> Good engineering dictates never running a square
> rise and fall waveform into a crystal filter.
> The square rise-and-fall waveform will cause
> filter ringing.
This is precisely the reason why strong signals sound clicky in the 1000MP
and probably other radios. Signals outside the crystal filter passband but
inside the roofing filter passband are not subject to AGC action. If a
signal is strong enough to overload the second mixer, it appears at the
mixer output as a quasi square wave with very sharp edges. These edges hit
the following 8215 kHz filter which produces a damped oscillation of a few
cycles on each edge. This is easily observed with just an oscilloscope.
These oscillations (or ringing) pass through the rest of the receiver and
come out in the audio as key clicks having nothing to do with the
The solution is simple. Turn down the RF gain or insert some attenuation in
front of the receiver. There is no perfect radio.
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