K2 receive audio harmonic distortion

[K. Rice]

I have been thinking that the receive audio from my K2 (#4314) was just
OK, especially on CW when compared to other rigs including a Drake R8
and a K1.  It's not terrible by any means, just not as good as it might
be.

Today I fed an S9 CW signal into Spectrogram and find that its third audio
harmonic is down approximately 10 dB from the fundamental.  The 5th,
7th, 9th, are down about 4 db sucessively from the next-lower odd
harmonic.

This is easier to see than to say:

   http://world.std.com/~rice/k2_harmonic_distortion_05.jpg

This is observed when driving headphones or speaker to a comfortable
listening level (red trace on the spectrogram above).  Decreasing
the RF or AF gain so that the audio level decreases ~30 dB (blue trace on
the spectrogram) eliminates the problem, but the signal is then too
faint to copy effectively.

The vertical red line is the center frquency of the widest CW filter
(1.2 kHz) set per N0SS's instructions.

AGC on/off and DSP on/off make no difference. It seems to be pretty
far downstream in the audio chain.

Sidetone audio behaves the same - no worse.

Is this level of harmonic distortion normal?  If not, what should I
look for to correct it?

Thanks and 73,

Ken Rice
K3VV
Coopersburg, PA


 
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[Sverre Holm-2]

> -----Opprinnelig melding-----
>
> Today I fed an S9 CW signal into Spectrogram and find that
> its third audio harmonic is down approximately 10 dB from the
> fundamental.  The 5th, 7th, 9th, are down about 4 db
> sucessively from the next-lower odd harmonic.

Just a few questions to understand your measurement: How did you feed the
signal into your PC for analysis, using a microphone or from e.g. the
headphone output?

In my experience, it is very easy to overload the sound card in the PC. The
result is the same kind of distortion that your plot shows. A simple check
is to try to adjust the PC input level (software controlled slider) and see
if that changes the distortion.


73

Sverre
LA3ZA
http://www.qslnet.de/la3za/
 

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[Mike Harris-9]

G'day,

| Today I fed an S9 CW signal into Spectrogram and find that its third
audio
| harmonic is down approximately 10 dB from the fundamental.  The 5th,
| 7th, 9th, are down about 4 db sucessively from the next-lower odd
| harmonic.

Sounds like overdriving the sound card to me.  I never run Spectrogram
above -30dB for that reason.  Turn down the line input on the PC.

Regards,

Mike VP8NO

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[Don Wilhelm-3]

Ken,

I do not believe that harmonic distortion is coming from the K2 - that
pattern looks like a soundcard overdriven condition to me.

When you reduce the AF Gain at the K2, you reduce the input to the sound
card, and at some point, the soundcard no longer distorts.

Use the PC soundcard mixer controls to reduce the gain settings on your
soundcard (mic or line-in, whichever you are using, and the master volume)
to somewhere between 1/2 and 2/3 of the max settings (if you are using the
mic input, you may have to go as low as 1/4 or less), then bring up the gain
on the K2 to produce a reasonable size display,  As others have noted, -30
dB is probably as high as you can go before the soundcard distorts.

73,
Don W3FPR

> -----Original Message-----
>
> I have been thinking that the receive audio from my K2 (#4314) was just
> OK, especially on CW when compared to other rigs including a Drake R8
> and a K1.  It's not terrible by any means, just not as good as it might
> be.
>
> Today I fed an S9 CW signal into Spectrogram and find that its third audio
> harmonic is down approximately 10 dB from the fundamental.  The 5th,
> 7th, 9th, are down about 4 db sucessively from the next-lower odd
> harmonic.
>
>

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[K. Rice]

Thanks Sverre, Mike and Don.

Doh!

After cutting the PC's software gain control to near-minimum, it does
appear that most if not all of the odd-harmonic distortion was due
to overdriving the soundcard.  To make certain, I will repeat the
experiment on a USB sound card that has bona fide line inputs and
better dynamic range, but I expect to find that the K2's audio
is just fine.

Thanks again for putting the experiment right.

73,

Ken
K3VV
Coopersburg, PA


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