VTVM DC (Isolating) Probes

[van lincoln]

thank you mike, i appreciate your observations.  and input.
wd8aam


At 06:03 PM 4/23/98 -0700, Michael Donnelly wrote:
>Gang, a few weeks back (before I had E-mail and list access) there was a
>discussion about the DC probes for VTVM's. The discussion talked about
>the need for the 1 MEG resistor in the probe and why not put the resistor
>'inside the case'. And in fact I guess someone said there was a Heath
>factory authorized modification to do just that. I am somewhat surprized
>this was indeed done. From what I can find out the resistor is in the
>probe for 2 important reasons, besides just for impedance match.
>
>I'm going to quote verbatim from a book called 'The VTVM' and written by
>Rhys Samuel and published in 1956 by Gernsback....
>
>"Isolating probes are used for the measurement of DC voltages. For
>example, DC voltage measurements from 0 to 1000 are made with a shielded
>cable terminated with an isolating resistor. The shielded cable
>eliminates the pickup of any stray fields about the test bench, which
>might otherwise result in inaccurate indication on the VTVM. The
>isolating probe resistance prevents the shielded cable from acting as a
>capacitive shunt across tuned and high-impedance circuits, such as the
>local oscillator in a receiver. If it were not for the isolating effect
>of the resistor, connection of the VTVM test leads might detune the test
>circuit so greatly that circuit operation might cease altogether. The
>cable capacitance is isolated from the probe tip by the 1 megohm
>resistor, which cuts down the effective value to approximately 1 or 2pf.
>This is a large reduction, as compared with the approximately 75pf of
>cable capacitance. Hence, the DC probe can be applied across a local
>oscillator circuit without disturbing circuit operation. "
>"The equivalent circuit is also a low pass filter. This is an essential
>feature, since DC voltages at the grids and plates of tubes often must be
>measured in the presence of large AC voltages. If the AC signal is passed
>along to the bridge circuit in the VTVM, it might be rectified and cause
>an erroneous DC reading. Most VTVM's include another
>low pass filter at the grid of the VTVM tube to make certain that no AC
>voltage reaches the grid."
>
>So.....I dunno about putting the resistor inside the VTVM. Even modern
>solid state oscillators might be affected by the cable capacitance if it
>were not for the isolating resistor. I hope I haven't offended anyone but
>if I have in any way violated list rules or etiquette please let me know.
>I just wanted to pass on some info that I ran across.
>BTW....I was able to make up a DC probe for my Heath V-5 VTVM using
>readily available Rat Shack parts. If anyone wants that details feel free
>to E-mail me.
>
>
>Mike Donnelly
>MJATS at JUNO.COM
>
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