement a duplexer, but all rely upon the
characteristics of different types of RF filters. Specifically:
- Bandpass filters which allow a specific range of frequencies
to pass through them. The filters are designed and tuned to a specific ‘center
frequency’ and ‘pass band’ with relatively low losses to desired frequencies and
higher losses that increase as the deviation from the center frequency
increases.
- Reject or Notch filters which operate opposite of a bandpass
filter. These are designed to cause high losses at the center frequency and
lesser losses as the frequencies increase from the center frequency.
- Specialized filters such as TX RX Systems “Vari-Notch” (c)
filter, which has characteristics of both a bandpass and notch filter in one
device.
The filters are usually tubular or square cavity type filters but other types of construction such as combline, ceramic, etc. may also be used in some cases. Cavity type filters offer the best overall balance of performance, simplicity and costs. Combline and ceramic filters have some space and size advantages at higher frequencies. Ceramic filters may have power limitations and higher cost. Although this discussion centers on the more commonly used cavity type filters, the basic principals will apply to any type of filter used in duplexers.
DUPLEXER AND FILTER TERMINOLOGY
Decibel (dB):
A decibel is a logarithmic scaling value that is used in most RF engineering
work because of its universal acceptance and simple manipulation in calculating
signal and power levels. A decibel is a relative number, not an absolute value. For example, a +20 dB difference in a power level is the same as
saying the level change is 100 times the starting value. If the change was -20
dB, the change would be 1/100th the original level.
Decibel/one milliwatt (dBm):
This signifies an absolute (real) value, with 0 dBm being one milliwatt of
power.
Selectivity:
Selectivity is a measurement of the ability of the filter to pass or reject
specific frequencies relative to the center frequency of the filter. Selectivity
is usually stated as the loss through a filter that occurs at some specified
difference from the center frequency of the filter. For example; “- 3 dB bandwidth is +/- 250 KHz.” means the output
level of a signal frequency at + or - 250 KHz from the center (tuned) frequency
of the filter will be at least 3 dB less than the level of the same signal IF it
was at the center frequency.
The greater the selectivity the greater the attenuation of frequencies other than the center frequency. The greater the selectivity the narrower the lowest loss ‘window” of the filter and the need for good temperature and mechanical tuning stability in the filter design. The larger the diameter of a cavity filter the greater the selectivity, assuming similar materials and construction of the filters being compared.
Tuning Stability:
Tuning stability is the ability of the filter to remain at tuned at the desired
frequency over time and variations in temperature, orientation and vibration.
Many aspects of filters are designed to overcome these variables, such as the
use of temperature compensating metals, elimination of threaded tuning rods
which can store mechanical torque stresses, added cooling, fine tuning
adjustments, etc.
Insertion Loss:
Insertion loss is the minimum amount
of loss to the signal passing through a filter at a designated frequency. For
example, a filter may have 1 dB insertion loss at its center frequency and if
two filters are used in series in a duplexer, the duplexers insertion loss would
be 2 dB.
Insertion losses occur in both the transmit and receive paths of a duplexer and they may be different amounts. The greater the insertion loss, the less the output level. Higher insertion losses generally increase the selectivity of cavity filters. (i.e. A filter bandpass might be +/- 200 KHz at 1.5 dB insertion loss and +/- 100 KHz at 2 dB insertion loss. The greater the insertion loss, the greater the power dissipation and temperature rise of the filters. High insertion losses may reduce the power capacity of a filter.
Receiver Desensitization:
Receiver desensitization, commonly called ‘receiver desense’, is caused when
high RF signal levels enter a receivers antenna input. When desense occurs, the usual symptom is as though the desired
signal was reduced; the signal becomes noisy or even fades out completely. The frequency of the desensitizing signal can be considerably
different than the frequency the receiver is tuned to. The interfering signal
can be wideband noise and/or spurious emissions from the associated transmitter
or other nearby transmitters. The susceptibility of a specific receiver to off-frequency
signals is dependent upon the receiver design and any external filtering added
to the receiver.
Transmitter Noise:
Every transmitter emits signals other than those on the desired frequency. The
frequencies and amplitudes of these undesired signals varies greatly and is
dependent mainly upon the transmitter design and the modulation used. The amount
of transmitter noise can be reduced by external filters and/or physical
isolation between the transmitter and any receivers.
