[hpsdr] Penelope hum like component

[hartfuss]

Hi,
just realized a strange behaviour which I haven't seen or overlooked before:
My hpSDR-transceiver (Atlas, Ozy, Mercury, Penelope) shows a hum 
component around 100 Hz which is about 10 dB above the TX noise level in 
the Panadapter (possibly it's not a clean 100 Hz spectral component but 
more 1/f like but stronger decaying, I can't clearly distinguish). The 
relation to the noise level is independent on Mic Gain setting. No 
external connections (Mic, Line out, Control from DB-25) are present; 
there is no influence if connected. The boards are placed away from the 
power supply's transformer stray field (no effect if positioned near 
by). The voltage supplies (+-12V, +5V) are clean.
Of course, it can be made zero by setting the low frequency cut-off in 
the Transmit Profile > 140 Hz.
Now the strange thing: when activating the AF monitor function (MON) it 
disappears completly if the AF monitor setting is higher than about 10. 
If set to zero, MON does not affect the hum component. Nothing external 
connected, besides the computer.
Has anybody else seen this or has even an idea how to cure ??
Thanks, Hans, DL2MDQ.

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