Water probe: improvements

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Original poster: "Denicolai, Marco" <Marco.Denicolai@xxxxxxxxxxx>

Hello all,

During the past days I played with the smaller water probe prototype (a
table top, 70 cm tall) I have at home. The step response time had a
decay as bad as the one of the bigger probe at the university (about 50%
straight, then a 20 us decay).

It was nice to see how sliding my hand along the water column I could
either increase the decay time or reduce it, up to get some negative
peaking from overcompensation. But luckly I decided to first try
something else than brutal force.

99% of the decay was actually due to the coax cable impedance. I built a
small video amplifier fed by two 9V batteries, including an EL2244,
protection schottky diodes, input AC coupling and compensation
possibility. Output is balanced for a 50 ohm coax. Gain is now set to 1.

And voila'. The step response time went down to 20 ns (nanoseconds).
That's the fall time of the fastest 30V pulse I could generate at home.
This corresponds to a probe bandwidth of 0.36/20E-6 = 18 MHz. I had to
compensate a little bit with a trimmer to balance the capacitance of the
schottky diodes at the input (about 100 pf).

Yesterday I went to the university lab with my mighty videoamp, full of
hope and...

Well, the videoamp improved quite much the step response, but nothing as
radical as at home. I also tried sliding my hand along the water column
(now 2.7 m long, hanging from the roof at 6 m height) but I couldn't get
the "overcompensation" with negative peaking.

The benefit of the videoamp was also that now I could locate the
oscilloscope on the floor, far, at the end of several meters of coax
without a noticeable signal degradation. And that's what I did. I
noticed that removing the elevator from the TC vicinance bettered the
response. And also rerouting the probe grounding horizontally straight
to the wall helped. Then I moved the whole pulse generator box 4 meters
far from the coil and, still, an improvement.

To make it brief, I ended up with a response straight to 75% and with a
small "bump" lasting for about 10 us. With all the "stuff" near the TC
the bump whould be a decay from 60% lasting for 15 ns. So, once the
macroscopic problems from the coax are eliminated I bumped into the
isotropic capacitance problems. After the TC was freed from nearby
objects I was left with...what?

I think I figured it out this morning. It must be the dielectric (water)
polarization.
As the step is generated from feeding 250V to the probe for 0.3 s and
then shorting it to gnd, I think the DC component of that is responsible
for the little bump I wasn't able to counterbalance.

Tomorrow we'll add a 1uF capacitor in serier to the pulse generator and
I really hope to get rid of that bump too.

I have scanned a printout of the probe step response. It's at the bottom
of the page at:

http://www.iki.fi/dncmrc/work/hv_divider.htm


Best Regards