i think you are going to see two main components
bad isolation/grounding/shielding
poor decoupling
bad isolation/grounding can include everything from capacitive coupling to RF interference picked up by bad chassis design and much more.
poor decoupling is really part of the bad isolation/grounding/shielding if you think about it.. proper PS filtering should have taken care of most if not all problems with noise coming into the box through grounding or on the line itself, anything induced on power by digital switching means that the filtering/smoothing of the powersupply doesn't have enough capacitance to keep the rails from drooping, and digital noise on ground means that your PS is not properly decoupled from the various analog and digital grounds. this is not even taking into account current noise and everything else that a bad design might include..
so the question is.. is the grounding good enough to support the quiet environment that you desire, or is it really just moving the noise around screwing with the circuit?
:green:
