1701 (DAC?) noise
I'm trying to locate the source of this noise. As soon as I put a small signal like low freq on this output, it is gone. it is only there with no input signal. I programmed a volume slider before the output and it's still there.
first I thought it was limit cycles of the cross-over filter and I changed it to double precision. both peaks are 116 Hz apart and centered at at 19848 hz. my xtal clock is 12.288 Mhz. it's down in the noise, and not audible, but i'd like to find the root cause of it.
my initial thought go towards limit cycles of the DACs, lack of dithering ..
comments welcome !
Hello Koen,
Is the source of the noise from the ADCs? I wonder if you might be seeing ADC Sigma-Delta converter "idle tones" or "limit cycles".
Some info on this subject:
http://www.eecs.qmul.ac.uk/~josh/documents/PerezReiss-AES122.pdf
http://processcontrol.analog.com/static/imported-files/faqs/vol28n1.pdf
Do you hear the noise it if you mute the inputs, and generate a source by the DSP core itself (i.e using any available Sigmastudio-generated source such the sine tone generator)?
If idle tones are the problem, you can add a large pulldown resistor on the ADC inputs to add a small DC offset in tens of mV. The small DC offset pushed the idle tones in higher frequencies and are filtered out during the conversion process.
-JT
found it!
it was the agilent 54622 scope injecting noise into the gound leads. not much but enough to trouble...
when I turned the scope off it was clean..
Hi, Glad to hear you found the noise source. Nowadays, scopes have switching power supplies which spray more noise than I'd like to see in test gear. So do laptop power adapters, which is why I run mine on battery for critical tests.
Best regards,
Bob
Please log in to reply.