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Measuring sub 2Hz frequency with high accuracy
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ricko_uk:
Thank you all for the suggestions, all very interesting and useful!! :)
bdunham7:
To use your scope, slow it down so that there is at least one period on the screen and use cursors to measure the time from one edge to the next.  If your Rigol is like my old one, there are two different frequency counters--have you found and tried them both?

I'm not familiar with your frequency counter, but even my ancient HP 5316B correctly reads a 2 Hz square wave (I just tried it) to 7 or 8 decimal places with no special settings. 

Just for fun I plugged in my joke-quality FeelTech 6900 and it promptly read the frequency correctly, as shown.  You have to increase the gate time to 100s to see two decimal places, but if you just look at the cycle time of 4.99994 million nS (FeelTech wants you to be able to move decimal points in your head...) you can infer the Hz quite accurately and quickly.

I've no idea why, but Rigol probably owes you an apology.  Any reasonable device should handle this without any problem.
EDIT:  Check and see if the Rigol counter has a HF Reject option and try it if it does. 
ricko_uk:
Thank you bdunham7,
I just checked me Rigol MSO1104z and if the frequency is lower than 15Hz it just shows "< 15Hz". I assume because the internal counter overruns/rolls-over...
bdunham7:

--- Quote from: ricko_uk on June 07, 2020, 03:50:17 pm ---Thank you bdunham7,
I just checked me Rigol MSO1104z and if the frequency is lower than 15Hz it just shows "< 15Hz". I assume because the internal counter overruns/rolls-over...

--- End quote ---

The hardware counter on scopes is typically a simple gated counter, no frills.  There should also be a frequency entry under 'measurements'.  Set your trigger to DC coupled, HF reject (or noise reject) ON and timescale to 100ms (for 2 Hz) or whatever gives you 2 to 4 complete cycles on the screen.  Make sure it triggers reliably on the rising edge of the square and looks stable on the screen.  Then find Frequency and Period in your measurements menu and turn them on--you should get a two-decimal readout on each.  Not super accurate or precise, but good enough for most things.
ricko_uk:
Thank you bdunham7 :)

I just checked the manual and I think that's by design. Have a look at page 145 of the manual: https://www.batronix.com/pdf/Rigol/UserGuide/MSO1000Z_DS1000Z_UserGuide_EN.pdf
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