TYPES OF DUPLEXERS
Bandpass Duplexers:
Bandpass duplexers use several filters to reduce the bandwidths of the
transmitter output and the receiver input frequency bands.
Cavities 1, 2 and 3 tuned to pass 458 MHz. 80 dB loss at 453 MHz.
Cavities 4, 5 and 6 tuned to pass 453 MHz. 80 dB loss at 458 MHz.
Cable lengths between cavity 3 and cavity 6 to "T" are tuned lengths.
The amount of isolation between the transmitter and receiver may be reduced or increased by changing the number of cavities and the size (efficiency) of the cavities. Note that since the transmitter output passes through bandpass filters, therefore transmitter noise and spurious emissions are also attenuated in a bandpass type duplexer, which can help reduce interference to other receivers at the same site. Bandpass type duplexers are best suited for moderate to wide transmit/receive frequency separations. Close spaced frequencies may require additional notch filters and/or separate antennas.
Notch Type Duplexers:
Notch type duplexers may appear physically similar to bandpass duplexers but
their operation and tuning is very different. There are two types of notch filters that may be used in a notch
type duplexer:
- The series notch filter, which has two ports (in and out).
- The shunt (or common) notch filter which has one port and is linked to the
other filter sections by a “T” connector. NOTE: Do not confuse this with the TX
RX Systems “T-Pass” filter which is a specialized bandpass filter.
Cavities 1, 2 and 3 are tuned to notch out 453 MHz.
Less than 1 dB loss near and at 458 MHz.
Cavities 4, 5 and 6 are tuned to notch out 458 MHz.
Less than 1 dB loss near and at 453 MHz.
Cable lengths between cavity 3 and cavity 6 to "T" are tuned
lengths.
Note that since the transmitter output passes directly to the antenna, therefore transmitter carrier and noise is only attenuated near the 453 notch frequency. This offers minimal reduction of interference to other receivers at the same site. Notch type duplexers are cost effective and operate at much closer transmit/ receive frequency separations than bandpass type duplexers. Shared sites may require additional bandpass filters and/or separate antennas.
Bandpass/Band Reject (BP/BR) type duplexers:
These types of duplexers are combinations of the two preceding duplexer types,
having many of the benefits of both and usually at some increase in cost. An
bandpass/band reject example; (Actual combinations vary widely)
Cavity 1 and 3 tuned to pass 458 MHz
Cavity 2 tuned to
notch (reject) 453 MHz.
Cavity 4
and 6 tuned to pass 453 MHz.
Cavity 5 tuned to notch (reject) 458 MHz.
Cable lengths between cavity 3 and cavity 6 to "T" are tuned lengths.
General Description
A Signal Booster is a device which can often be used to improve radio communications in an area where
normally good radio signal levels are degraded due to obstacles in the radio path. The most common
misconception in applying Signal Boosters lies in expecting a Signal Booster to operate in the same
fashion (and signal levels) as a common repeater station. Signal Boosters are especially useful in many
situations where conventional, trunking or paging radio signals are too weak to provide reliable
communications. Specifically:
Signal Boosters should be considered especially when:
FCC Requirements
Signal Boosters must be FCC type accepted and labeled as such when operating on any frequency regulated by the FCC.
The FCC has released specific rule for the use of signal boosters, effective July 19, 1996. These rules allow the
use of Signal Boosters within most FCC authorized radio systems without any additional licensing requirement. The
FCC has designated two classes of signal boosters:
Signal Boosters vs. Radio Repeaters
Although Signal Boosters are sometimes confused with regular repeater stations, there are important differences in the application and design of these very different devices.
Signal Boosters have the following characteristics:
Signal Booster Configurations
Signal Boosters are available in several design types;
a. Class B: Fixed Gain (Non-OLC), non-channelized types, where the gain and power levels are set to meet very specific or unusual requirements.
b. Class B: Automatic gain adjusting, or OLC (Output Level Control), non-channelized types, where the Signal Booster automatically sets the gain over a wide range of input signal levels.
c. Class A: Channelized versions that only pass specific radio channels on a channel by channel basis. These are more expensive than non-channelized types and used primarily at the over- the-air (OTA) interface in areas where many high level unwanted channels exist. In complex system, these may also be required to meet FCC out-of-band emission limit requirements.
