Hi Cortland:
> One can also use a pair of probes known to be well balanced and take the
> difference between the A and B channels, or
> use a differential input accessory or plugin. There is then no possibility
> of high-voltage on the instrument chassis.
"Balance" is really Common-Mode-Rejection-Ratio, CMRR.
There are two schemes for difference measurements. One
is measuring the high and low signals independently
(through Channel A and Channel B), and then subtracting
the two (A and B) signals. This requires both channels
to have exactly the same gain (and phase shift) throughout
the frequency range of interest. A 1% gain difference
yields a 100:1 CMRR, which is quite poor -- almost useless.
Another scheme is the difference amplifier, where the high
and low signals are applied to the + and - inputs of the
same amplifier. A good difference amplifier is capable of
100,000:1 CMRR at low frequencies. The CMRR of a difference
amplifier falls off as the frequency increases. A
difference amplifier may not have sufficient bandwidth to
accurately measure the high frequencies in a SMPS.
I have found the portable, battery-operated scope to be the
most accurate and easy way of measuring waveforms within a
SMPS.
Best wishes for the New Year,
Rich
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