"DoN. Nichols" wrote in message
...
On 2013-07-29, Jim Wilkins wrote:

Surprisingly I needed only a space frame box with wide-open sides
to
fully shield it for the test and calibration fixture. The circuit
rejected 60 Hz and above very well and the seemingly gaping access
holes were too small for lower frequencies.

Electrostatic fields only -- not electromagnetic?

There was no inductance sensitive to it. We all knew to minimize the
physical size of the summing junction node to avoid such parasitics.

However the circuit I built to measure the dielectric absorption
current decay rate picked up ~40KHz audio from the security sensors.
The project manager, a brilliant physicist, calculated that the outer
panels were vibrating about a micron to cause the coupling we saw. I
cured it by creasing them on a brake.

From the mistakes I've seen on commercial products it seems that PC
board layout designers who understand electronics are rare. I could do
a high-speed digital board with dramatically less noise that the usual
products of commercial PC board companies. I debugged one failing
example that had 3V of ground bounce between two points on the same
plane, because it had been notched into a star configuration that was
wrong for the 74AS logic on it.

Hmm ... I remember at Transitron (early 1960s) one item on the
production line (long before computer controlled tests) was a
reverse
voltage leakage test for high voltage diode assemblies. Think about
the
size of a billy club made of a spiral of diodes potted in black
epoxy,
and perhaps 12" long with an anode cap (like from a tube) on both
ends.

AFAICT exotica like voltage multipliers, dielectric absorption,
magnetic amplifiers and ferroresonant transformers aren't covered well
in EE courses, but must be learned through job experience. I picked up
a lot of subtleties that new EEs didn't know. One of them insisted
that an open collector transistor driven from a gate didn't need a
series base resistor because the transistor model he had learned had a
current source in the base lead. I couldn't convince him that the base
was just a diode to ground.

He didn't stay long, the rumor was that he touched a large exposed
electrolytic with 480VAC while putzing with the three phase
connections to a power supply. I was working on a machine next to it
but out sick that day, and came in to find the floor and the side of
my machine blackened from the explosion.

jsw