hot probe

The numbers didn't make sense on my HV pulse generator. Then I found that the 100:1 scope probe was dissipating about 5 watts.

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This is a 1KV pulse at 3 MHz.

I guess I'll have to build something.

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John Larkin         Highland Technology, Inc 
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John Larkin
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Capacitive divider? Those can be had off-the-shelf.

Reply to
whit3rd

Years ago I got a supply of small 3kV-rated 3pF caps on eBay. With 99x higher coax + caps on the low side, they make excellent HV dividers. There's also some stuff to protect the scope.

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Reply to
Winfield Hill

I'm thinking that, for testing, I'll make a 10:1 capacitive divider to front-end a standard 200 MHz passive probe.

I also want to build a pickoff into the pulser itself, to drive a coax. That will probably need a fast opamp buffer; we haven't come up with any passive circuit to pick off my 1KV pulse and drive a 50 ohm coax without some sort of lethal gotcha. A 100:1 division ratio buries the sampled signal in ground loop noise. Lower ratios either pick off too much power, or have rotten frequency response.

I posted a thermal image of measuring my pulse output with a commercial 100:1 high-voltage probe.

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John Larkin         Highland Technology, Inc 

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Reply to
John Larkin

Depending on the bandwidth of concern, you may want to check the Zin of the probe. Many of them have more than a single RC break. Probably can be fixed but it may not be as simple as you'd hoped. -F

Reply to
Frank Miles

I meant 1000:1 there. 100:1 has different problems.

Lower ratios either pick off too much

The 100:1 HV thing is a cheap Amazon probe. But scope probes use special coax with a skinny, resistive inner conductor. They have to jam more current into that coax at high frequencies, to keep the response flat, so that loads the signal more as frequency goes up. So yes, it's probably not a simple R+C.

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John Larkin         Highland Technology, Inc 
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Reply to
John Larkin

Have you considered using a 10:1 probe, with an additional 10:1 interface at the scope end? Obviously high voltage can be a problem in handling so you would need to make sure the probe itself is rated for the peak voltage. There are circuits for these things here and there.

Or:

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John

Reply to
John Robertson

I have a 100:1 high-voltage probe. It's OK at low pulse rates, but it gets hot as I ramp the rate up. I'm thinking that no 10:1 probe would survive kilovolt pulses at MHz rates.

The fact that it's getting hot means that it doesn't look capacitive to my source. So what I see on the scope isn't realistic.

Nice, but I need at least 100 MHz bandwidth, preferably 200 or so.

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John Larkin         Highland Technology, Inc 
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Reply to
John Larkin

Then you're probably looking for something like a Yokogawa "701944"

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Costs some 35000 JPY (=$320) in Japan, likely somewhat more overseas.

Note the derating over frequency however. It is significant above 1 MHz.

See

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Reply to
Dimitrij Klingbeil

Yikes. At 20 MHz, the impedance is 1K! At 100 MHz, it's 200 ohms! A

950 ohm Caddock axial resistor would be a better probe, into a 50 ohm load.
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John Larkin         Highland Technology, Inc 
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Reply to
John Larkin

Well, it did specify a 7.5 pF input capacitance. At 100 MHz that 7.5 pF alone will make 212 Ohms. As expected, most of the "Z" is capacitive.

If you need Hi-Z at these frequencies, it most likely means own design.

Reply to
Dimitrij Klingbeil

I guess so. I need a kilovolt fast fet probe!

Maybe a 1pf to 250 pF divider followed by a fast fet opamp, like an ADA4817 or something. Then 50 ohms out to a coax.

Current-mode opamps are like scope probes. They get their bandwidth by loading down the input signal.

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John Larkin         Highland Technology, Inc 
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Reply to
John Larkin

I'm not sure what sort of dielectric will give you a reasonably small

250pF with an ESR as low as you need (ideally
Reply to
Frank Miles

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