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Re: Water probe: improvements

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

Hi Marco,

> Original poster: "Denicolai, Marco" <Marco.Denicolai@xxxxxxxxxxx>
> Ground hearted. Not a problem to have it floating with an insulation
> trasformer, but that usually doesn't help. Would a differential probe be
> better?


> The water is a so called "polar" dielectric. Its polarized molecules > take time to organize according to the changing E field. I haven't yet > found a thorough source for this, but dielectric absorption seems to be > related with polarization. Dielectric absorption is responsible for the > memory effect of certain capacitors (gaining back voltage after having > been shortcircuited) and can be modeled as an RC series in parallel with > the ideal capacitor. > Now, this is just what I am seeing in those waveforms. It is not a decay > any more, is more an extra bump following a sharp falling edge

I don't think you have to worry about dielectric absorption according to
this link the relaxation time of the water molecules is 8ps
 check this link out http://www.lsbu.ac.uk/water/microwave.html
>  > 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.
> I guess you mean what I call "overcompensation", that makes the falling
> edge peak to the negative side of the voltage scale. Well, if I
> overcompensate that (I did it to see how it goes), the bump stays there
> but the horizontal part after the bump starts bending to the negative
> side. Encreasing the overcompensation (i.e. reducing even further the
> resistance on the divider lower voltage arm) the horizontal part bends
> very well down but the bump is still there (!).
> Consider also that the two traces have same setup, only different time
> scale for a better insight look.

I don't understand what you mean.
I would have called it under compensation when initial rise is less than the
final rise to the top of the step and over compensation when the initial
rise is greater than the top of the step.
This is caused in the first case by lower gain at hf frequencies than at dc
and the second case by higher gain at hf  than at dc.
I believe a hump after the initial rice is incorrect gain at intermediate
frequencies or resonances.

I tried to find some straight forward references to leads lags and
compensation but did not.  I you want me too a will jot some notes down with
the graphs and equations then get Terry to store it and post a link to it.