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

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 you haven't changed the offset (or the scope didn't do that because it thought 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'd send this info to Tektronix support. If they can't answer I'd ask the sales 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 a real scope?" and out came a big old Tektronix boat anchor where people took a step back when I pulled the power switch handle ... bzzzt ... TUNGGGG ... blueish green traces showed up and ... tada ... fuzz could be seen that the DSO simply was unable to show.

Here I have a Tektronix 7704, a Telequipment D66 and I also still have my old Hameg that I had as a highschool student (but that doesn't get used much anymore). I would not want to be without a real scope. They don't make them like that anymore. So if your workplace does not have something like a Tek 7000 series or a Tek 2465 I'd ask them to go on Ebay and get one :-)

Somehow this reminds me of the old Sorcerer in Aladdin crying out, "New lamps for old. New lamps for old," trying to recover that special, magic lamp.

(Replace lamp with oscilloscope....)

:)

Jon

Hmm, just found an app note from Tektronix on this:

Overdrive seems to be the problem

However it is typically specified as less than 20ns, i am seeing a lot more. But perhaps thats due to excessive overdrive. Funny enough the DPO5034 does not mention anything about that in the datasheet

I have been in contact with Tek, and they keep telling me to update the SW of the scope :-)

Regards

Klaus

Interesting - it sounds like the overdrive issue Jim Williams has addressed several times:

But I would not have expected it to show up so badly for such a modest overdrive.

Did you run a virus scanner? :)

I believe I've seen similar things with DSO's. Sometimes negative 'overdrive' is different than positive 'overdirve'. Which can also lead to you seeing things that aren't really there. The older DSO's in general seem worse... An old HP 54600B is note worthy.

George H.

Interesting. Just a question of understanding (I'm not a user of Tek scopes, hence apologies if I miss the obvious information): in the screen shots there are different numbers of acquisitions displayed (86 vs. 226). Is the scope set into averaging mode? And what is the coupling you selected, AC or DC?

Besides set I only can tell you that I never ran into this problem with my LeCroy scope and I did that kind of overdrive measurements on an almost daily basis. So, if this is problem is for real, then I dare conclude that not all scopes are affected by it :-)

Klaus

Quote "There is a misconception that some oscilloscopes can tolerate overdrive conditions, whereas others cannot."

Tell Tektronix that this is not a misconception but it is true. Or better yet, tell them to visit some nursing homes around Beaverton during lunch time and ask anyone who was an engineer at Tektronix to raise their hand. Then they should ask them how it's done right :-)

I just tried it out. I have to send my Instek GDS-2204 (digital scope) into 10x overdrive until it distorts. It shouldn't even distort there but, oh well, this is a

It shouldn't be. There are risetime limits, of course, but it should not suddenly and drasticaly alter the waveform or lose the offset. If I had designed ultrasound frontends that way they'd have fired me. Those are expected to remain true even in a +40dB overdrive situation.

That's the standard answer I guess. Like when you call your telco because the broadband went out. "Sir, please re-install the software". Yeah, right :-)

Overdrive it past 10x and see. If it can do 100x without flinching then they did the amp design about right.

Klaus Bahner wrote in news:4e81ca00$0$56771$ snipped-for-privacy@dtext02.news.tele.dk:

there's no substitute for the old TEK 7000 series and the 7A13 or 7A22 differential plug-ins!

Well, I don't have access to this scope any longer (sigh), so I can't check it again - would love to, because before this thread I wasn't aware of this overdrive problem at all. And of course I wonder now, whether this is because the scope was free of those problem or whether I did a bad job on analyzing my results :-(

However, a case I clearly remember was a 2kV pulser design, where the major design goal was fast and clean recovery to 0V after the pulse. An "offset" voltage in the 1V range was significant, hence I made tons of measurements with lots of overdrive (>100x). Didn't observe any strange things during these measurement with that scope (Waverunner 6050).

Please, don't tell me now that this design was just a perceived success because overdrive issues acted in the right direction :-(

Well, the pulser worked fine in its intended application, so I take that as an argument for that the scope didn't lie. :-)

Cheers, Klaus

That would be nice if it had the performance of the good old analog scopes. What I personally do not like about many LeCroy scopes is the sluggish Windows interface.

Who knows, but the main thing is that your circuit works and that the firefighters did not have to come out there :-)

Sometimes such situations can be remedied by using a separate amplifier in front of the scopes. An amp from the good old days when they still knew how to design them well. This is one of the reasons why I have an HP dual amp with 20dB each here. Small, so I can carry that to clients where it is likely that I have to do pulse measurements and they only have "modern" analyzers. Fits easily into my carry-on bag but the security folks sometimes want to know what it is.

Well my cheap TEK 1001B has a recovery time of 2-3 us. With anything more than 2 volts on the 200mV/div scale. It craps up the top of the pulse too! My pulser won't do much more than 10V peak so I can't check the 500mV/div scale beyond that.

My Rigol DS1052 is even worse... again it craps out at 200mV/div... but it takes like 10 ms to recover!

George H.

What on earth are they doing wrong? 10msec sounds like they screwed up majorly, with some supply rail going out of whack. 2-3usec might be the use of wrong clamping diodes. Someone should teach those engineers, and soon. Anyhow, I'd send those two scopes back for a full refund unless you are past the time limit for that. Then buy something better.

My Instek scope does distort a bit if I go past 10x. It is fairly minor saturation which isn't nice but at least I cannot see any recovery time. It's instant. If I go beyond 100x the trigger "loses it".

Of course, the trusty old Tex 7704 and its 7A26 insert sail through all that without flinching. I don't know whats so difficult about that. In ultrasound we build amplifiers that must listen to microvolts and they get hit with pulses of up to 200V. Takes some time but I didn't find such designs particularly difficult to do. It's not rocket science.

The number is number of aquisitions made, in essense the number of triggers (screen plots) since you last pressed "Run". I did not use averaging, but if I had, then you can see by that number how many data sets the current plot is made from.

It could be interesting to try that with a Lecroy. I will try to see if we have one in the lab somewhere :-)

Regards

Klaus

That why I didn't believe my measurements was correct when I first saw it, seems logical to me that I can do whatever with the gain setting without the scope representing the data incorrectly. At least such a scope with a lot of SW whould have waved a flag in front of my nose that I cannot trust current measurements due to saturation effects.

Today I asked them to find a analog guy from Tektronix, I won't spend any more time from guesswork by the first line of defence at Tek :-)

Regards

Klaus

r.

..

I saw the same thing with the 12bit USB Picoscope, took forever to return to the correct level. Really crappy design. It has 12bit resolution however, so I could zoom in on the vertical axis and get resonable resolution for low signals

Like Joerg says, this is not rocket science, but I guess it maybe a test the Scope engineers may omit intentioally to save a buck or perhaps they are not aware that some engineers/users would like good performance at such an stimuli.

Regards

Klaus

Hey, its even cheap, goes for 200 USD at ebay, has a geek factor too

If it breaks, you can just buy a new one :-)

Regards

Klaus

Keep in mind that any 7000 series scopes are back-breakingly heavy so the shipping charges will be high. The fact that it has a handle does not mean it is portable :-)

The flat R-versions are probably less heavy but also much more expensive.