Roy Lewallen wrote:

Most novice designers get burned somewhere along the line by failure to
realize what Ian is describing -- that all amplifiers are differential
amplifiers, and calling one of the inputs "ground" doesn't impart magic
properties. Once that lesson has been learned (alas, usually the hard
way), the probability of making circuits work as designed increases
dramatically.

Even so, though, a surprise sometimes comes along at the last minute. I
know of one expensive, high quality test instrument that had a ground
path broken at the last minute because of ground current that was
induced in the signal path by the field from the display CRT deflection
yoke. Forewarned, I discovered 100 mA of induced current in the shield
of a multi-turn delay line from the same source, in an instrument in
whose design I was participating. Simply soldering the turns of
semi-rigid coax together reduced the effective transformer secondary to
one turn from 20 or so, making it possible to reduce the coupling and
its effect to an acceptable level by other means.

But I remember that shortly after I went to work at Tektronix, I was in
the main lobby admiring the example of the first oscilloscope model
produced by that company, which was on display there. Its success and
the reputation it gathered as a quality test instrument was instrumental
in launching the company. It had two chassis, one above the other,
supported by four L - cross section aluminum legs between the chassis.
One of those four legs had very neatly been cut, leaving a small gap. A
sturdy piece of phenolic was mounted between the gapped ends, again very
neatly, with countersunk flathead screws. I remember that seeing this
gave me a good feeling that ground currents could be a problem for the
best of 'em.

The 'scope in the lobby was a production model, but somewhere, I'm sure,
there was a prototype much like the Racal device -- with a hacksaw cut
across the leg.

Roy Lewallen, W7EL

This thread has got me wondering if I could reduce some of the residual
'birdies' in my R8B (there aren't that many) which I suspect are caused
by one or more common mode ground loops involving the synthesizer, by
implementing the chassis cutting technique. I'm not about to take a
hacksaw (Dremel tool) to it using the trial and error method, so how
does one analyze the situation to get a feel for what might be going on?


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