Hi butterwaffle,
If your loop time is set to 100 mS, you won't be able to measure any frequencies that are faster than 5 Hz. 1000 ms / 100 ms = 10 Hz = sampling frequency. Sampling frequency divided by two is the maximum measured frequency you will be able to capture, according to the Nyquist Criterion. Your loop time will inversely correspond to your sample rate, so decreasing the loop time will increase your sampling rate. If you are sampling in the 0-1 kHz range, your loop time needs to be 0.5 ms or faster. This means using a microsecond timer instead of milliseconds. Any signal that is sampled slower than 2X its frequency will show up as zero from this analog period measurement VI.
