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Hi,
nice quiz. Can't wait for the solution!
On Friday 28 May 2010 17:37:47 fons@kokkinizita.net wrote:

quoted text
> 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=20

switching power supplies produce distortions of 100Hz (and of much higher=20

harmonics).
Here I think these affect the op-amp of the input by modulating the supply=

=20

voltage and/or the reference-voltage. This could probably be fixed by more =

or=20

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=20

stabilized far better then the consumer psu that is shipped.

b) Someone swapped the high-quality capacitors for lower-spec ones at the e=

nd=20

of the development cycle to cut costs.

c) The capacitors in your special device just broke their smoke-seal. Bette=

r=20

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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