Re: Water probe: improvements

["Tesla list" <tesla@xxxxxxxxxx>]

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

Hi Marco,

I just reread your post and realized you had some compensation for input C
of 100pf that's huge figure to compensate for and the amp may have run out
hf gain hence the wrong edge height.

I don't know your circuit but if the comp ration is not correct that could
be the source of your problem too similar if not the same as above.

Bob
----- Original Message -----
From: "Bob (R.A.) Jones" <a1accounting@xxxxxxxxxxxxx>
To: "Tesla list" <tesla@xxxxxxxxxx>
Sent: Monday, January 03, 2005 11:42 AM
Subject: Re: Water probe: improvements


> Hi Marco,
>
> I checked it is correct,  Assuming that the problem is C and R ratio
> differences.
> No simple way to determine what the corner frequency of the lead lag terms
> are.
>
> 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.
>
> Bob
>
>
> ----- 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
> > >
> > >
> >
>