"How to measure very low frequency?"

[virgo]

I'm using Labview 5.1 and a data acquisition card PC-LPM-16PNP. How can I measure a low frequency signal like the human pulse of 72 pulses/min or 1.2 Hz. Thanks.

[BruceAmmons]

Virgo,

When measuring very low frequencies, I recommend measuring the period between pulses and converting to frequency.

To measure the period, there are a few methods. For something like a heartbeat, I would probably use a peak detector to locate the peaks and measure the distance between the peaks. For other signals, it may be easier to locate zero crossings. Sometimes filtering or smoothing is required to remove noise and improve the results.

Bruce

[Faraclas]

You can also do this in the frequency domain. The only trick is to sample sufficiently low enough. The lower your sampling rate, the more resolution you will have for the low frequencies. Just make sure that you include an analog Low Pass Filter (ideally set to half the sampling rate) before the A/D. Once your signal is captured you can either take an FFT of the signal or use the "Auto Power Spectrum" .vi