Re: PSK31 & DM780

Posted by Jim Brown-10 on Feb 20, 2009; 9:12pm
URL: http://elecraft.85.s1.nabble.com/PSK31-DM780-tp2354428p2361366.html

On Fri, 20 Feb 2009 10:49:40 -0800, Mike Scott wrote:

>Jim, your K3 and mine seem a bit different. Before I put the audio
>modification in my K3 [ser 508] I would see obvious IMD on the DM780
>waterfall or Spectrumlab with Line Out set anywhere above 3, 10 was
>problematic. Even at 3 I could see harmonics of S9 tones.

I doubt that our radios were different, only a question of degree. The
harmonics at an output setting of 3 were in the noise level of my
measurement. For Line Out, the level of distortion at 3 (or even 5) is VERY
unlikely to be a problem.  

>After the modification I run Line Out about 10 and see I see slight IMD
>above band noise 60-70dB below the carrier tone, passable.

Remember, there's a transformer in line. 60-70 dB is WAY down, and quite
respectable IMD performance! Of course, I'd still like the transformers to go
away. There's no good reason for having them, they add to cost, and they
subtract from performance!

>I have played with sound card gain to try and make sure that the sound card
>is not being over driven but I have to admit that I don't really know how to
>distribute the gain between the sound card and the K3 Line Out. DM780 has a
>soundcard Receive Signal Level; I run it in the lower 25% of the scale and
>seem to see nice traces.

Some sound cards are better than others. Based on having used them with some
high power audio testing software, I can say that the one in my IBM T41 is
better than the one in my T22. I suggest that you run the K3 as cool as you
can (that is, a low value of Line Out gain) and adjust the input of the sound
card to get good decoding.

73,

Jim Brown K9YC


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