Re: High voltage probe, odd NST measurements

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Ed Phillips <evp@xxxxxxxxxxx>

Tesla list wrote:
>
> Original poster: "Gerry  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>
>
> Hi Ed,
>
> The stray capacitance of the resisters have not been part of the
> discussion. The resisters in question are 1Gohm and using a string of
> 10 resisters you have 10 Gohms of resistance in parallel with 100
> Gohms of capacitive reactance all feeding a 1Mohm in parallel with
> the 10-30 pf of meter capacitance.  Actually, this stray capacitance
> will help the response.  If the impedance ratios of the capacitances
> matched the resister ratios, you would have a compensated probe.
>
> If you ignore the stray capacitance of the resisters (worst case
> senerio) and thevenize the circuit, you will see that the 1Mohm and
> meter input capacitance determine the frequency response - one pole
> roll off at 16000Hz for a 10 pf meter (or scope probe)
> capacitance.  I have indeed used such a probe with a scope and gotten
> this good of a response.
>
> Gerry R

	I agree if the numbers are as you say, but I can imagine layouts where
the capacitance from HV might "over-compensate" the divider and give
falsely high readings.  If you had a 100 kV square wave generator you
could probably equalize the thing by adjusting capacitance across the
scope.

Ed