Thanks to all who responded. Sorry I didn't think to point out that, yes,
the noise blanker was off.
I made some more measurements last night, and it looks like the problem
may be high order spurious responses within the roofing filter passband.
I applied a single input signal of about 10 mV (-27 dbm), and found some
spurious responses only about 50 dB down. Depending on the precise
frequency, the spurs were 500 to 1000 Hz from the signal. Their strength
varies linearly with input, so they're not intermod products. They tune
faster than normal, so must be related to VFO harmonics. In some cases
they have a strange slow beat note, which could be what sounded like
intermod. I don't know the cause yet, but there's something strange
happening.
73,
Scott K9MA
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