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Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
When you're dealing with just a few picofarads, I should think that stray capacitance can stick its snout in from all sorts of directions. Measurement of very low capacitance isn't something I know much about, I'm afraid, Patrick.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
I've just had a rummage in a box of low pF silver micas for you, and see that below 10pF the percentage tolerance system gives way to accuracy expressed directly in pF e.g. 4.7pF ±0.5pF.
Where a very precise low pF capacitor is required - say in RF tuning - the usual approach is make C larger than needed, and to put Ctrim in series with it to bracket the desired range, or make C smaller than the desired capacitance, and put Ctrim in parallel with it.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
I think the best way to test a probe like this is fire it up and look at the response to a square wave input. You're testing the capacitance in exactly the way you intend to use it. If you can get a nice square wave, ship it.
Because of linearity, the response will be the same for all voltages, so you don't need a 10,000 volt square wave to test it. Just the smallest that your scope can resolve. Maybe 100V or even 10V would do. Most DSOs have an averaging feature that you can use to help resolve a smaller signal.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Steve McConner wrote ...
I think the best way to test a probe like this is fire it up and look at the response to a square wave input. You're testing the capacitance in exactly the way you intend to use it. If you can get a nice square wave, ship it.
Because of linearity, the response will be the same for all voltages, so you don't need a 10,000 volt square wave to test it. Just the smallest that your scope can resolve. Maybe 100V or even 10V would do. Most DSOs have an averaging feature that you can use to help resolve a smaller signal.
does the capacitence effect the rates of change or the whole wave? can you draw waveforms of ideal and then of annomoulus conditions?
I may not be able to generate a 100 v squarewave real well. should i compare a known good probe to my HV probe whi;e looking at the same square input?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
If the probe is "just right" the waveform will be a perfect square wave.
If it's not, it will be some bizarre shape. The simplest case is that the leading edge either overshoots or undershoots.
If the shields in your HV assembly have the wrong capacitance, to a first-order approximation, it has the same effect as a regular scope probe's compensation trimmer being set wrong. Turning that logic on its head, there isn't just one right capacitance for the HV shields: rather a range that you can compensate by twiddling a trimmer in the compensation box you'll have to build.
You should do this so you can have a look at all the other time constants in there that you'll need to compensate eventually.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Steve McConner wrote ...
If the shields in your HV assembly have the wrong capacitance, to a first-order approximation, it has the same effect as a regular scope probe's compensation trimmer being set wrong. Turning that logic on its head, there isn't just one right capacitance for the HV shields: rather a range that you can compensate by twiddling a trimmer in the compensation box you'll have to build.
You should do this so you can have a look at all the other time constants in there that you'll need to compensate eventually.
Ok ill post waveforms here when I get them for evaluation, these tests and builds are expensive and difficult so being wrong is no light issue.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
I have solved the capacitence problem, it was due to complex fringing field flux wandering so badly. My physics book held the answer.
Capacictors are most predictable with wide plates at narrow distances, thus the calculus of the large uniform field overwhelms the little non-uniform (fringe) flux at the edges of thin plates. This was the problem, two thin wall pipes acting with only 0.040" plate breadth, with 2.4 inches of circumference across a gap of 0.570". The 0.040" dimension was too insignificant compared to the 2.4", and 0.570" so the field was unstable and difficult to control, varied wildly from 1pf to 4pf quickly when all demensions were held constant.
I will post pictures to describe the problem and then the solution soon.
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High voltage probe for O-scope, 1 of 2, ( DEVELOPMENT ).
