Hi Paul, Markus,
OK, here are the results from additional tests. I took a record from CH1
and CH2 again, this time corrected by vttime which is getting PPS from CH3.
I concentrated on CH1 this time. The top of the spectrogram shows CH1 at
1 pixel/FFT bin the bottom shows CH1 from 0...12 kHz (24 kHz sample rate).
The noise background spectrum is quite clean i find! No 1 kHz multiples
having 20 dB SNR ore more, like many other soundcards have.
This time i choose 2.97 kHz :-) While changing the amplitude, i
disconnected the inputs, so it becomes clear when i did it.
You can see harmonics from the carrier, which is due to the VFO, not due
to the soundcard (something is wrong with the IC that produces the sine
output on the DDS board, it has an unwanted DC component, so there are
likely some nonlinearities)
The signals levels were (relative to full scale): -82 dB, -52 dB, -38
dB, -32 dB and -24 dB. The AM sidebands are always at ~ -60 dBc.
BUT now, the interesting part is the last one! I took cooling spray and
sprayed onto different regions of the board. It seems to have an effect.
The AM sidebands are gone, there are FM sidebands however. Maybe i need
to use a cleaner signal source in the next tests.
A picture of the board, with some modifications ( :-) ):
Enough for today here...