How to measure frequency by USB 6211?

[baicy]

Dear all,

I'm using USB 6211 to measure the input analog voltage signal, then use a counter to yield the frequency. Please find the VI figure in the attachement.

The problem is the insteal of measure the right frequency, which is from 300Hz to 1000Hz. There is a lot of high frequency noise from thousand to milliion Hz. I think it pick up the high frequency harmonics. How to avoid it or use a more effective way to achieve the measurements.

Regards,

Alan

[waldemar.hersacher]

You have to suppress high frequencies on the analog side of your signal. You must set it to a frequency which is not higher than half of your aquisition rate. If you sample with 10 kHz your filter must have a cutoff frequency of max. 5 kHz. Otherwise you will see the signal mirrored at the half of the sample rate. A 6kHz signal will lead to a 4 kHz signal after the sampling.

[baicy]

Based on my VI, it is determine the rising edge of the signal. How to limit the magnitude of the voltage. because the high frequency noise normally get low voltage magnitude.
Or there is better way to measure the frequency

[rainfalls]

Because there's no filter or level judgement circuit in Counter, you may use AI to acquire the signal, then use software filter and fft to measure the frequency of the input signal. Could you accept that?