I run across this same situation when trying to copy weak signals in the
QRP Fox Hunts. I rarely use the NB here so it usually surprises me when
it happens.
The K3 NB is made up of two parts: IF and DSP. For the IF NB, the
spectrum is sampled ahead of the roofing filter and is quite broad. This
allows the IF NB circuit to take out very short pulses. But when there
is a strong signal (or many in the case of VK9WA) in that bandwidth it
will cause what you are seeing. The strength of the effect somewhat
depends on the IF NB level setting.
The DSP NB, of course, is implemented in the DSP code which is after the
roofing filter so it's not as susceptible unless the strong signal is
inside the roofing filter passband. So if you can select a narrower
roofing filter the effect will pretty much go away.
Incidentally, this effect isn't unique to the K3. I've noticed it in my
other receivers as well.
73,
Brian, K0DTJ
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