Re: High voltage probe, odd NST measurements

["Tesla list" <tesla@xxxxxxxxxx>]

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

> Original poster: Ed Phillips <evp@xxxxxxxxxxx>
>
> "Original poster: "Gerry  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>
>
> Hi Michael,
>
> Correction:  BW = 1/(2*pi*RC) or about 1600Hz
>
> Gerry R."
>
> I haven't seen anything in this discussion (might have overlooked it)
> about the effect of capacitance across the SERIES resistors.  AT 60 Hz
> the reactance of an 0.27 pF capacitor would be only 10 Gohm if I haven't
> slipped a decimal point.  That's an awfully high series resistance for
> an AC divider and I can't imagine the divider giving accurate results at
> power line frequencies.
>
> Ed