Helmut Wabnig at wabi@mail.carinthia.co.at says...
>
>> I was thinking of taking a glass tube, and stacking lots of
>> glass-body resistors in series to drop the voltage down to
>> a managable level. Would this work? ...
>
> That is exactly right. You have the correct plan already.
> Go, make it.
> The scope input has a certain resistance, eg 10 Megohms.
> Parallel is a capacitance, several picos.
> Must be taken into the calculations, the capacitance
> will considerably reduce bandwith.
>
> Scope probe schematic:
>
>
>
> !------------------<scope input
> !
> !
> <-------------RES1-----------RES2---------!!ground
> ! ! ! !
> !--CAP1--! !--CAP2--!
>
> tau1=R1*C1 must be equal to tau2=R2*C2,
> then the frequency response is correct and you
> don't waste bandwith.
>
> where R2 and C2 are the added values of the
> scope input and the probe.
This is classic low-voltage probe architecture and it doesn't
work for HV scope probes, unless 1) you're willing to have an
overly high capacitive loading, or 2) you don't care about
mid-frequency or pulse-shape response accuracy. This is
because the RES1 value will be very high, 100M or more likely
1000M ohms, and physically long and large. So the real
circuit is like:
________ CAP1 __________
| |
<------- R -*- E -*- S -*- 1 ------ etc
| | |
Cs Cs Cs
| | |
gnd gnd gnd
Because the RES1 is so high, the probe becomes a good antenna,
and a shield is mandantory. Therefore the Cs "stray"
capacitance is higher than you might think.
I think you see the problem.
One solution is to make C1 very large, but it's just a matter
of specs - if you want 1% performance over the whole range, C1
is a severe load. There is a good overall solution, which I
think is fairly clever (after thinking of it, I discovered the
experts had beat me to it!); see my earlier posting in this
thread.
--
Winfield Hill hill@rowland.org
Rowland Institute for Science
Cambridge, MA 02142
Date: Mon, 07 Oct 96 17:16:57 EST
Original Subject: Re: Can I build a HV probe?