> I have two counters on my 6025E DAQ card.
Counters can only detect specific trigger levels. If you have a complex waveform you may have problems. The Analog inputs (12-bit A/D) can help you with more complex waves.
> Can I use one counter method in the power spectrum.vi.
No, the power spectrum requires an analog input.
> I am actually measuring only one low fequency between 0 to 100 Hz.
> Can I reject all other frequency outside this range?
With analog inputs and the power spectrum, yes. (You may get readings above 100 Hz, just ignore them.)
With the Counter input, you just get the frequency or period result. Proper set-up of signal conditioning is up to you. You can use hysteresis to eliminate higher frequency components - as long as the higher frequencies have lower amplitude than the low frequency amplitude.
> Where can I find the power spectrum.vi?
It's in the professional version of LabVIEW. (A lot of things are - they make life easier and are worth the extra few bucks.)
