[Elecraft] Low Frequency Modulation of Side Tone.

[G3VVT at aol.com]

In a message dated 24/01/05 02:07:20 GMT Standard Time,  
jeff_list at burnshome.org writes:

Problem  found. I added a fixed audio out port to the K2. Disconnecting the
wire to  the sound card stopped the modulation.  An audio transformer in  line
also eliminated the problem. 



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A similar solution was of value in clearing up my problems in using  
Spectrogram.
 
I was getting spikes on the Spectrogram display at 50Hz (our power freq)  and 
every 100Hz from 150Hz upwards. This was believed to be due to my PC having  
a power line filter installed to clear hash from the PC switch mode PSU and  
this left the PC case floating above ground at RF. Series  capacitors had no 
effect, where a transformer placed in the audio path  completely cured the 
problem.
 
Used a miniature audio transistor interstage transformer for the purpose  
with the higher impedance side to the PC sound card. Would guess a transistor  
output transformer with the usual 1.2K to 4 ohms windings may have been  even 
more suitable, but did not have one in the junk box. The only proviso in  going 
down this path is to use a transformer that has a response well below  300Hz 
and 50 Hz if possible to ensure correct placement point of the BFO is  seen.
 
A totally different problem, though in the end with the same  solution.
 
Bob, G3VVT