Re: [LAD] A little quiz about audio measurements...

Hi,

nice quiz. Can't wait for the solution!

On Friday 28 May 2010 17:37:47 fons@email-addr-hidden 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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