HiSo I wanted to check the VCE of a transistor in a small converterrunning at 100kHz, expected to see about 100mV above ground as thesaturation voltage of the NPN, increasing linearely since magnetisingcurrent increasing.I saw nothing like that, debugging and found the scope (Tek DPO5000)somehow saturates with high voltage inputs. The signal is a squarewave of 100kHz, 25V peak to peak, offset so low voltage is close toGND (measured by a 10:1 probe)See plot (green trace):Low value is about 400mVOk, changed the scale div from 3V/div to 2V/div then this:Now low value is about 2.4V !I checked it was not my setup and all is fine.So I then hooked up the HP33120A function generator, with a BNCdirectly to the scope, now the problem still exists, but just atanother volt/div setting (probably since the probe P500 has internalscaling etc). The signal, square wave, 9Vpp, 4.5V offset, 100kHz) (alevelshifted 9Vpp signal above GND)My suspicion is that the input stage of the scope is saturated sincethe top level of the signal is way out of bounds. If I reduce thefrequency to 1kHz, then the signal starts to look good again, whichtells me the input stage of the scope has settled and recovered fromthe overdrive.Then for a sanity check I hooked up a Tek TDS3032B and it showed thecorrect signal all the way down to 20mV/Div settingThen I took a Tek DPO4000 scope, and it had the same fault, allthoughat a different volt/div settingTried with a TDS220 (low end scope), it has the fault beginning atbelow 2V/divTried a PicoScope 3000 series, had the fault at below 2V/divTried a Agilent DSO 4000 series, had the fault at AFAIR 5V/divThen dusted off my first scope, the Hameg 205-3, it worked all the waydown to 100mV/div :-)To the question, do any of you guys know the front-ends of scopes andif so is my assumption that the input stage is saturated correct (sothe amp takes time to recover)?Seems I should throw out the 10k USD scope and bring my 1k USD Hamegto work for some real...

I believe you set it from 3V/div to 600mV/div for channel 4.Can't follow that but I think it's 400mV. Which is weird, assuming youhaven't changed the offset (or the scope didn't do that because itthought you might have wanted to ...).That is rather surprising.I've never trusted those things.Well, good old German quality :-)In a nutshell, yes. Maybe not throw out the $10k scope though but I'dsend this info to Tektronix support. If they can't answer I'd ask thesales rep who sold it to you to come out there.I can't count the times where I've had to ask clients "Do you have areal scope?" and out came a big old Tektronix boat anchor where peopletook a step back when I pulled the power switch handle ... bzzzt ...TUNGGGG ... blueish green traces showed up and ... tada ... fuzz couldbe seen that the DSO simply was unable to show.Here I have a Tektronix 7704, a Telequipment D66 and I also still havemy old Hameg that I had as a highschool student (but that doesn't getused much anymore). I would...

How good is your scope? (problem with capturing offset signals)

Sep 26, 2011 41 Replies

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Joerg 14 years ago

[...]

This would probably be the point where I couldn't wait any longer, crack open the probe enclosures, and get into trouble :-)

There'd have to be some sort of semiconductor involved here. Maybe some sort of limiter circuit?

Regards, Joerg http://www.analogconsultants.com/

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Fred Abse 14 years ago

7A13 with attenuator set to 1mV per division recovers to within 2mV in 1us, and 1mV in 100us after a pulse of +/- 10V. (Per published spec)

And it does, too. Better, in fact, with the five I have around the lab, and have personally calibrated. It's part of the performance check procedure.

Effective offset range of 10,000 divisions.

I must try the same test with a 7A26. There's one that needs my attention for unequal trigger delay between channels. Probably another of those goddamn made-in-Tucson trimmer capacitors again. I'll try when I fix it.

"For a successful technology, reality must take precedence over public relations, for nature cannot be fooled." (Richard Feynman)

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