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I'm experiencing an issue with my Rigol DS1202 oscilloscope. When using the math function to measure the RMS voltage of the difference between two channels (A - B), the RMS value is displayed in gigavolts (GV) instead of volts (V). This is puzzling because the RMS measurements for Channel 1 (CH1) and Channel 2 (CH2) are correct and displayed in volts. When there is no power across the resistor I am measuring, and there is just noise, the VRMS for the math mode jumps between 0 and some megavolt value...

Here's a summary of my setup and the steps I've taken to troubleshoot this issue:

Setup:

Oscilloscope: Rigol DS1202 Probes: 1x attenuation, correctly compensated. I am measuring the signal across a resistor (CH1 on one side, and CH2 on the other, to calculate the RMS voltage drop across it. I haven't connected the grounds to anything)

Signal: CH1 measures a signal with approximately 1V RMS, and CH2 measures a signal with approximately 6V RMS. The signal is a burst modulated waveform at 25.5kHz carrier, and 200Hz modulation.

Issue: The RMS measurement for CH1 is around 1V, which sounds right. The RMS measurement for CH2 is around 6V, which sounds right. The RMS measurement for the math function A - B (CH2-CH1) is incorrectly displayed in gigavolts (e.g., 2.52GV). I have previously made correct measurements in volts, and now the numbers seem correct, but the units in GV so it seems to be scaled up for some reason.

Troubleshooting Steps:

  • The waveform is stable and properly triggered on CH2, with a holdoff of 6ms so that it waits till the next burst to trigger
  • Verified that both probes are set to 1x attenuation.
  • Ensured the oscilloscope settings match the 1x probe configuration.
  • Used cursors to isolate one complete cycle of the waveform for accurate RMS calculation. - Ensured the cursors were correctly positioned around the signal period.
  • Performed a factory reset on the oscilloscope.

Screenshot: I've attached a picture showing the VRMS measurements at the bottom of the screen. As you can see, CH1 and CH2 are in volts, but the math A - B measurement is incorrectly in gigavolts.

screenshot of the settings for CH1 CH2 and Math mode enter image description here

I would appreciate any insights or suggestions on what might be causing this issue and how to resolve it. Has anyone else encountered a similar problem with the Rigol DS1202 or other oscilloscopes?

Thank you for your help!

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    \$\begingroup\$ What might Over=***** mean? \$\endgroup\$
    – Andy aka
    Commented Jun 19 at 19:36
  • \$\begingroup\$ Your setup sounds reasonable to me. Have you checked if the vertical resolution of the math channel is also displayed in GV/div? In this case, you might have overlooked a "hidden" multiplicator in the math function. Otherwise, it looks to me like a software bug. You could then check if there's a firmware upgrade for your scope or contact rigol directly. \$\endgroup\$
    – Mau5
    Commented Jun 19 at 19:38
  • \$\begingroup\$ that is just a deleted measurement, can be ignored \$\endgroup\$
    – raymysterioso
    Commented Jun 19 at 19:38
  • \$\begingroup\$ thanks Mau, I have contacted Rigol with my same question. Will update here if I get a response. I can play with the vertical scale which is showing 5V and 10V, so I don't think there is a multiplier there, but good point. \$\endgroup\$
    – raymysterioso
    Commented Jun 19 at 19:45

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