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| Measuring sub 2Hz frequency with high accuracy |
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| ricko_uk:
Hi, I need to measure a steady 5V square wave frequency between 2 and 5 Hz with the precision of at least 2 decimal points, better if 3. 1) I tried using the digital oscilloscope Rigol MSO1104 but unless the whole signal fits in the display it does not measure it and if I set the scope's timescale to fit the whole period in the display then the accuracy is not there. Any suggestion to doing such measurement with a scope? 2) I tried using a frequency counter part of a Rigol DG1032Z but despite the waveform being very clean for some reason the reading keeps moving around by a lot (sometimes even several Hz) despite having tried various settings. Any suggestion why that is and what settings to do such measurement? 3) any other suggestions? Many thanks |
| ejeffrey:
What settings have you tried? I haven't used the frequency counter on the Rigol generator but according to the manual it can be DC or AC coupled and AC is default. Make sure you are DC coupled. |
| CaptDon:
One way slow signals were counted was measuring a PLL that was running 10 or 100 times the base frequency and measuring PLL frequency. Not very stable unless the input is stable. Another way was gating the counter for 10 seconds or more also not good as it averages the signal over time. The best way I found was to set the counter up in more of a 'timer' fashion and gate the counter from the signal you want to measure and have the counter counting a fast signal like 10khz or 100khz that way you measure your frequency as a time interval of 100us or 10us or even 1us steps and then do the reciprocal calculation to get frequency with resolution two or three digits to the right of the decimal. The square wave is in your favor because the gate circuit will be less effected by noise and decision point on the slope. |
| TimFox:
Most “universal” digital counters have a “period averaging” mode, where it times 10 or 100 cycles. Some will compute the frequency from that period. |
| StillTrying:
--- Quote from: ricko_uk on June 05, 2020, 07:40:48 pm ---I need to measure a steady 5V square wave frequency between 2 and 5 Hz with the precision of at least 2 decimal points, better if 3. 1) I tried using the digital oscilloscope Rigol MSO1104 --- End quote --- I'd say trigger off one edge of the square wave with the trigger X position in the center of the screen, then keep moving the trigger position until the next edge is in the screen center, the period is then the X position off-set, ..or something. |
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