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Oscilloscope screen

I'm measuring pulses from a device with a constant frequency. The oscilloscope's usual frequency indicator shows 600 Hz. But when I use "Measure" button and then Type is set to Frequency, then that indicator shows around 300 Hz.

Then I measure it with a DAQ board and analyze it with MATLAB and I find again 300 Hz.

Whats going on here? Does oscilloscope's generic frequency indicator (at the right bottom corner in mine) interpret edge rise and fall as one signal? Why would it read double?

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  • \$\begingroup\$ A picture would help - possibly the trigger threshold point you use is close to either the top or bottom of the waveform as displayed. \$\endgroup\$
    – Andy aka
    Commented Apr 3, 2014 at 9:08
  • \$\begingroup\$ @Andyaka I uploaded a picture. As you can see the frequencies measured by the same oscilloscope is different. One is 860 and the other indicator says around 430. And my DAQ says also 430. Can it be the bottom indicator reads each ups and downs as 1 signal instead of rising edge time differences? \$\endgroup\$
    – user16307
    Commented Apr 3, 2014 at 9:25
  • \$\begingroup\$ Btw I just checked with a signal generator bottom indicator showed right. Then I autoset it and tried again this time it shows correct frequency. \$\endgroup\$
    – user16307
    Commented Apr 3, 2014 at 10:03
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    \$\begingroup\$ You see the horizontal arrow on the right vertical edge of the scope picture - it is quite close to the bottom edge of the waveform - it's called the trigger position and you can move it up and down from the trigger menu - try repositioning it and see what happens. \$\endgroup\$
    – Andy aka
    Commented Apr 3, 2014 at 11:08
  • \$\begingroup\$ thanks! exactly what you said was the problem. when I adjusted the trigger position above enough it shows the correct frequency. is that because when we lower it too much it calculates from the noise. why does it happen? btw I would choose as an answer but it is a comment. if u copy paste it as an answer i will do that. \$\endgroup\$
    – user16307
    Commented Apr 3, 2014 at 12:13

1 Answer 1

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You see the horizontal arrow on the right vertical edge of the scope picture - it is quite close to the bottom edge of the waveform - it's called the trigger position and you can move it up and down from the trigger menu - try repositioning it and see what happens: -

enter image description here

I suspect on your scope there are two methods of calculating the frequency - one is based on the trigger threshold and the other is based on some other mechanism.

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  • \$\begingroup\$ hej Andy. sorry one more thing maybe you have an idea. i did the same test with a calibrated 80Hz signal generator. Changing the trigger level didn't effect the frequency read by the scope. My guess is that for this situation to happen the noise should be periodic as well. Would you have a comment? Thanks \$\endgroup\$
    – user16307
    Commented Apr 3, 2014 at 14:19
  • \$\begingroup\$ The only thing I noticed was that there are faint shadow images in the picture above that may imply half the cycles were not triggering properly but other than that it's difficult to say. It is rather odd though. \$\endgroup\$
    – Andy aka
    Commented Apr 3, 2014 at 14:23

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