Hi,
For a LNA in a receiver, how does the receiver handle signals that are strong enough to saturate the output of the LNA? Is this usually does with some AGC in the transmitter, or is it also done in the receiver? It seems more efficient to do this by lowering the transmit power, but that only works if the signal is two-way. Any other simple ways to handle this? :)
cheers, Jamie
Low Noise Amplifier refers to an amplifier at the input to a receiver. The LNA must itself have a low noise contribution and enough gain to boost the signal well above the noise due to the rest of the receiver stages. The LNA usually does not have a lot of selectivity - it passes through the desired signal and any signals adjacent in frequency.
Once you drop the gain of the LNA, two things tend to happen. 1) The LNA noise performance degrades and 2) the signal out of the LNA is no longer large enough to dominate the noise of the receiver. So, if your desired signal is weak, you must run your LNA at high gain, even if there are other unwanted signals of large amplitude on nearby frequencies. So LNAs tend to have an additional requirement - the ability to amplify weak signals as well as strong signals on nearby frequencies. This capability is often specified by the "3rd Order Intercept".
The input amplifer (LNA) on some receivers have awesome strong signal handling ability and are more like power amplifiers than small signal amplifiers - you can get 100mW or more out of some RF input stages !
So most LNAs operate flat out. Some have a gain pot. Most aren't connected up to the AGC. Typically LNA refers to an amplifier mounted on a satellite receiver dish, running flat out. The satellite has not got power to waste, and the dish is as small as the TV company can make it - hence the LNA is as good as it can be at low cost.
Roger
Sometimes the technique is simply to turn it off or bypass it and have an associated pre-programmed gain step in the AGC algorithm. Naturally, LNA bypass increases the noise figure. All lowering of gain tends to raise noise figure, especially (of course) as it gets closer to the front end. The trick is simply to have the gain decrease faster than the noise figure increases. This isn't problem if the AGC "starts" at the "back-end" and moves towards the "front-end" as more gain reduction is required. The Friis noise equation tells why.
If the cause of overload is off-channel and unwanted energy, there are only two obvious and general techniques:
Phase nulling techniques using directive antennas with additional circuitry are also possible (sometimes even with on-channel interferers), but this can be tricky and is practical only for fixed installations (including the jammer) and specific circumstances.
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