Re: K3 Receiver Noise Level -- Evaluation and Adjustment

Posted by Julian, G4ILO on Dec 20, 2009; 4:58pm
URL: http://elecraft.85.s1.nabble.com/K3-Receiver-Noise-Level-Evaluation-and-Adjustment-tp4187070p4195177.html

John Lemay wrote

I have deviated from white or pink noise, because most of the time we are
listening to real signals (or qrm!). I decided to inject a signal: I used my
K3 and XG1 to perform the test. The line out from the K3 was connected to
line in on my PC.

With reference to a fundamental audio signal at 0db, a third harmonic of
this was observed at about -40, and a fifth harmonic at around -50dB. The
even order harmonics are well down in comparison.

Hi John.

Thanks for posting this. It reminded me while I had Spectran set up to take a look at something that has often bugged me - the appearance of harmonically related ghost signals when using narrow filters in CW or PSK31. I used the XG1 as you did. With a wide bandwidth selected, the harmonics were lost in the noise. When using a narrow filter they can be seen quite clearly.

However - and this is the interesting bit - the level of the harmonics relative to the fundamental can be varied by changing the LIN OUT level. On my K3 reducing LIN OUT to 003 and they just about disappear. (Obviously that also depends on the strength of the input signal.)

It seems to me that there is some non-linearity in the audio stages that sets in at a fairly low level. I'm not an expert either, but might the isolating transformers in the line output be the cause of this?