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Re: Water probe: improvements

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.


----- 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 > >