In the "real world," overload can occur when 2 or more signals cause the apparent total output power of an LNA or a mixer to come within about 20 db of it's rated 1 db compression point, or when one strong signal causes the RF device to behave in a non-linear manner (what goes in comes doesn't come out the same, ie; distorted).
The rated output of an amplifier is the point where the signal being amplified is compressed by 1 db, and the amplifier is no longer linear. If multiple signals are present, the result can be INTERMOD (mixing of signals whose products fall within the passband of the receiver) and DESENSE (reduction of range caused by increased "noise" generated from signals mixing together and raising the "noise floor" (the minimum detectable signal a device can work with), or causing the device to go into conduction, so as not to respond to weak signals). The amplifier, mixer, or other component(s) no longer function in a linear (square law) manner.
When you have 2 or more signals passing through an RF device, you need about 20 db of headroom for CLEAN signal amplification. Signals start mixing well below the 1 db compression point. The problem is that at some point in time (instantaneously), all the signals seen by the amplifier or mixer are going to be IN PHASE, which could result in the amplifier, mixer, or other device, going into saturation, but an RMS type meter or spectrum analyzer does not see this "instantaneous power", and even some design engineers are fooled! GOOD FILTERING AHEAD OF THE LNA IS A MUST.
A COFDM Signal, which consist of many individual carriers, will have its recovered Soft Decision Data Stream corrupted, if its own RF signal level is too high. The many carriers will mix together and cause Intermod!
Your ENG and STL LNA System is the first item to replace. High-Power Transmitters (including TV, Paging, RADAR), PCS, XM Radio at 2.3 GHz, the new 2110-2150 MHz Land Mobile Band, ITFS, 2.5 GHz Wireless LAN, Wireless Cable, Radar, Paging, and other signals that can easily exceed -30 dbm will be hitting the average 24 db gain microwave antenna. If the signals fall within the system's passband, the resulting 24 db gain from the typical antenna results in a -6 dbm signal being applied to the LNA's filter. Most commercial LNA systems use 3 to 6 section filters. These filters attenuate VHF, FM, and UHF very well, but only attenuate some PCS frequencies less than 1 db. This leaves us with a signal level of -7 dbm.
This -7 dbm PCS signal will now be applied to the input of a typical 30 db gain Low Noise Amp! Since new LNAs typically achieve about +10 dbm, and older pre-amps are capable of -5 dbm output at 1db compression, your new LNA is badly overloaded by ~13 db! This saturated condition of of your LNA causes weak signals to be masked, and even moderately strong signals become noisy. In COFDM and other digital transmissions, Bit Error Rate rises, range is degraded, and drop-outs and video "FREEZES" increase drastically.
Overload can also occur in Duplexed TSL/STL systems when old receivers are replaced with new ones with high sensitivity. This is usually caused by reflected power from the antenna. In virtually all cases, the Duplexers can be retuned for increased rejection. Call 1-954-850-1016 for more information.
Overload can also occur by RF Signals entering the LNA through Power and Switching Cables. This is more common than you think. While I was assistant chief at WTVJ-TV in Miami, we found that a VLF Beacon Signal 15 miles away, was modulating the 7 GHz LNA. The TV Broadcast Tower itself was resonant near this VLF Beacon, and it was actually modulating one of the audio subcarriers.
In another case, a PCS and a nearby UHF Channel reduced range on KABB-TV's Ultrascan Antenna to 10 miles on high power. After upgrading their Ultrascan Antenna System with our SuperFilterLNA System, they were able to receive a 2 watt signal 35 miles away.
WSOC-TV in Charlotte NC, now uses 5 of our SuperFilterLNA Systems to keep PCS from reducing range and causing noisy video and audio on weak signals..
WAVE-TV in Louisville KY, uses our SuperFilterLNA System to do 2 GHz 60 mile ENG shots, while other stations in their market are sending out their SNG Trucks and trying to buy satellite time.
NO OTHER MANUFACTURER COMES CLOSE TO OUR SPECIFICATIONS!
PCS Rejection increases to over 120 db at the center of the PCS Band.
2 GHz System Performance: Frequency vs. Rejection
|1800 Mhz||>140 db||35 db||87.1 db|
|1900 Mhz||>140 db||7 db||71.2 db|
|PCS "A"||1930 Mhz||
|PCS "B"||1950 Mhz||
|1965 Mhz||>140 db||1 db||30.9 db|
|PCS "C"||1975 Mhz||>140 db||1 db||19.8 db|
Below is the actual response of the latest Ultrascan TM filter from Microwave Radio TM (sitting on top of the Vector Network Analyzer).
