Frequency Measurement

[mama007]

Hello,
How Do we calculate frequency with discrete samples measured by NI-4060 DMM.
Can I get any sample code,using FFT
Thanks

[ryank]

The 4060 is not a good instrument for measuring frequency. To accurately measure frequency you want a digitizer, or something with behavior similar to a digitizer, like a Multifunction DAQ board, a FlexDMM (407x), or a DSA board. For one thing, you generally use software times sampling with a DMM. For another, the maximum reading rate of a 4060 is 60 samples/sec. Therefore, they nyquist theorum states that you're going to alias any signal above 30 hz. This means that unless your signal is under 30 hz, you're pretty much out of luck luck (technically, you could calculate the actual frequency by using the apparent frequency, but that's a pretty crude solution, and if I remember the formula right, you'd already have to know the frequency of the signal +-15 hz or so).

Regards,
Ryan K.

[Tristan_J]

Hi,

Ryan is correct in his post, performing accurate frequency analysis with the 4060 is not really possible. You will need to look to perhaps a multifunction card (eg M-Series) to achieve what you are after.

Kind Regards

Tristan
Applications Engineer
National Instruments UK & Ireland

[mama007]

Hai,
Thanks for your replies.I actually don't need to calculate the frequency of a unknown signal. All I need is to check is weather there is signal or not? So far I do take a a set of measurements with DMM and compute standard deviation. And I think frequency calculation with FFT may be a better idea. That's why I requested for some sample code

[ryank]

Hrm, this reply doesn't really make sense, but you can't calculate the frequency of the signal based upon your measurements from the DMM. First of all, the measurements are probably not taken with a high enough reading rate, and second, it's going to be difficult to accurately determine the change in time between your measurements as they are most likely software timed. An FFT will not help you.

Regards,
Ryan K.