Unable to Measure 80MHz wave in my ScopeFun Hardware

Hi,

I am trying to measure a signal (83 MHz approx.) in my camera board and get 1Hz in the measure window of ScopeFun where as probing the same signal using Instek Oscilloscope I get 83MHz (expected). Attaching the Camera ON images for your reference. Can anybody let me know if I need to adjust any settings in ScopeFun to get the measurement? I am using a 100MHz oscilloscope from Pomona (https://www.digikey.com/en/products/detail/pomona-electronics/6500/737763).

When I probe same signal at lower frequency (@2.9MHz approx.), I get same reading in both ScopeFun and Instek. Refer Camera OFF images attached.

Thanks,
Sanat

Hi, could you try to measure with AC input coupling and observe the result? I think there could be a low frequency signal present that overrides the measurements.

Hi DEJAN,

I have tried in AC mode as well. What I see is the DC shift goes away. The waveform stays same.

Best Regards
Sanat

That is correct, but what about the measurement, does it still read 1 Hz? If you have it in AC mode you can also change scale, to "zoom in" vertically.

Hi DEJAN,

In AC, When I zoom in the frequency increases but still I don't get a proper reading or measurement. Even, the waveforms look different. Please have a look at the images of ScopeFun and Instek in AC mode.

Probing is done at same signal but different waveforms and measurement are recorded. Instek is showing sine wave with frequency is 84MHz (expected). But ScopeFun is showing a very low voltage square wave with 0.425MHz frequency which is not correct.

Best Regards
Sanat

Hi DEJAN and others,

Do you have any further suggestions as to why the ScopeFun is not measuring the 84MHz wave?

Best Regards
Sanat

Hi, this looks like a bug in the way that frequency is calculated in measurements panel.  By looking at the shape of the signal, I see that the signal period is about 3 samples, which is 12 ns (which gives the frequency 83,33 Mhz for this signal). So clearly the frequency value on the measurements panel is wrong for this specific signal. We will need to check why the frequency is not calculated correctly.

Regarding signal shape. Your Instek oscilloscope is probably using sin(x)/x interpolation. This is used to help approximate the signal shape between samples. We don't know for sure if the measured signal is in fact sine wave, but this is an approximation that can be done based on the selected sampling speed and oscilloscope bandwidth. We are not doing any king of interpolation between samples on our oscilloscope, only raw sample values are displayed. This is why you see a sinus shape on Instek but not on our oscilloscope.

Hi Dejan,

You are correct, when I measure manually with cursors, I get frequency as 83.33MHz that is acceptable. So, as you said, there must be some bug in displaying the frequency.

However, I also see the voltage levels reduced to just 0.5V which is strange. I am not sure why this is getting caused. It is not the case in Instek oscilloscope. Do you see any reasons for this behaviour? I am attaching a picture of the measurement from ScopeFun for your reference.

Best Regards
Sanat

If the probe is compensated, then the difference again is most likely due to interpolation.