The suggestion is to use the two identical receivers in the K3 to null
noise and interference. The idea is to connect each receiver to a
different antenna so the signals arrive with different phases. The
phase difference will generally be different for the desired signal and
the interference or noise, so by combining the two signals with the
proper amplitude and phase you can null out the undesired signal.
There are units you can buy today that do that by combining the RF
signals from the two antennas before they are sent to the receiver. The
question is whether you can do it at audio frequencies rather than at
RF. Theoretically you can, since the demodulated SSB signal is just a
frequency translation of the RF. If that works, it would be simply a
question of new firmware in the K3, since the audio from the two
identical receivers is already available to the DSP.
The issue is that the amplitude and phase responses of the crystal
filters in the two receivers are not identical. To get a good null, the
difference in the phase shift through the two filters would have to
track within much less than 360 degrees across the passband. In other
words, the variation of the difference in group delay would have to be
much less than 1/8 usec with an 8.215 MHz IF.
A 2.5 kHz bandwidth filter probably has on the order of a msec group
delay. I doubt the two filters typically track each other to better
than a few usec across the passband, if that.
You probably could get a good null at a single CW frequency, but I don't
see how you could hope to get the null to hold across the passband.
Alan N1AL
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