Posted by
G3VVT on
URL: http://elecraft.85.s1.nabble.com/Low-Frequency-Modulation-of-Side-Tone-tp373879p373884.html
In a message dated 24/01/05 02:07:20 GMT Standard Time,
[hidden email] 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.
--------------------------------------------------------------
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
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