[PLUG] Fun and Yum! Signal integrity HUM and NOISE!

[Guy Letourneau]

>> Ground each piece of equipment to your electrical ground in the 
>> conduits. For multiple boxes, be sure that each ground path branches off 
>> from the ground wire but does NOT form ground LOOPS. 
> 
> I very much doubt it is a ground problem. Would not a ground problem
> produce a hum? Specifically a 60 Hz hum? I know what a 60 Hz hum sounds
> like and this is nothing at all like that. This is a wailing that
> varies in frequency and intensity as you move around or as a bird flies
> overhead.

Aha! it's not the path(s) to ground, but rather that if you create a 
LOOP which may then act as a receiving antenna for HIGH frequency 
emissions from unshielded equipment. The resistance to ground at low 
frequencies (60hz, 120hz, etc.) may indeed be nearly zero, but the 
impedance at high frequencies is another matter.

The loop gets excited by receiving the RF, and SHARES it with all the 
participating components. The noise you hear is the frequency of the 
emission source plus or minus any fun overtones you get from the 
resonance frequency of the loop (which depends on the size of the loop.

Moving your body (a watery mass) with respect to the loop also changes 
its high frequency characteristics. Bullet chronometers work this way - 
the metal projectile moving PAST two V-shaped antenna a known distance 
apart causes impedance variations; the first acts as a trigger (pulse) 
and the second stops the timer. The time of flight is calculated from 
the two events a known distance apart in time and space, and vee-o-la, 
(viola!) you get a reading in feet per second without touching the 
actual bullet in flight.

If you have an RF noise source and a loop in any signal paths (not just 
grounds) then that loop may pick up the emitted noise or a harmonic 
multiple of it...