Again, this document only deals with Class B, Non- channelized, OLC type Signal Booster designs as this type is more commonly used to date. Non-OLC versions, Channelized types and customized Signal Boosters are also available from TX RX Systems.
The Components of a Class B Signal Booster
The specific organization of a Signal Booster can vary greatly according to the specific application and
installation requirements. In general, a Signal Booster with OLC contains:
Signal Boosters may be constructed for either:
Signal Boosters may be applied in a manner similar to a coaxial cable RF signal distribution system. Non-radiating (common) coaxial cables and/or radiating coaxial cables (sometimes called "leaky coax") can be used to route the RF signal between Signal Boosters and radio coverage areas. Multiple Signal Boosters may be connected in series to provide radio coverage for multi-story buildings, long tunnels or separated areas of communications.
Basic One-Way Class B Signal Booster
The components and operation of a one-way Signal Booster shares many aspects
that are common to a two-way Signal Booster, therefore a good understanding of
the one-way Signal Booster is important to understand two-way Signal Boosters.
A one way OLC type Signal Booster is organized as follows:
The operating frequency band and bandwidth of the Signal Booster is determined by the characteristics of the bandpass filters (cavities) used. These characteristics are engineered to meet the specific applications requirement. Bandwidth values generally range from 250 KHz to 20 MHz and typically 2 to 10 MHz.
The amplifiers will amplify all signals within the bandpass, therefore it is important to reduce the bandwidth to the minimum practical bandwidth for optimum operation. Note that 'practical' also relates to size and cost.
In all TX RX Signal Boosters the amplifiers are special ultra-linear amplifiers with a preset gain value. Common OLC Signal Booster models are available with gains ranging up to about 80 dB. The maximum output power of the output amplifier is set below the amplifiers 1 dB compression point ( +35 dBm) to prevent damage to the amplifier and minimize undesirable RF output signal components. This results in a typical operational output of about + 25 dBm for one or two channels input, depending on the specific model used.
The output power level is monitored by the OLC circuit and a gain adjusting control voltage is fed back to an electronic attenuator at the preamplifier input. The dynamic (output level control) range of the OLC circuit is typically 30 to 40 dB depending on the specific model used.
Since the amplifier gain and OLC control dynamic range are fixed values, coaxial pads should be inserted before the electronic attenuator when the input signal levels are high enough to constantly cause attenuation at the electronic attenuator. The addition of coaxial pads, if required, restores the full dynamic operating range of the Signal Booster and makes the output signal levels more consistent and coverage more predictable. Fixed value coaxial attenuator pads are preferred over variable attenuators due to the infrequent need to alter the settings, the higher cost of variable attenuators and the lesser possibility of noise being generated within the attenuator itself.
The use of coaxial pads instead of amplifier gain adjustments allows the amplifiers to operate within the best possible operational range, which is often compromised in circuits using current or voltage adjustments to control the gain. The amount of relative attenuation is easily observed by measuring the DC voltage applied to the electronic attenuator control input. The factory provides a verified chart of control voltages with each Signal Booster shipped.
Decoupled RF test points are also provided in standard versions of TX RX Systems Signal Boosters to permit performance testing or alignment while the Signal Booster is in operation. Non-disruptive testing is a common feature in most TX RX Systems products.
As no input-to-output frequency conversion occurs in a Signal Booster, the RF output must not be close coupled to the RF input to prevent feedback saturation of the Signal Booster. The isolation within the Signal Booster amplifier itself is well over any possible gain of the Signal Booster and does not present a concern so long as the factory connections and shielding is maintained. The amount of external output to input isolation requirement is usually not a problem since the input and output are normally located on different sides of the signal blocking obstacle (wall, ground, etc.).
If insufficient isolation exists, antenna placements, input pads and/or amplifier stage gain reductions must be implemented.
When a radiating cable, or "leaky coax", is used at the input and output ports of a Signal Booster there is little potential of feedback due to the high coupling losses of such cables.
A good rule of thumb is a minimum output-to-input path loss equal to the maximum operating gain of the Signal Booster plus 10 dB. For example, a Signal booster with 55 dB overall maximum gain should have minimum of -65 dB output-to-input isolation.
OUTPUT SIGNAL LEVEL:
The non-channelized Signal Booster will amplify all signals within the input filters bandpass therefore the effective output power per single channel will change as more channels are amplified.
In other words, the maximum output power level is shared by each input channel. The exact proportion of 'sharing' is determined by the number of channels AND the power level of each channel in relation to all the other input channels. A chart illustrating the effect of multiple channel amplification is included in the Addendum.
When using a non-channelized Signal Booster where many undesirable channels may be present at the input also, the effect of those channels may be reduced by several means;
- Use a directional antenna to direct the signals to the desired repeater site. In some cases, a corner reflector may be used to increase rejection of unwanted signals from the sides of the path. This increases the desired signal level and greatly reduces the power loss effects on the desired channel at the output of the Signal Booster caused by undesired signals.
- Use more selective bandpass filters in the signal path coming from the antenna. Some filters may have to be placed outside of the Signal Booster or in a larger cabinet because of their physical size.
- If the above steps are inadequate, then channelized, Class A, signal boosters should be considered.
Note that, in many cases, any additional filters only have to protect the INPUT from the 'outside world' because only 'desirable' signals normally exist inside the area to be covered.
INPUT SIGNAL LEVEL:
The
input signal level is perhaps the greatest item of confusion between
conventional repeater stations and Signal Boosters. A conventional repeater is
designed to provide radio coverage for great distances and requires only a few
microvolts of signal input. A Signal Booster is designed to "fill-in" an area
that would normally have a very good signal level and therefore operates best at
higher input signal levels.
A nominal input signal level of -60 dBm can be used as a guideline input signal level for most standard Signal Booster configurations, although Signal Boosters will operate at much lower levels with the acceptance of reduced output signal levels.
As examples:
A -60 dBm input level applied to a Signal Booster with 75 dB gain would have a maximum output level of +15 dBm.
A -85 dBm input level to the same Signal Booster would provide a -10 dBm output signal level.
Sensitivity and quietening level specifications are not applicable to Class B Signal Boosters due to the relatively high level of the input signals and the lack of any frequency conversion or demodulation circuits.
NOISE:
The noise figures of the amplifiers are normally of no concern except
when many high gain Signal Boosters are placed in series. This is minimized by
reducing the gains of the Signal Boosters and losses between the Signal
Boosters. The first amplifier is the dominant contributor to the noise figure
any design using amplifiers in series. The factory will provide assistance in
determining specific noise figures for complex systems using several Signal
Boosters in series, as the resultant noise figure will be much less than the
simple addition of the individual Signal Boosters noise figures.
Note that the "system noise figure" specified by TX RX Systems is the worse possible overall value, with the maximum gain of all stages applied.
POWER SOURCES:
Signal Boosters
are normally configured to use a 120 or 240 AC power source. Optionally, they
may be operated from a DC source, which requires a DC voltage converter if the
DC source is less than 24 volts.
Non-disruptive automatic transfer battery backup options are available.
If power is not available at the Signal Boosters physical location, the Signal Booster may be powered by inserting a DC supply voltage on the coaxial cable at a point where the cable is near a power source. This requires the addition of optional coupling devices that are available from TX RX Systems and an external DC power supply.
ACCESSORIES:
Additional supporting devices, such as power splitters, antenna
decouplers, indoor antennas, terminating loads for radiating cables, etc. are
also available from TX RX Systems to complete almost any possible application of
Signal Boosters.
TWO WAY (Bi-Directional, Boadband) CLASS B SIGNAL BOOSTERS:
Two-way Signal Boosters are basically two, one-way signal booster sections in the same cabinet with one section operating in one frequency band and direction and the other section operating on another frequency band in the opposite direction.
All the previous one-way Signal Booster criteria applies to two-way Signal Boosters.
Two way Signal Boosters use the bandpass filters in a manner similar to a duplexer to maintain separation and isolation between the two frequency bands while using a single coaxial port for the RF signals on each side of the amplifier sections.
In frequency bands below 450 MHz, the variable and small amount of separation between transmit and receive frequencies frequently requires larger filters and cabinets. These are engineered, quoted and specified on a case by case basis by the factory.
Naturally, Signal Boosters cannot amplify a simplex frequency signal in both directions.
