Re: Digital Equalizer Design for a Radio Receiver

Mark wrote:
the carrier going down and sidebands fading differentially CAN be
corrected by an adaptive equalizer (not the same thing as an audio guys
call a graphic equlaizer) that trys to keep the channel flat...this is
done all the time with QAM signals..... but my question was how do you
expect to be able to do this on a voice modulated carrier AM signal
that has no "expected" characteristics...For example, in the simplest
case, when there is no modulation, you have only carrier so there is no
way an AE can have any information about selective fading in the
channel....

OK maybe if some sort of low level pre-defined reference signal was
sent by the Tx, but this is not done...

In practice, one doesn't make it good, one settles for making it better. There's a lot of redundancy in voice that the brain can use if overall volume is held fairly level and distortion is avoided. Early exalted-carrier receivers had a sharp, narrow peak in the center of the IF, so that the carrier amplitude at the detector was 20 dB over 100% modulated sideband; this prevented most distortion from fading carrier. Modern practice achieves the same result by injecting a phase-locked oscillator. Various AGC schemes are used in the IF strip, but final AGC is done on the demodulated audio. The scheme wouldn't do for audiophiles, but it works for mobile dispatch. For the base station, diversity antennas provide even more improvement.

Jerry
--
Engineering is the art of making what you want from things you can get.
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