RF Noise Canceller ?

[Geoffrey Mackenzie-Kennedy-2]

Interesting thought Marinos. There are receivers (non amateur) in which any
DSP,s ( if there is one) main job is to provide a large number of audio
filter bandwidths and some additional hetrodyne removal.  The removal of QRN
noise and QRM  within the IF filters passband is done in the early stages as
it is too late to do this at the back end.

Evasive Noise Blanking works well, with the tunable noise receiver tuned to
a QRM free frequency near to the operating band. Its output noise pulses can
be used to 'gate' the LO drive to the first signal mixer, easy to do with
the logic circuits producing the 50% duty cycle square wave LO required by
diode ring mixers etc.

There are also circuit techniques to reduce substantially or eliminate 'Zero
Beat' QRM and /or QRM within the IF passband. Some use a combination of
phasing, hard limiting and reinjection or mixing. The effects of QSB can be
a problem if the QRM is one moment stronger than the desired signal, and the
next moment weaker, Strange to say these ideas appeared in two or more
countries at almost the same time. But after playing with one technique here
I believe that there are solutions.

Thus if noise of any type (not QRM) is the target and starting design
thinking from the above, there could be a solution - although White Noise
could be tough to deal with, if indeed possible.

IMHO it would be difficult to add what you suggest to the K2. Any undesired
coupling to other circuits would not be acceptable. Then there is the
question of phasing adjustment from the front panel. I would think that a
stack of back to back varicaps at high bias might not be useful.

Just a few thoughts in haste as I leave the country very soon for two weeks
or so

73,

Geoff
GM4ESD

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