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

This experiment was done in Europe with 50Hz AC line current. When rectified by a non-switching power supply, that ends up being 100hz "hum". Alternately, it's 50 hz ground loop noise that is being rectified or has cross-over distortion resulting in 100Hz noise. This "rectification" could take place through a number of unexpected means, including oxidized contacts that act as semiconductors ( http://en.wikipedia.org/wiki/Rusty_bolt_effect ) , low-quality electrolytic capacitors being used for Ac-coupling, and output transformer hysteresis. ( http://en.wikipedia.org/wiki/Intermodulation#Passive_intermodulation ) The 100hz signal due to it's regularity, could also be caused by "periodic jitter"... i think more info is needed to determine whether the situation you're seeing is caused by jitter or modulation by external hum (like testing at a bunch of difft freqs and mixing two sine waves to see the IMD behavior). Perhaps this periodic jitter is the effect of AC line hum interfering with the sampling clock?? http://wolfsonmicro.com/uploads/documents/en/Specifying%20Jitter%20Performa… .... 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. .... Niels http://nielsmayer.com