This is the High Accuracy ANRITSU 37247B Network Analyzer with the latest (1/14/'98)
Microwave Radio Filter sitting on top. After a full 12-Term Calibration was performed
(precision references were used), the Marker Readout (attenuation) is shown below.
|MARKER 1||2 Ghz ENG Band||Center|
|MARKER 2||Block C PCS||Upper part of Band|
|MARKER 3||Block C PCS||Middle of Band|
|MARKER 4||Block C PCS||Low end of Band|
|MARKER 5||Block B PCS||Upper edge of Band|
Broadcasters will soon face new problems with the upcoming auction of the 2110 to 2150 MHz region for Land Mobile Communications. Presently the Personal Communications Service, or PCS, affects virtually all broadcasters using 2 Ghz microwave. PCS CELL Sites are typically 2.2 miles apart, so the furthest you can be from a PCS Site is 1.1 miles. No one mentioned that large structural glass uses iron for strength, and that 2 Ghz isn't the first choice for penetrating trees and brick buildings. But now that all these investors have spent billion$ to win the "spectrum lottery", they have to re-coupe their investment. And when the signal is weak, they simply add more power.
The closer you are to a metropolitan area, the more severe your problem will be. The Charlotte, North Carolina market, was one of the first to experience PCS. WSOC-TV and WBTV-TV lost live News shots, and WSOC-TV could not even use their downtown site! Both Stations now use our equipment. WSOC uses 5 SuperFilter/LNA Systems in their operation
How much of a guard-band has the FCC assigned for the broadcaster's protection between "C" Block (1975 to1990 MHz), and the 2 Ghz ENG Band? Not 1 kc! The PCS "C Block" ends at 1990 MHz, the start of channel 1.
The PCS Band is shown above. It starts at 1850 Mhz, and meets the lower edge of ENG channel 1, at 1990 Mhz. This band is divided into three distinct segments; a 60 Mhz wide receive band 1850 to 1910 MHz), a 20 MHz wide unlicensed band, which also serves as a "guard band" (1910 to 1930 MHz), and a 60 MHz wide Base Station (cell) transmit band (1930 to 1990 MHz). Each transmit and receive band is further divided into six additional segments: three 15 Mhz wide blocks: A, B, and C, and three 5 MHz wide blocks: D, E, and F.
The PCS tower site transmitters operate between 1930 and 1990 MHz. The PCS sites receive 80 Mhz below their transmit frequencies. The transmitters at the PCS towers are allowed to have an EIRP of 1,640 watts, but typically run ~500 watts EIRP for each loaded 1.23 MHz wide CDMA channel, and ~800 watts for the 200 KHz wide TDMA channels. Most sites break up power distribution into 120 degree sectors.
CDMA channels ideally handle no more than 25 simultaneously conversations in good practice. When traffic increases, additional transmitters, called "expansion" channels, handle the additional traffic, each handling an additional 25 conversations. The base station PCS transmitter output power for the "pilot" of a CDMA channel, with no conversations present, is normally ~1.2 watts. This is doubled to around 2.4 watts in difficult terrain. This low power, however, is going to a 17 db gain antenna that handles a 120 degree sector, so multiply the transmit power by 50.
But that isn't the maximum power! As more customers come on line, each PCS base station transmitter's power output increases. ONE typical PCS Channel will increase its transmitter output to around 16 watts per CDMA channel. Factor in about 2 db line loss, and multiply the remainder by 17 db (by 50), and you have around 600 watts EIRP for every loaded transmitter on an average PCS site. This higher EIRP from a loaded channels, accounts for the unpredictable interference to the ENG band. As more PCS Companies and subscribers, ENG interference potentially increases. Even in today's "state of the art" systems (except ours), the noise floor increases, and weaker and even moderate ENG signals and their audio subcarriers are masked or distorted.
TDMA is a different case. No matter the number of users, TDMA still has the same high power output per channel.. TDMA has lower channel capacities, so they need more channels. Each 200 KHz wide TDMA channel around here (South Florida) is running around 800 watts EIRP.
There are other types of modulation, but these seem to be the most popular so far.
The broadcasters who use Channel 1 with a 2 Ghz full-band LNA and a frequency agile receiver, will have the most problems.
New High-Power Problems for the
1). XM-Radio at 2337.485 & 2340.015 MHz
2). Land Mobile Radio Service - 2110-2150 MHz
Even if you use upper ENG channels, you still have a problem! When the signal from your live shot comes into your receiver, it is mixed (heterodyne principle) with a fixed frequency, which is dependent upon the channel you use. This is called the local oscillator frequency, or L.O. The resulting, much lower, Intermediate Frequency (I.F.), is then filtered (selectivity) and processed with less expensive, lower frequency components. An agile receiver overloads when signals to the mixer exceed about -30 dbm.
70 MHz is the I.F. used by most receivers. Signals 70 MHz above and 70 MHz BELOW the L.O. produce the same 70 MHz Intermediate Frequency! The unwanted frequency coming into the receiver is called the image frequency, and must be filtered out ahead of the mixer. PCS signals just happen to occur at ENG image frequencies for 'A' & 'B' Block. At "C Block" frequencies, 1975 to 1990 MHz, filters in frequency agile receivers cannot remove them. ENG signals 70 MHz above the L.O. and the PCS signals, 70 MHz below the L.O., are mixed together!
