Looking for a scope probe for high capacitance input

[Mike G8GYW]

One of my scopes is a 1980s vintage VuData PS950 (rebadged by Racal). The input capacitance is 30pF and I'm struggling to find a x10 probe with sufficient adjustment range to tune it out. I bought a Hantek PP150 which claims a range of 15 to 40pF but this was the best it could do on the compensation waveform.

Any suggestions please?

Mike

If it were mine I would unzip the probe and add a few pF, a gimmick (twisted wire pair) capacitor would do.

[Mike G8GYW]

Interesting idea, thanks.

Mike

[factory]

It's quite hard to tell from the picture, but have you got the input coupling set to DC?
To me channel 1 looks to set to AC, if channel 2 is also set this way it would explain why you can't get a square trace.

David

To me channel 1 looks to set to AC, if channel 2 is also set this way it would explain why you can't get a square trace.

An interesting observation, in that case the droop would be at the trailing end of the square wave and (as it is) non existent at the usual 1kHz "calibration" waveform.

[6AL5W-Martin]

the old Tektronix Probes P6006 are useful for your scope.
greetings.
Martin

[Mike G8GYW]

Originally Posted by factory

It's quite hard to tell from the picture, but have you got the input coupling set to DC?
To me channel 1 looks to set to AC, if channel 2 is also set this way it would explain why you can't get a square trace.

David

Yes it was set to AC, but it doesn't make any difference to the waveform.

[Mike G8GYW]

Originally Posted by 6AL5W-Martin

the old Tektronix Probes P6006 are useful for your scope.
greetings.
Martin

Thanks Martin. Not many for sale in the UK but I will keep looking.

Mike

[cmjones01]

I've got a bunch of the cheapo "P6100" type probes and I don't know what their official compensation range is but they (just about) tweak up OK on the 1M/47pF inputs of a Tek 7A22 differential amplifier. I have a couple of old RS/Coline ones which work OK at high input capacitances too. I've seen them for sale on eBay.

Chris

[Mike G8GYW]

Originally Posted by cmjones01

I've got a bunch of the cheapo "P6100" type probes and I don't know what their official compensation range is but they (just about) tweak up OK on the 1M/47pF inputs of a Tek 7A22 differential amplifier. I have a couple of old RS/Coline ones which work OK at high input capacitances too. I've seen them for sale on eBay.

Chris

Thanks Chris, that's good to know. There are indeed Coline probes on eBay, with prices ranging from a reasonable £10 to an eye-popping £59.99!!

Mike

[prumpfy]

something NQR here.
The usual input is 1 meg//47pF.
If the input of the amp is 1meg//30pF, then the input will be over compensated and will show a differentiated pulse shape.
The trace shows slow to rise.
Looks like a dodgy probe to me.
prumpfy

[factory]

There is an error in the first post, the Hantek PP150 probe is actually 15 to 35pF (not to 40pF), but I would have thought it should just about be OK, not sure why it isn't in this case?
The lower bandwidth Hantek PP80 & PP90 claim to be OK to 40pF.

I checked a few probes here with my HP 1402A (1MΩ 43pF input) and the only probes that can be set properly were the original HP types & two RS branded Coline ones.
The later HP 180A with 1801A (1MΩ 25pF input) I've repaired recently, is fine with some more modern probes I have.

David

[Mike G8GYW]

Originally Posted by factory

There is an error in the first post, the Hantek PP150 probe is actually 15 to 35pF (not to 40pF)

Not an error on my part, I was quoting the figures from the data sheet that came with the probes (I bought two and both gave the same result).

My other scope is a Tektronix 2340A with 1M/15pf input and the probes worked fine with that so my assumption is they don't have enough compensation range.

Mike

[Mike G8GYW]

I managed to find a used Coline SP150 at a sensible price but on my VuData scope (1M/30pF) it is no better than the image in my first post.

When I connected it to my Tektronix 2430 (1M/15pF) the adjuster altered the amplitude but the waveform remained a perfect square wave throughout. Not sure I understand that.

So the search goes on for a probe that works into 30pF. It could end up being rather expensive (although the scope was free ...).

Mike

[PETERg0rsq]

Are you sure the issue is the probe?

Not sure how to test, but could you simulate the load in some way?

What about using a 50 ohm termination at input, and feeding from 50 ohm source? Does that give a distorted input, or a clean one?

[Mike G8GYW]

I think the scope is OK because both channels behave the same and the calibration signal is clean with a x1 probe and 50 ohm termination.

My Tektronix scope has a calibration source that varies in frequency from 50Hz to 5MHz according to the timebase setting. Using 50kHz gives a clean square wave on the VuData scope with the Coline probe but the amplitude is slightly low with the compensation adjustment at maximum.

I guess I need a vintage probe for a vintage scope ...

Mike

[cmjones01]

Originally Posted by Mike G8GYW

When I connected it to my Tektronix 2430 (1M/15pF) the adjuster altered the amplitude but the waveform remained a perfect square wave throughout. Not sure I understand that.

This is a significant catch with the Tek 24xx series scopes and their calibrators. The calibrator changes frequency to match the timebase, so you have to be really careful to make sure the timebase is set to the right speed for compensating the scope probe. If it's running too fast, the calibrator frequency will be too high (50kHz is definitely too high) and you'll see the amplitude of the whole waveform vary as you adjust the probe compensation. The most convenient frequency for the compensation procedure is usually 1kHz, so set the timebase on the 2430 to 1ms/div before starting. That way you should get the expected adjustment on the leading edge of the square wave.

The reason for this is that the probe compensation is basically adjusting the high frequency response of the probe. To get the response at high frequencies to match that at low frequencies, you need a waveform containing both. A 1kHz square wave does the job nicely. If the square wave is too high in frequency, you'll only see the compensated high frequency response and have no low frequency component to compare it to, hence why the whole waveform appears to grow and shrink.

Chris

[Mike G8GYW]

Chris, thank you for that clear explanation. It's helped me to understand what I'm seeing.

Mike

[Oldcodger]

Might be of interest to folks setting up/lining up Radio etc . Years ago in MSDS, we experimented with tuning on the RT353 on recievers and transmitters, when aligning both with a scope and a sweep generator. Spec called for a x10 probe, but in the repair section we found that we could get a better response from both units using a wire single line probe .