[K3] [Elecraft] Harmonic Distortion

[Julian, G4ILO]

Mike Scott-7 wrote:
> 
>>The "problems" people are having with "harmonics" when they listen to CW
> with a 2.7KHz filter/DSP setting seem to be easily solved by just reducing
> the DSP bandwidth.
> 
> If you were paying attention to the spectrogram plots you would have
> noticed
> that the IMD and harmonic products are showing up far outside the
> bandwidth
> of the DSP filter, very far. The distortion products are being produced in
> the analog domain after the DSP digital to analog conversion. No amount of
> DSP tweaking will fix this.
> 
> 
I am looking, right now, at the second harmonic of a CW signal. The signal
is at 660Hz audio frequency, the harmonic is at 1320Hz. The "problem" is not
solved by reducing the DSP bandwidth. That simply makes it more apparent,
because the harmonics then show up against a dark background whereas with a
wider setting, noise would mask them unless they are extremely strong.

I should point out that with a normal sound card program I have to turn the
line output up to an excessive level to see these harmonics on the
waterfall. And I certainly can't detect their presence audibly. So I don't
think this low level harmonic distortion is a problem, even for PSK
decoders. These harmonics do seem to cause some confusion to the CW Skimmer
program. But I guess that most people who are willing to pay $75 for that
software will be using it from an IF output to get a wide bandwidth, not the
K2 audio feed, bypassing the problem.

I don't think this issue has anything to do with the unpleasant distortion
that a few have been complaining of. It has been suggested to me that this
is caused by some DSP artifacts occurring at around 12KHz and not
harmonically related to the signal. That is probably higher than most of us
can hear, which is why there have been few complaints about it.

-----
Julian, G4ILO. K2 #392  K3 #222.
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