Not just homebrew ... any receiver with a high level mixer.
Here is the description that Elecraft gave for their "SIG RMV"
function to eliminate harmonically generated birdies:
" 'SIG RMV' is a function we created that
shifts both the BFO and VFO by the same small amount in order to
shift a spur out of the passband.
The reason this is effective for many spurs is because they're
the product of harmonics of the signal sources, not the
fundamentals. For example, you may hear a spur that results from
mixing of the 3rd harmonic of the VFO beating against the 9th
harmonic of the BFO (any combination is possible). That's the
nature of high-level mixers in superhet transceivers.
SIG RMV shifts the VFO and BFO by the same amount *at their
fundamentals*, but the spurs end up shifted by a multiple of
this amount. Using the previous example, a 100 Hz shift at the
fundamental might shift the VFO 300 Hz and the BFO 900 Hz.
(Often the multiples are much higher -- I've seen them up to
21.) Doing this often moves the spur out of the passband, while
having an insignificant effect on the filter center frequency
(these are fixed because of the crystal filters)."
It is of course possible to have other internally generated sources
of unwanted signals, but if the birdie being heard sounds like a
chirp instead of a tone when you tune across it, it's because the
birdie is actually moving as you tune and it is caused by the
condition described above.? And as mentioned above, the birdies
themselves are virtually unavoidable because of the very strong BFO
signal necessary to get high dynamic range out of a mixer.? The
mixer needs to act as close to a perfect switch as possible.? The
birdies WILL get created, but this technique deals with them
effectively so they aren't a bother.
As somebody just mentioned, I suppose that it would be possible to
calculate where such birdies would be found and create a hash table
of VFO and BFO shifts to pre-emptively deal with them.? Elecraft
didn't do that ... they just created a procedure where the user
could manually set up the shift for any birdie that was
problematic.? When I did the procedure on my K3, I found that
sometimes a shift in one direction was better than a shift in the
other direction ... I don't know if that would be easy to predict.?
And as Elecraft says above, any combination of harmonics is possible
so it could end up being an unnecessarily large hash table since not
all possible combinations might be a nuisance.? In any case it was a
simple matter to do it manually.
73,
Dave?? AB7E
On 6/19/2023 11:20 PM, HA3HZ wrote:
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The question is, has the description of this feature been
published somewhere?
It would really be a big help, since all home brew sdr have these
birds.
--
Gyula HA3HZ
HA3HZ
Raj vu2zap