> 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
AGC is a problem, but if the radio has overload anywhere before
the narrow filter you are in deep trouble with far more than clicks
and it would take more than AGC to fix it.
The receiver will also have severe IMD problems. Clicks would be
the last worry when a receiver is several dB into gain compression
before the narrow filter!
My R4C's had this problem (all of them do, especially the crummy
MOSFET mixer models), and even the 600 Hz Sherwood filter
wouldn't help really close signals. What I wound up doing was
replacing the second mixer with a diode high-level double-balanced
mixer, with +13 dBm 1 dB compression, but I still use the 600 Hz
filter.
Now the receivers are immune from overload on signals less than
around +10 dB or so input, even when they are only 200 Hz away.
The stock R4C isn't as good as my modified FT1000(D).
73, Tom W8JI
W8JI@contesting.com
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