[Elecraft] [K3]PSK31 & DM780

[Jim Brown]

On Thu, 19 Feb 2009 17:59:38 +0000, Dave Van Wallaghen wrote:

>Has anyone else using DM780 with a K3 noticed that very strong 
psk 
>sigs produce other images on the waterfall? It is about 2k 
further 
>up or down from the original signal and can be decoded on the 
>screen although weaker. I'm also running LP-Pan but don't notice 
>the images on the panadapter.

That's harmonic or intermod distortion. Something in your audio 
chain is being overdriven. Could be the sound card. Or if it's an 
older K3, it could be the radio. Older versions of the K3 produce 
a LOT of audio distortion at the Line Out for settings of Line Out 
gain greater than about 10. Distortion starts showing up weakly 
with a setting around 5, and is bonkers up around 50. I've heard 
that's been fixed in recent production, but don't know what serial 
number. I think it's also fixed with the audio mod that you can 
buy. 

To fix your situation, first turn the Line Out gain at 3. Now, 
turn the sound card input gain down until you no longer see 
signals, then bring it back up until they all start decoding well. 

This same distortion vs level thing occurs on transmit, so be VERY 
careful not to overdrive the sound card or the radio. If you 
overdrive anything, you'll produce multiple copies of your own 
signal. 

One good way to set levels on your sound card is to listen to the 
sound card with headphones while you tell it to transmit. You 
should hear some extra sharpness or raspiness when the sound card 
level is too high. Listen carefully to it and pull the sound card 
playback level down until the raspiness goes away. That should be 
a good setting. You can also look at it on a scope and adjust the 
level so that there's no squaring or rounding of the top of the 
sine wave. Or you can crank the gain up until the waveform clips, 
then back it down until the waveform is half that height. All of 
these methods should get you to clean audio out of the sound card. 

Once you have that level right, hook it up to the K3, pull the 
Line In gain down, then bring it back up slowly as you watch the 
RF level. When the level stops increasing, you've gone too high. 
Back it off a bit. That should get you close. 

73,

Jim K9YC