Hi, nice quiz. Can't wait for the solution!
On Friday 28 May 2010 17:37:47 f...@kokkinizita.net wrote: > On Fri, May 28, 2010 at 08:30:54AM -0700, Niels Mayer wrote: > > Audio ADCs and DACs have three important inputs; > > the signal input, the voltage reference, and the clock. > > Noise and interference on the voltage reference causes > > amplitude modulation, and jitter on the clock causes > > phase modulation. The resulting modulation products > > look very similar in the frequency domain. One of the > > authors once spent several days trying to track down a > > low-frequency jitter problem, only to find that it was > > in fact a problem of LF noise on the voltage reference. > You're close. See the extra hint in a previous post. I would say it is an effect of the power-supply. As others stated, 50Hz switching power supplies produce distortions of 100Hz (and of much higher harmonics). Here I think these affect the op-amp of the input by modulating the supply voltage and/or the reference-voltage. This could probably be fixed by more or better capacitors stabilizing the +5V (or +12V). My guess why the development engineers didn't catch it: a) They used a high-quality lab-power supply during development which is stabilized far better then the consumer psu that is shipped. b) Someone swapped the high-quality capacitors for lower-spec ones at the end of the development cycle to cut costs. c) The capacitors in your special device just broke their smoke-seal. Better refill them soon:-) BTW: The graphs would be better to interpret if both had the same x-axis- scaling. Have fun, Arnold
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