TSSP: List Archives

From: Paul
Date: Wed, 29 May 2002 21:23:41 +0100
Subject: Re: [TSSP] Re: Top V Probe Design

Terry wrote:

> http://hot-streamer.com/temp/HVProbe-1.gif

At first glance I really like the look of that - it carries
all the advantages of the shielded tube.

> One may want to shorten the windings so that the affect of the
> internal can is uniform along the secondary to make modeling
> easier.  Just a uniform capacitance to ground along the coil.

Don't worry.  We can model any old shape of innards, so long as
it has cylindrical symmetry.   As for calibration, just put a 
step signal into it and capture a trace of the output signal.
A bit of signal processing will deliver the phase and amplitude
response over the range that we need (which isn't very much to
begin with, I think - say 20Mhz would do very nicely).

> if one is willing to calibrate it with math rather than
> precision hardware, it is easy.

Naturally we would do both as a cross check.  We'd probably have
to guess the permittivity of the oil and fiddle it to match
the observed probe response.

Sounds like there are lots of ideas and techniques out there
used by the professionals - but our main requirement goes beyond
these in that we want to achieve minimal coupling to both the
secondary and the space charge around the topload.  IMO Terry's
plan seems to fit the bill and we need to put some numbers to it.

The only number that really needs a careful decision is the diameter
of the grounded shield.  Too wide and the secondary has too much
extra C loading.  Too narrow and the field around the input conductor
may break down the oil dielectric.  Beyond that, neither the
frequency response nor the division ratio, nor the extra
C loading added to Ctop, are particularly critical.  To a large
extent we can work with whatever it comes out to be.

Whatever is made, we just need to be absolutely sure that it won't
form any sort of breakdown within the oil, so that when we see Vtop
departing from the idealised behaviour, we can confidently attribute
it to the streamer loading.  In other words the probe must be
linear at TC operating voltages. That's probably our most important
requirement.
--
Paul Nicholson,
--


Maintainer Paul Nicholson, paul@abelian.demon.co.uk.