Re: Water probe: improvements

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Bob (R.A.) Jones" <a1accounting@xxxxxxxxxxxxx>

Hi Marco,

What's the hash on the edge?  It looks like you don't have something
correctly terminated or you have current loops on the grounds.
Is your scope a diff input or grounded/earthed reference.

Can you expand on what you mean by dielectric polarization?

The last trace looks like the C division is lower than the  R division. i.e.
if you have a C divider and R divider in parallel and the C divider has a
lower ration than R divider, then the rising edge will rise quickly to a
particular level determine by C division ratio followed by a slower rise to
R division ratio. You will be able to compensate for this in the video amp
with the correct lead lag terms (hf boost) I think. I would need to check
that analytically to be certain.

Bob


----- Original Message -----
From: "Tesla list" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Monday, January 03, 2005 6:42 AM
Subject: Water probe: improvements


> 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
>
>