RE: RF AC Voltage Probe Question


From: rohre (
Date: Fri Oct 13 1995 - 14:43:09 EDT

Ted and the group,

You want the Peak to Peak(P-P) volts demodulator (detector) probe for your VOM
or DVM to NOT load down the circuit you are measuring. Hopefully, the circuit
connects to something that provides the 50 ohm load or if interstage, some
higher impedance load. You do not want the probe to be 50 ohms, for that
would load down your meter, or scope from reading properly as well as the

Here is the circuit of an old RCA WG-291 probe to illustrate a 550 kHz to 250
M Hz rating RF probe to go to your scope or a high impedance DVM, or even your
VOM. Exact impedance matching is not necessary, as you are usually looking
for a peaking or null measurement, and you are trying to measure without
loading effects.

Because I am not good at these ASCII drawings, I will make a verbal
description that hopefully can be drawn and followed by all:

First lightly draw the outline of a box. Now we are going to fill in
components that form the sides of the box, OK? For the top of the box, draw
in a 220K ohm resistor. The bottom side of the box is left as a line for
ground bus. The upper right corner of the box is a connector to your scope,
(BNC center) or meter hot lead. The right hand side of the box is the ground
from that connector to the ground bus bottom right side. Now the fancy part:
The left side of this box consists of two parts in parallel, so draw side by
side, a resistor of 120 K ohms, and a 1N34 diode, (which will have lower
voltage drop than the 1N914 silicon originally in Ted's drawing.) The bar
side of the diode, (cathode) is UP to the left end of the 220K resistor (top
line of the box, remember?) The 120K, and diode cathode, besides joining the
left end of the 220 K resistor, also at the top join the right end of a
horizontally drawn 270 pf capacitor that will extend off the top left corner
of the box forming a probe tip with its left side. OK, hope all are still on
board, because the last thing is to extend a ground clip off the bottom left
corner of the "box" which of course connects to our circuit under test ground.

Now, this probe will handle demodulation of 30 to 5000 Hz, so would be good to
look at SSB as well. The input capacitance loading on the circuit under test
will only be 2.25 pf says RCA. The equvalent input resistance varies with
frequency, but is adequately high for ham work, varying from 23, 000 ohms at 1
M Hz to 10, 000 ohms at 50 M Hz. and finally to 2500 ohms at 200 M Hz.

Maximum input voltage is 20 V rms, 28 V peak. DC: 250 Volts. The cap can be
mica, or ceramic. The original uses 1/2 watt resistors, but you can use
whatever you have. Radio shack has the 1N34 diodes, and this thing was in a
cylinder case of polystyrene, thus you can do the same or use a small perf
board. I have used bus wire for probes, or the pins that come out of new
shirts, but those are sharp so be careful. Hope this helps.

72 Stuart

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