TX RX Systems has FCC type accepted Signal Booster models in the VHF , UHF and
800 - 960 MHz bands.
Antennas, Radiating Cables and other Radiators:
The 'external' antenna used for the path between the Signal Booster and the distant base/repeater station should be a directional, gain antenna which will improve the signal path and reduce undesired signal that may occur in other directions. Parabolic and corner reflector type antennas often have better rejection of unwanted signals that appear to the side of the antenna.
Non-radiating cables should be low loss, well shielded coaxial cables fpr best results.
The amount of radiation and signal levels inside the area served by Signal Boosters can be controlled by selection of radiating coaxial cables (i.e. "leaky coax") or antennas as radiator elements. In some cases both may be used.
The choices of radiators is based on several factors;
- The topography of the coverage area. (Long and narrow areas versus large open areas).
- Costs of radiators and installation.
- Maintaining input levels to the Signal Boosters within the OLC control range.
Radiating cables provide coverage that can be easily controlled and is especially applicable to tunnels, stairwells, passageways, etc. The disadvantages of using radiating cables is the cost of the cable, its additional installation concerns and the high amount of coupling losses. The primary advantages of using radiating cables inside passageways and tunnels is due to the fact that the distance between the cable and portable radios does not vary greatly and the high coupling losses are usually acceptable because the portable is always near the cable.
Antennas provide the minimum coupling loss and often lower installed cost. Generally, unity gain omni-directional antennas are preferable.
More than one antenna may be coupled into a non-radiating cable to distribute the signal over a large area. Multiple antennas should be placed such, or decoupled, so that the signal levels coming into the Signal Booster are as uniform as practical.
Decouplers, 'taps' and 'splitters' are used to connect antennas and cables to the main feed lines. These devices are avaiable in many types and values, including multi-port and multi-band models with coupling ranges from 3 to 40 dB per port. These, along with simple coaxial RF pads, allow the system designer to distribute the signals equally throughout an area.
Internal gain antennas can be a disadvantage when the radio users roam from side to the center of the antennas beam, causing great signal level variations.
Isolation between the Signal Boosters output-to-input ports may become more difficult to achieve when gain antennas are placed near outside windows, doorways, etc.
An example of a system using both types of radiators: Radiating cable is used inside a stairwell that leads to a large open underground parking garage. The end of the radiating cable is in the garage and connected to a antenna placed high on one side of the garage. The high antenna placement prevents a radio from being close to the antenna to minimize signal level variations to the Signal Booster.
Multipath effects:
Some system
engineers initially express concern over the possibility of conflicting signals
coming from both the direct path and the Signal Booster output. In practice
however, the multipath effects when using a Signal Booster has been found to be
less than that normally experienced in any mobile radio system. The output of
the Signal Booster is usually much greater than that found in the direct path,
allowing inherent FM capture effects to minimize any interaction at the
receivers when the radio is operated inside the coverage area provided by the
Signal Booster. There is always the possibility that the differential between
the two signals will not be adequate for FM capture at some location at the
outer edge of the Signal Boosters coverage. The most likely place for this to
occur is when the radio user is near a doorway or an outside facing window.
Other factors also aid in reducing multipath resulting from Signal Boosters because of the nature of the users and the application itself; (1) Signal Boosters are usually used to improve signals in areas where portable radios (as opposed to mobiles) are primarily used. (2) Portable radio users instinctively make minor adjustments in their radio positions to get the clearest reception. This is also the location of lowest multipath.
Signal Booster multipath is similar to that inherent in any mobile/portable radio system used in an urban area and often with a much smaller area of potential equal level (non-capturing) multipath signals.
SPECIAL APPLICATIONS:
There are some cases not covered by the more common
Signal Booster configurations. TX RX Systems has a wide range of experience in
solving unusual applications and will provide their assistance upon request if
you have a special case. In fact, every TX RX Systems product is factory tested
on operating frequencies and levels when the customer supplies operating
parameters.
SIGNAL BOOSTERS IN SERIES:
In
large facilities and long tunnels, Signal Boosters may be connected in series.
