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Inconstant Frequency readings

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Inconstant Frequency readings

Postby GusIA_USA » Tue Apr 12, 2016 11:54 pm

Hi
I happen to be checking the clock output of a Atmel ATiny 9.
I seem to get a different frequency reading depending on the collection time. If I collect for 500ms the frequency reading is about what I would expect 8Mhz.
If I collect for 1ms the frequency for the whole trace is 1.4Mhz. But if I do a ruler measurement over about 90% of the trace I get 8Mhz again.
Why the difference and what am I over looking? I have uploaded images of the scope.
Thanks
GusUS_IA
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Re: Inconstant Frequency readings

Postby AndrewA » Thu Apr 14, 2016 8:57 am

Hi GusUS_IA,

It maybe that Picoscope sees to many samples when trying to calculate the frequency. Also the sample rate is lower which is most likely the cause. You would need to zoom in to the waveform to see what the scope is actual seeing per cycle.

In any case I would increase the timebase so its between 1us/div to 100ns/div. So you see the 8MHz (125ns) cycles on the screen. (The number of samples need to be set to +1kS) You can also use the zoom tool.
Regards Andrew
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