Bill,
There is a better way if you have a dual-channel scope. Use both
channels in ADD mode and set one of them to INVERT and DO NOT connect
the probe ground to anything. That way both probes are "floating" and
either one can be used for hot, ground or anything else.
Yes, that's a valid technique, but it has important limits. One is the
voltage rating of the probes, another is that of the scope inputs, and a
very important one is the headroom of the vertical amplifiers. If you
will be using two 100:1 probes rated for 1kV, to measure the drain
waveform of a line-connected amplifier, that should be fine. But if you
want to measure the gate waveform, which is only a few volts, these few
differential volts are mounted atop several hundred volts of common mode
voltage into each scope channel. If you set the scope to a gain that
allows you to see the few RF volts with a reasonable height on the
display, most likely the vertical amplifier stages that are before the
summing point will totally saturate from the high common mode voltage,
and you will see nothing more than a blurry mess, if anything at all.
So, it's a technique one should be aware of, but it's not usable in all
cases. You would still need some of the other techniques, to measure the
smaller signals in such an amplifier.
A good technique is to use a small, but well insulated RF transformer
between the scope probe and the circuit. It can be wound on a small
ferrite toroid. It needs a series coupling capacitor, and for higher
voltages it also needs a voltage divider in front of it. It all can be
built into a special, homemade, galvanically insulated, magnetically
coupled RF scope probe.
This won't let you see DC, of course, but it shows you the RF signal
nicely. The DC component can be measured separately with a common plain
multimeter. Almost all present-day multimeters are rated for direct use
at line voltage, at least on home circuits.
Anyway most people I know just float the scope and connect it directly
to the circuit under test, trying to remember to unplug the device under
test before touching the scope! We even did that at the job. If somebody
forgets to unplug, the GFI will come down as soon as the scope is
touched, forcing a trip to the electrical panel to switch it back on.
GFIs are fast enough that the shock is barely noticeable. I know from
experience, because I am somewhat forgetful, and on my past job I often
had to service high power motor controllers that were directly line
connected. The funny thing is that the panel was down the corridor, past
another 6 office doors that usually were open, and my colleagues knew
why I was trotting down to the panel making a sheepish face!
Manfred
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