I just bought a new MSO2000A series Rigol scope, 300MHz bandwidth version. When you set the timebase to 20ns or below, there is an oscillation present.

Can anyone else with this series of Rigol scopes observe this issue?

More info below

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You can see it at all amplitude ranges, the oscillation always appears approximately the same size on-screen regardless of vertical scale.

This oscillation is in-phase even though it's not triggered on it, and it looks to be 500 MHz on the nose, so I strongly feel it's intrinsic to the scope, and either an artifact of the sampling itself or crosstalk from something in the clock domain of the digitizer.

Restarting the scope causes the amplitude to change randomly. Sometimes it is very obvious, sometimes not there at all. It is less obvious on Ch 2, which makes me further think some crosstalk/EMI problem as opposed to just a display artifact.

It is not affected by 50-ohm vs 1M input. BW limit has literally no effect (even on 20 MHz limit you can still see a 500 MHz oscillation!). Probe ratio selection makes no difference. "Invert" does invert the polarity of the oscillation. Setting coupling to AC has no effect, but coupling to GND causes the oscillation to disappear. Connecting a BNC short looks identical to a floating probe or no connection (as shown).

Turning on channel 2 (which decreases the sample rate from 2GS/s to 1GS/s) has a noticeable effect, slightly attenuating the oscillation. But still present.

I applied an external signal, and the oscillation was less obvious due to external noise, but still apparent, clearly a 500MHz periodic component on top of the signal.

Here it is again after a restart, looking like a good clean line. Nothing was changed other than turning it off and on.

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Oscillation is still visible in dots mode. Here is the FFT showing a clear peak at 500 MHz.

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Here's another observation, which I think might be caused by the same thing. When you zoom out the horizontal the baseline signal actually shows two strong bands, instead of one band strong in the center as you would see from a regular noise spectrum. This looks more like a sinusoid or bistable signal + noise. Shown below in both vector and dots mode.

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The banding effect is also reduced when turning on Ch 2 (2GS/s down to 1GS/s shared sampling), similar to the oscillation amplitude being reduced.

  • \$\begingroup\$ youtube.com/watch?v=Znwp0pK8Tzk \$\endgroup\$ – G36 Jul 5 '20 at 18:39
  • \$\begingroup\$ i am no expert in this, but i wonder if it is intentional ... it may have something to do with the least significant bit in the ADC ... maybe it is a 1/2 LSB ripple ... I'm not sure where I'm going with this, but something nebulous is in the back of my mind \$\endgroup\$ – jsotola Jul 5 '20 at 20:54
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    \$\begingroup\$ You're probably thinking of ADC dithering, but that needs to be white noise \$\endgroup\$ – Keegan Jay Jul 5 '20 at 21:02
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    \$\begingroup\$ A guess, and a suggestion...many digital 'scopes curve-fit from sample-to-sample. Could it be a failure of Rigol's curve-fitting algorithm? Does this still show up in straight vector or dot-per-sample display modes? Does it show up in FFT display? \$\endgroup\$ – glen_geek Jul 5 '20 at 22:09
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    \$\begingroup\$ I wonder if this is due to imperfect interleaving of multiple ADCs. Maybe there is a calibration routine that helps reduce it. \$\endgroup\$ – Spehro Pefhany Jul 6 '20 at 4:48

My Rigol ds1054z also shows this "oscillations"

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But I think that this is normal and we don't have to worry about it. But I'm not an EMI expert.

  • \$\begingroup\$ Thank you! So I don't need to return it then. Going to leave question open for a little while to see if anyone has comments on why Rigol scopes show this. Seems like it must be related to the ADC \$\endgroup\$ – Keegan Jay Jul 5 '20 at 19:47
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    \$\begingroup\$ But what if our scopes are buggy? \$\endgroup\$ – G36 Jul 5 '20 at 19:57
  • \$\begingroup\$ Good point. Hopefully more people can confirm or else say they do not see it. \$\endgroup\$ – Keegan Jay Jul 5 '20 at 21:53
  • \$\begingroup\$ Even some analog Tektronix scopes (2240 series) used to show small amplitude noise (spikes from their own SMPS) if you turned the gain up to max. (That's why I bought a Hameg 605 instead). If it's a 1 LSB ADC noise it may well appear/disappear as the scope warms up due to DC drift. \$\endgroup\$ – Brian Drummond Jul 5 '20 at 23:56
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    \$\begingroup\$ @KeeganJay here's a set of screenshots for my MSO1104: imgur.com/a/nGm0zEt . As you can see, I also have some oscillations, but their amplitude does appear to depend on vertical scale. This scope being 100MHz, the FFT peak (not shown in screenshots) appears at 125MHz. \$\endgroup\$ – Ruslan Jul 6 '20 at 7:46

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