[Elecraft] K3, diversity and noise reduction

[Alan]

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