How to measure a clock signal frequency using picoscope 6404c

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tnvmurali
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Joined: Mon Apr 29, 2019 6:43 pm

How to measure a clock signal frequency using picoscope 6404c

Post by tnvmurali » Mon Apr 29, 2019 6:56 pm

Hi,

I am using Picoscope 6404c to measure a clock signal frequency. Every time I measure with a different Times per Division scale, I am reading different frequencies.

Picoscope settings:
Times per division(collection time) -> Varying
No. of Samples -> 1MS (fixed)
Resolution -> 8bits
No filtering has been applied on the channel

I am trying to measure a signal in the frequency range of 5 to 40Mhz

Expt-1:
Times per division: 200ms/div
Clock frequency read = 5Khz

Expt-2:
TImes per division: 2ms/div
Clock frequency read = 5Mhz

Please help with the settings that I should be using while measuring a clock signal frequency and the necessary calculations.

Thanks,
Murali
Attachments
5Mhz.jpg
Yellow colored channel, 2ms/div
5Khz.jpg
Yellow colored channel, 2ms/div

Gerry
PICO STAFF
PICO STAFF
Posts: 443
Joined: Mon Aug 11, 2014 11:14 am

Re: How to measure a clock signal frequency using picoscope 6404c

Post by Gerry » Wed May 01, 2019 1:23 pm

Hi Murali,

You ideally need to using a sample rate that is at least 5 times the highest frequency that you need to measure. If you are trying to measure a clock of 5MHz then a Timebase of 200ms will only give you a sample rate of 4.883MHz (which is actually less than the clock frequency). What you need to do is go to the 'View' menu, and click on 'View Properties', which will open a properties pane on the right of your display showing you values for all of the important parameters that you need to keep an eye on.

So as you are capturing a square wave which has harmonics, you need to be sampling at a bare minimum of 10 x 5MHz = 50MHz, or better (depending upon the reason for your measurement, i.e. if you just want to measure its frequency or if you want to check exactly how good a square wave it is, in which case from somewhere like the 6th harmonic, at 11 times 5MHz or up to the last harmonic will give you increasing accuracy, so you would then need 55 times the sample rate or more).

So, the answer really depends upon what your measurement goal is (i.e. why are you trying to measure a clock frequency)

Regards,

Gerry
Gerry
Technical Specialist

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