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Rigol DS1000 weird problem

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c4757p:
(Oh no, not another fix-my-test-equipment from him!)

Actually, I have a pretty good hunch where the problem is. Just wondering if anyone has seen this. Suddenly, I am getting very strong noise on Ch 1 on my DS1052D (converted to 1102D), at almost precisely 100 MHz plus a wide scattering of other frequencies. The exact waveform changes with scope function, so I'm sure it's internal interference from one of the digital circuits. The peak-to-peak amplitude is approx. 0.3 divisions (regardless of vertical scale) when the vertical position is at center, increasing as the position moves upwards, so whatever is causing it is after the attenuator and probably is, or is related to, the DC offset circuit. The less the scope is doing, the rougher it gets. (When I turned on FFT to look at the frequency components, it became almost a perfect sinusoid with just a bit of a second harmonic many dB down.)

Before anybody asks, no, warranty repair is out of the question.

Just wondered if anybody else has seen this same issue and can point me in the right direction. Also - I remember seeing that somebody on here had reverse-engineered the analog frontend and had a schematic, but I'm having a hell of a time finding it. Anybody better with the search tool than I am?

Thankfully, I've still got my trusty Tektronix to help me track this down  :-+

ivan747:
Well, just a checklist:
*Ground coupling: is the effect the same?
*Physical inspection, inside and out.
*Bring it back to 1052E see if the problem persists. If it persists, the problem is after the input section.
*Turn on bandwidth limiting, see if the problem persists. If it does, and the BW limit is in hardware, you know where the problem is physically located.
*Self-calibration, after 30 minutes of warm-up (it really needs the warm-up)
*Broken shielding causing it to capture FM radio broadcasts internally? (almost out of question)

That's all what comes into my mind.

c4757p:

--- Quote from: ivan747 on May 09, 2013, 12:57:26 am ---Well, just a checklist:
*Ground coupling: is the effect the same?

--- End quote ---

I am almost certain that ground "coupling" in this scope is done in software, or at least it disconnects the input and then additionally zeroes out the trace. I have never in my life seen such a clean 0V trace - I wouldn't even expect that from a properly functioning scope. It is literally perfectly blanked. (Out of curiosity, to find out whether there is a proper hardware disconnect, sometime I will get around to pumping RF into one channel, setting it 'ground', and seeing if the cross-coupling changes. That's not going to help this, though.)


--- Quote ---*Physical inspection, inside and out.

--- End quote ---

Done, except under the shielding can. (I'll dig into that next week. I have exams to study for now.)


--- Quote ---*Bring it back to 1052E see if the problem persists. If it persists, the problem is after the input section.

--- End quote ---

Good idea. Pain in the ass, but I'm sure this will be, whatever the problem is...


--- Quote ---*Turn on bandwidth limiting, see if the problem persists. If it does, and the BW limit is in hardware, you know where the problem is physically located.

--- End quote ---

Damn. That didn't even occur to me.  |O Thanks! That narrows it down significantly. The BW limit has absolutely no effect on the noise, so it must be a hardware limit and the noise must be coming in after the limiter. The separate software filter does reduce the noise, as expected, so I haven't hosed the digital section or anything.


--- Quote ---*Self-calibration, after 30 minutes of warm-up (it really needs the warm-up)

--- End quote ---

Done multiple times.


--- Quote ---*Broken shielding causing it to capture FM radio broadcasts internally? (almost out of question)

--- End quote ---

I can't get this much radio noise on the scope with a foot-long antenna sticking out of the BNC! Also, there's no effect at all when I take off the large, main shielding can around the whole scope, so I doubt it.


--- Quote ---That's all what comes into my mind.

--- End quote ---

Thank you very much!

c4757p:
Awesome! Here is that analog frontend schematic I mentioned. God, I love this forum...

c4757p:
One more hint - I'm 99.999% sure that it's an internal clock, because it appears to be in phase with the acquisition clock. If I zoom out beyond 200ns/div, it seems to disappear, but switching to peak detect mode reveals that the disappearance is just an aliasing effect that happens to be perfectly synchronized with the acquisition.

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