The path between Signal Boosters may be either coaxial cables (both
non-radiating or radiating) or antennas. It is general practice to set up one
Signal Booster to 'talk to the outside world' and this is where the levels are
adjusted so that the 'inside' portion has balanced levels of signal between the
input and output paths. Balanced, in this case, usually means the following
Signal Boosters will all have the same configuration of pads and gains. The
greatest differential in signals are those over the air to and from the external
repeater station. This technique becomes more apparent as the system design is
developed.
MULTIPLE FREQUENCY BANDS:
It is sometimes desirable to operate the Signal
Boosters on more than one RF frequency band in the same direction. This cases
usually use one coaxial cable to reduce cable costs or due to other limitations
at the installation site.
In these cases, there are several approaches;
1. Bandpass filters are paralleled to pass each band through the same amplifier
sections. Four bandpass filters would be required for a two band two way Signal
Booster.
2. Two or more single band Signal Boosters are operated in parallel using crossband coupler devices. This arrangement allows maximum signal level control for each frequency band and is the preferred method, as each band can be controlled independently.
There are some systems that operate with more than two bands through the same Signal Booster amplifiers. Specially engineered combinations are available.
Some two band combinations (UHF + 800, UHF + 900, 800 + 900) are becoming more common.
Some VHF or UHF combinations (152 MHz + 173 MHz or 450 MHz + 470 MHz for example) may also use parallel filters to reduce the number of undesired channels between the desired bands.
Another case would be to pass a paging channel one way while passing two way communications in another frequency band.
All of these possibilities require factory engineering assistance due to the unique operation of each combination. TX RX Systems will provide a prompt and free preliminary technical and cost evaluation of any requirement.
DOING YOUR OWN PRELIMINARY SYSTEM DESIGN:
Worksheets and examples are available to assist you in the preliminary design of
a Signal Booster system in the Appendix. These worksheets allow you to organize
and analyze the various system design factors and to get a better understanding
of the principals used in Signal Booster system designs. Perhaps as important,
the worksheet information will provide the basic information required by the TX
RX Systems engineers to assist you in your requirement and establish common
terminology.
DETERMINING SIGNAL LEVELS:
The primary objective of a signal level survey is
to determine where the radios cannot communicate or the signal levels are too
marginal to be reliable. Remember: If only one critical location in a building
cannot get the signal, you will probably need a Signal Booster, regardless of
any other higher signal levels in that same structure.
Test the most probable low signal areas first. Once it has been determined a Signal Booster is required, the layout can often be arranged to accommodate other areas of lesser priority.
System engineers can minimize initial measurements to those areas where Signal Boosters would probably be required in any case. Typical examples are;
- Second level or lower basements.
- Tunnels over 2000' or without end to end visibility.
- Windowless, thick walled structures. Especially stairwells and elevator areas.
It is not difficult to estimate the approximate signal strength that would be normally be present in the general vicinity surrounding an obstructed area using common 'over the air' propagation engineering methods, but it is more difficult to determine the signal strength inside an obstructed area by theoretical engineering models alone.
There are many variables in the nature of the materials causing the blockage and other obstacles within the area to be covered. For example, wall and floor construction attenuations can range from about 3 dB to over 90 dB. Even the type of glass used in a building window can have attenuation factors from near 0 dB to over 20 dB.
A Signal Booster system that is designed with little reserve and marginal signal levels limits the future flexibility of the system. The area to be covered can also be a dynamic and changing situation. What may be adequate today may not stay that way as station sites are changed, structures are altered and departments relocate.
Signal level measurements are more accurate when the test signal levels are relatively similar to those expected when the Signal Boosters are installed. Excessively low level testing measurements can be influenced by undesired signals and noise, especially when a high gain, wide bandwidth spectrum analyzer is used.
Measurements made with a high quality receiver (not a broadband scanner), which has had its RF signal level test point (i.e. RSSI) calibrated, is usually accurate enough for the tolerances required for a system design.
Sometimes, potential Signal Booster requirements for a proposed system can be based on simple 'walk-throughs' of areas using portables in the same band that communicate with an existing or similar radio system base station in the same frequency band located close to a proposed radio systems station site.
In critical "must talk" coverage areas, there is no good substitute for on-site signal measurements using close simulations of the anticipated distant base station facilities.
It is strongly recommended that the system design include considerable safety margin to accomodate future changes in the system. A 15 dB 'use factor' derating is commonly used.