[Elecraft] [K3] Manual Notch Reconsidered

[Jim Brown]

I have no clue what the K3 does, but the slickest way to do this is to 
first sense the frequency of the tone you want to notch, then create a 
tone of the same frequency, shift its phase so that it is exactly 180 
degrees out of phase with the interfering tone, make it equal to the 
interfering tone's amplitude, and add it to the signal. That will cancel 
the tone with no other effect on the signal. That ain't easy, because it 
must track the drift of both the TX and the RX and the varying strength 
of the interfering signal, but with DSP, it IS possible.  The beauty of 
this technique is that it has NO effect on the desired signal -- it 
simply cancels the interfering carrier.

The alternative technique is to add a narrow band notch filter, which 
MUST add phase shift that distorts the audio. The deeper the notch and 
the narrower the filter, the greater the phase shift, and thus greater 
distortion of the signal.

These are fundamental concepts -- as my friends back home used to say, 
"you cain't get no better" than this, :)   and you must be very good 
(and have the processor cycles available) to do it well.

73, Jim K9YC