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Re: Water probe: improvements

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

Hi Marco,

I checked it is correct,  Assuming that the problem is C and R ratio
No simple way to determine what the corner frequency of the lead lag terms

You could do it experimentally and adjust the hf boost until the initial
rise height is correct then fiddle with the corner frequencies until you get
the best flat top after the rise.  There will be quadratic terms  which
inevitably you can not compensate perfectly  for with 1st order lead lags
but they may be small or at a much higher frequency.

Can you do a complex FFT on the step response to get the frequency response?
The signal does not look very clean for it.


----- Original Message ----- From: "Bob (R.A.) Jones" <a1accounting@xxxxxxxxxxxxx> To: "Tesla list" <tesla@xxxxxxxxxx> Sent: Monday, January 03, 2005 10:58 AM Subject: Re: Water probe: improvements

> 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 > > > > >