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.The heart of the Receive System is it's "Noise figure" A high noise figure can be compared to a man yelling to a friend during a baseball game, with the friend sitting on the opposite side of the field. A low noise figure is the equivalent of the same two people being alone in the same stadium at 2 a.m., with no other people, or noise sources present. The one person would not have to yell (low power gets you farther), and the other person could hear much further (less noise around you yields a higher sensitivity).
Now let's compare this to a receive system. If the antenna system or receiver has an RF amplifier in it, electrons passing through the device create random noise, some of which falls on the frequency you wish to receive. The more intense this internally generated noise is, the more signal you need to overcome this "noise figure." Increasing the gain will not improve the received signal if the noise figure of the circuit is high. It would be like turning up a hearing aid at a baseball game when someone hits a home run.
A cheap compromise to lower the noise figure in an RF amplifier, is to lower the current going through it. Unfortunately, this leaves you with an amplifier that has very low power output, and poor IP3, which are the basic factors that cause a device to mix signals together and cause intermodulation (or intermod). The result is a system that invites even moderate signals to overload and degrade the entire system. The bottom line for the manufacturer is a cheap device that works well under weak signal conditions, but fails miserably at medium to moderate signal levels.
Most receivers use a mixer stage to convert the higher frequencies to a much lower "Intermediate Frequency," or "IF." This "Superhetrodyne" conversion process results in a typical 6 to 8 db signal loss when diode mixers are used. Therefore it is desirable to amplify the signal ahead of this type of mixer, to make up for this loss.
Limiting the signals entering the amplifier and mixer stages to frequencies you wish to receive, greatly reduces spectrum noise from other transmitters and RF noise sources. This increases the sensitivity for your desired frequency or channel(s).
Other companies use filters and isolators that add to the system noise figure. Their LNA may have only a 2 db noise figure, but the TOTAL SYSTEM NOISE can be many times higher. Some of our filters have an insertion loss as low as 0.5 db, and we can give you an amplifier with a noise figure of less than 0.5 db, for a total system noise ~1 db! Suddenly a four foot microwave dish works like an 8' dish!
Gain alone means nothing! IT'S THE NOISE FIGURE THAT COUNTS!
A well designed LNA System must not exceed a net gain of 20 db for receivers without built-in LNAs, This means that if you have a -60 dbm signal at the connector on the receive antenna and amplify it, the amplified signal level, minus the loss of the transmission line, connectors, etc, must not be any higher than -40 dbm at the receiver. In congested areas, a net gain of 10 to 15 db is practical in order to mask the receiver's noise figure (conversion loss plus transistor noise, plus other losses in the receiver). LNAs used with receivers with built-in LNAs, should exceed a net gain of 6 db.
In no case should any signal, on frequency or off frequency, exceed -30 dbm at the input to the receiver. If the receiver has an internal LNA, the highest level signal applied to the receiver must not exceed -40 dbm. The reason for limiting strong signals, including any adjacent channels, is to prevent INTERMOD (signals mixing with each other and actually modulating each other. This usually occurs in the mixer stage of the receiver). This is especially critical in COFDM modulated systems, where many carriers are present per channel, and have the potential of mixing together, thereby corrupting the Soft Decision (data) Stream, resulting in video "FREEZE."
At @ GHz, a 4' dish having an LNA with 40 db of gain, driving 1500' of 7/8" line, should be capable of cleanly handling 100 milliwatts output at 2 GHz! If it is capable of handling only 0 dbm, a signal of -30 dbm would cause the amplifier to overload and generate intermod! All of our Super-LNAs are individually designed to match the special filtering, gain, and power needs of your individual systems.
Installing one of our custom built Super-LNAs in your system will greatly increase your range and reliability. KOLD-TV, Tucson, Arizona, used a 105 mile, 7 GHz Path. Transmitter power was 5 watts!
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