Hi Brian,
I have a Teletronics 500mW SmartAmp, the receiver gain is 12dB typical. If I use it with a radio which has receiver sensitivity of -94dBm, will it increase the receiver sensitivity to -106dBm?
Short answer: no.
(I had avoided getting into this subject in my previous post as it is pretty complex and there is really no way to simplify it.)
The problem is that the preamp has noise and the signal coming in has noise too. The result is that the noise gets amplified along with the signal so you have a point where you just can't make things any better no matter how much you amplify the signal. The goal is to get the best signal-to-noise ratio possible, not just the strongest signal possible.
See, the receiver itself adds some noise to the signal. Let's say its internal noise is about 6-8 dB above the theoretical thermal noise floor (noise figure). In that case, if you have a really low-noise preamp, say one with a noise figure of about 1dB (about as good as you are likely to get), you can amplify the incoming signal and get it above the receiver's noise floor. Now the preamp/receiver combination can receiver a signal that is weaker by about 5 dB. That is about what I would expect from a preamp with 12 dB of gain and a 1 dB noise figure feeding a receiver with a 6dB-8dB noise figure. The total system noise figure will end up being about 1.5 dB.
Using a preamp that has too much gain doesn't help because all it is doing is amplifying the noise too. Once you get enough gain to overcome the receiver's internal noise, the excess gain in the preamp hurts rather than helps. Every receiver has a maximum signal it can handle. You don't want strong signals to mess up the receiver for the weak signals. If your preamp amplifies too much it makes the strong signals way too strong and wipes out the weaker signals (receiver desensitization).
I apologize if this seems too complex but it is a complex problem. The bottom line is that, if you want to improve the performance of a receiver you need a low-noise front-end with just enough gain to overcome the noise of the subsequent stages. This means that the preamp gain needs to adjusted to be just right for the rest of the system. But now we get into the issues of receiver design and since none of us are building receivers, there isn't much we can do.
Also remember that the radios we are dealing with are designed to sell for $50, not $5000. This means there are some serious trade-offs, mostly in the quality of the receiver. Adding a bunch of gain is probably really going to make things a lot worse rather than better.
So if you want to make things a lot better, spend your money on an antenna with lots of gain in the direction you need it and use short cable runs.
-- Brian Lloyd 6501 Red Hook Plaza [EMAIL PROTECTED] Suite 201 http://www.lloyd.com St. Thomas, VI 00802 +1.340.998.9447 (voice) +1.360.838.9669 (fax)
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