On Fri, 10 Mar 2006 11:51:51 GMT, Cecil Moore wrote:

Wes Stewart wrote:
The VVM probes are comprised of a quad diode sampling bridge followed
by an FET amplifier. They are nominally coaxial, although without the
BNC adapters, they have an exposed pin (very delicate) and at lower
frequencies they can be used much as a high impedance scope probe is
used.

Thanks, Wes. When you say "lower frequencies", does that include
4 MHz?

When I wrote last, I was nursing a big toe that had just suffered the
trauma of having a 4' x 8' sheet of 3/4" plywood dropped on it edge
on. So I didn't want to hobble into the shack to search for the
manual.

Now I have it before me. The nominal impedance of the probes is 100
Kohm shunted by 2.5 pF. If this doesn't upset your measurement then
you're good to go.

I think that when I was remembering probing circuits with the bare
probes I was thinking of the HP Vector Impedance Meter more than the
VVM. It was a lower frequency instrument designed for that purpose.


The instrument uses a phase-locked oscillator to drive the samplers
with the "A" probe being the reference. One meter can be switched to
display the amplitude of either channel and the second meter reads the
phase difference between them.

I was planning to use toroidal pickups and a Lissajous figure
for the phase measurement. Did you know "Lissajous figure"
is described in my 1957 ARRL Handbook but not in my 2000
ARRL Handbook?

Well, they gotta leave something out so they can included the latest
PIC controlled-automatic-rig-to-computer-interface and coffeemaker
doodad.


My main concern is how to ensure there are no reflections
present during the measurement. I need to put the 75m
bugcatcher coil in an RF loop where current is flowing
in only one direction. That's easy to draw on paper but
I'm concerned about it. How would you set it up?

Can't help you.