vtvm question??

[Alan Dean Martin]

At dc, from the VTVM side of the circuit, the RC filter (composed of the 1 Meg
resistor and cable capacitance) doesn't really do anything other than give the
extra megohm of input resistance and the lower sensitivity. This lower sensitivity
is offset during the dc calibration step.

At high frequencies, from the VTVM side of the circuit, the low pass
characteristic has no notable effect.

However, to the circuit under test, the 1 megohm resistor "isolates" the circuit
under test from the cable capacitance. This substantially reduces the effect of
taking the measurement at a high impedance node such as the grid of the tube, gate
of a FET,  an op amp input or feedback point. Connecting the cable capacitance
directly to a high impedance node might cause the stage to malfunction. Such a
malfunction may or may not cause a change in the dc reading. But if the stage is
no longer functioning correctly then the reading isn't representative of the
circuit condition. A further, more subtle benefit is that it's less prone to
conduct hum from the probe cable and test prod into the measurement point.

The 1 meg ohm helps; and it needs to be at the probe end of the cable.

For ac readings, things are different. If the low pass remained, the meter reading
would get lower as the signal went up in frequency. Thus, the 1 meg gets switched
out for AC. The input impedance drops to 10 meg. because they used the same
resistor string on the range switch. BUT the circuit under test has to tolerate
the cable capacitance.

One the the best tube type VTVMs was the HP unit with the FAT AC probe handle.
These had the AC detector diode tube mounted out in the probe. (Built-in hand
warmer). Essentially no cable capacitance on AC with this scheme. As I recall
these had measurement bandwidth out past 100 MHz.


Gary L. Sanders wrote:

> Why would the grid Voltage be affected by capacitance in the DC mode? In an AC
> mode, there is a low-pass filter produced by the 1Meg resistor and the cable
> capacitance, which I don't see reducing the problem of cable capacitance
> affecting readings.
> --
> Regards,
> Gary L. Sanders, Staff Analog Eng., gsanders at csidaq.com
> Celerity Systems, Inc. --- http://www.csidaq.com
> Cupertino, CA USA (408) 873-1001 fx (408) 873-1397
> Ultra Fast Acquisition & Data Generation Systems
>
> Alan Dean Martin wrote:
>
> > Additionally the 1 meg resistor helps isolate the effect of cable
> > capacitance from the circuit under test. This permits testing grid d.c.
> > voltages, for example, which might otherwise become erroneous reading if the
> > cable capacitance were connected without the 1 megohm isolation.
> >
> > Alan Martin
> > Linear Technology Corporation
> >
> > Gary L. Sanders wrote:
> >
> > > The 1Meg resistor in the probe prevents dangerous high Voltage shorts if
> > > the probe is carelessly disconnected while on a high Voltage. This
> > > divider results in the meter having to be calibrated to compensate for
> > > the .90909 loss. When a Hi Voltage probe is used, a 909Meg resistor is
> > > used in the probe resulting in a 100:1 divider that is only off 1%. If
> > > there is some leakage around the big resistor, such as sweat, oil, or
> > > grime, then the accuracy is off in the right direction to get better.
> > > --
> > > Regards,
> > > Gary L. Sanders, Staff Analog Eng., gsanders at csidaq.com
> > > Celerity Systems, Inc. --- http://www.csidaq.com
> > > Cupertino, CA USA (408) 873-1001 fx (408) 873-1397
> > > Ultra Fast Acquisition & Data Generation Systems
> > >
> > > Steve Lords wrote:
> > >
> > > > greetings
> > > >  does anybody know why heathkit on the
> > > > im-13 vtvm put the extra 1 meg resistor in the
> > > > probe and not in the unit,without the probe
> > > > the vtvm is only 10 meg
> > > >
> > > > when reading articles about rf probes
> > > > they mention the probe will work with
> > > > 11 megohm vtvms however without the probe that
> > > > comes with the im-13,the input is only 10 megohm
> > > > which will affect the reading,
> > > > maybe the 1 meg should be put into the rf probe
> > > > when using the im-13...
> > > > thanks
> > > > steve
> > > > wa7isl :)
> > > >
> > > > --- --- --- --- --- --- --- --- --- --- --- --- --- --
> > > > To subscribe: listserv at listserv.tempe.gov
> > > > and in body: subscribe HEATH yourfirstname yourlastname
> > > > To unsubscribe:  listserv at listserv.tempe.gov
> > > > and in body: signoff HEATH
> > > > Archives for HEATH: http://www.tempe.gov/archives
> > > > --- --- --- --- --- --- --- --- --- --- --- --- --- --
> > >
> > > --- --- --- --- --- --- --- --- --- --- --- --- --- --
> > > To subscribe: listserv at listserv.tempe.gov
> > > and in body: subscribe HEATH yourfirstname yourlastname
> > > To unsubscribe:  listserv at listserv.tempe.gov
> > > and in body: signoff HEATH
> > > Archives for HEATH: http://www.tempe.gov/archives
> > > --- --- --- --- --- --- --- --- --- --- --- --- --- --
>
> --- --- --- --- --- --- --- --- --- --- --- --- --- --
> To subscribe: listserv at listserv.tempe.gov
> and in body: subscribe HEATH yourfirstname yourlastname
> To unsubscribe:  listserv at listserv.tempe.gov
> and in body: signoff HEATH
> Archives for HEATH: http://www.tempe.gov/archives
> --- --- --- --- --- --- --- --- --- --- --- --- --- --

--- --- --- --- --- --- --- --- --- --- --- --- --- --
To subscribe: listserv at listserv.tempe.gov
and in body: subscribe HEATH yourfirstname yourlastname
To unsubscribe:  listserv at listserv.tempe.gov
and in body: signoff HEATH
Archives for HEATH: http://www.tempe.gov/archives
--- --- --- --- --- --- --- --- --- --- --- --- --- --