The PHILLIPS MICROTECHNOLOGY SuperFilter/LNA solves these problems by providing a top of the line, specially temperature compensated filter/amplifier. With a design goal attenuation of >100 db in the PCS band, matched to a special Ultra Low Noise, high dynamic range LNA, our Super Filter/LNA keeps your system up and running in severe electrical storms, and has better sensitivity, and range than virtually any LNA currently in use.
Since 1996, WSOC-TV, WBTV-TV, the COSMOS Group Stations, including WAVE, WIS, WLOX, KPLC, and many others, all use our SuperFilter/LNAs. Some of these LNAs have been retrofitted in new Microwave Radio UltraScan Antennas. KCNC-TV, Denver and others, also use these highly selective filters to insure low Bit Error Rate for DTV transmission of live events.
When KMTV ordered a new RF Technology Antenna System, they specified that it would use our 40 db SuperFilter/LNA. They now have a relative signal reading of 85 from their remote bureau, 50 miles away, and clean audio and video.
Adjacent channel interference is usually caused by lack of receiver selectivity, which can be cured by upgrading your receiver with our SAW Filter Module, but sometime can also be caused by excessive bandwidth of transmitters operating on adjacent channels. This excessive bandwidth can be drastically reduced if stations operating on adjacent channels, agree to limit unusable high frequencies.
Many years ago, when hollow state devices were still popular, it was difficult to maintain video bandwidth out to 5 Mcs. Some of today's DAs have responses beyond 100 Mhz. With these modern video DAs, it is desirable to limit video frequencies above 4.5 Mhz.
The obvious reason is that they serve no purpose, and the information is unusable in present NTSC systems. More important is the fact that these frequencies contribute to unwanted sidebands, making your ENG or STL channel wider than necessary. This wastes precious fade margin and signal to noise ratio, by taking power from audio subcarriers and useable video information from your own channel. In addition to causing indirect degradation to audio subcarriers, some high video frequencies fall right at audio subcarrier frequencies, and are not totally removed by low-pass video filters. This is especially true in systems using 4.83 Mhz audio subccarriers.
Unnecessary high frequencies are generated in video and digital equipment that have high SLEW RATES (signals that change voltage rapidly over time, like a sync pulse, or Chyron generated text), contribute to modulation bandwidth, which subtracts from your video headroom, which limits your allowable video levels before clipping, and audio subcarrier cut-off (sync buzz in audio).
But this is only part of the story. In some cases, COMREX and other IFB systems that operate at 26 Mhz, have ridden into "remotes" on video cables, and were re-transmitted as sidebands, 26 Mhz above, and 26 Mhz below the channel that the ENG transmitter was on. Again, when you transmit any signal, it takes power, and transmitting unwanted or unusable frequencies takes away power from your useable video signal and audio subcarriers.
But worst of all, unusable high frequencies cause valuable power to spill over into the adjacent channels!
The purpose of all this is to point out the necessity to use a "brick wall" video filter at the input to the ENG or STL transmitter. This low pass filter should pass video up to 4.5 Mhz, and MUST be phase equalized.
Matthey makes an ideal filter for this application, model TBW446, which is priced around $625 (as of 2/18/'98). If stations operating on adjacent ENG channels use these filters, it will be to their advantage.
Now that the transmitters are clean, it is necessary to upgrade your receiver. Adjacent channel selectivity in most ENG receivers is only about 10 to 13 db. To increase it to beyond 70 db, give us a call at 1-954-850-1016.
Ingress is the process by which RF (at ANY frequency) enters your receive system by way of antenna line that is not 100% shielded, power cables, audio and video cables, et., and produces unwanted modulation in the system's video, data, or audio.
You can have the greatest RF filter, but if you use less than 100% shielded coax AFTER the filter in a heavy RF environment, some of that signal is going to reach your receiver. The same goes for ANY WIRE LINE that is connected to your system. Each has the potential of bringing RF into your system.
One broadcaster was blown off the air on his 23 GHz STL. The culprit turned out to be the new 200 KHz AC Current intercom system. The building's janitor used it to communicate with other people from the elevator penthouse of the building where the Broadcaster's transmitter site was located.
In another case at WTVJ in the late 80's, faint Morse Code was heard coming from the 7.5 MHz comm subcarrier of the station's 7 GHz STL. The tower & line acted as an antenna, and modulated the 7 GHz Tunnel Diode Amplifier, and a VLF Beacon, 15 miles away and operating at several hundred KHz, was coming through. There was an RF potential that was being developed from the TDA's ground to the power input of the TDA. A choke and cap on the power feed took care of this problem.
It is very important to decouple all RF going into your system from any wire or line connected to it.
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