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I have Micsig STO1104C oscilloscope. I was measuring Vpp and the generator was set at 1 V, but my Micsig said it was 1.3xx Vpp. So I measured it with my KEITHLEY DMM6500 and it was measuring 1 Vpp. So I made sure I had everything set right, tried all 4 channels (one at a time) and I was getting about 1/3 more volt an all of them.

So I took a probe from my other 100 Mhz scope (Aktakom ADS-2111MV) and it was measuring 1 Vpp. Are the included probes not accurate? Micsig is usually good quality, I think, so I don't understand what the heck is going on. I don't like that the original probes are fixed at 10X and I can't switch to 1X at all.

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    \$\begingroup\$ Based solely on the information you have provided, it sounds like your micsig probes are inaccurate. But there might be something going on that you didn't think to mention that could help explain this. I guess one thing worth checking is, do the probes have a little screw so you can adjust them to eliminate overshoot and undershoot? Most scopes have a built-in square wave function generator. You can connect the probe to the built in function generator and adjust the screw on the probe until the square wave has a nice flat top. Then measure Vpp again. \$\endgroup\$
    – user57037
    Commented Sep 4, 2022 at 0:22
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    \$\begingroup\$ Have you calibrated the probe compensation? And since you mention you don't like the probes because they are 10x only and can't be set to 1x, what is it that is so special you really need a 1x probe? \$\endgroup\$
    – Justme
    Commented Sep 4, 2022 at 0:47
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    \$\begingroup\$ Is your generator outputting 1 Vpp or 1 Vrms? Your Keithley DMM is measuring 1 Vpp? DMMs usually measure RMS, or equivalent RMS assuming a sine wave. \$\endgroup\$
    – Mattman944
    Commented Sep 4, 2022 at 1:09
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    \$\begingroup\$ Measure the probe resistance from input to output and see if it 9 M or something more. \$\endgroup\$
    – D.A.S.
    Commented Sep 4, 2022 at 1:22
  • \$\begingroup\$ I made sure I had everything set right, tried all 4 channels (one at a time) and I was getting about 1/3 more volts an all of them! I also rerun the internal calibration without the probes attached (if you do calibrate even with one probe connected, you'll get an error) and attached probe 1,2.... and squared the probes.... Nothing is helping at this point to improve the accuracy in Vpp. Do you, or anyone else reading this, knows why is this happening? (I'm using the original probes that came with my scope). I also updated the firmware/software and recalibrated again, but to no avail. \$\endgroup\$
    – Tony
    Commented Sep 5, 2022 at 19:36

1 Answer 1

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There are two possibilities.

  1. The probes are broken. It is not likely both have failed but I guess it is possible.
  2. The probes have not been properly compensated (applies only to 10X or 100X probes).

Read the manual to find how to perform the compensation. There is a screwdriver (non-metallic) adjusting capacitor on the probe that you can adjust based on a special square wave available on the front panel of the scope.

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  • \$\begingroup\$ The probes are in 100% new like condition and are not broken. They measure 9M when tested with my DMM6500 (see the accuracy information on the web) that has 6 digits after the ".", so I'm sure the probes are fine. Also, I did internal calibration and updated the firmware + all the probes are adjusted to perform correctly. As per the manual - I red the manual several times and researched the web, but to no avail. Mabie not that many people know about the Vpp inaccuracy, I just tested it and that is how I found this problem. Thank you. \$\endgroup\$
    – Tony
    Commented Sep 5, 2022 at 19:54
  • \$\begingroup\$ @Tony This is a cliffhanger. Could you post your final findings as an answer? \$\endgroup\$
    – Kuba hasn't forgotten Monica
    Commented Aug 23 at 17:48

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