How to measure the phase of a sinusoidal signal?

Question

I need to measure the frequency and the phase of a sinusoidal signal. I tried to use the Advanced Single Tone Measurements.vi but the phase measured in each iteration (each second) keeps changing (the measured frequency is not in integral Hz, so the first point of the next iteration is at a location different from the location of the first point of the current iteration...I guess this is the reason). How can really measure the phase of a signal (not always changing with time)? When I change the phase of the input sinusoidal signal, the measured phase should be changed though.
I'm using LabVIEW 7.1 and PCI-6110.
Thank you very much!
Marlon

Answer 1

Marlon,
Without DAQ hardware I cannot run your VI. DAQmx is not supported on my platform (Mac OS X), so I cannot examine your VI in detail.
1. The AI VI will wait until it has the amount of data specified. So if you are collecting one second's worth of data at a time, it will wait one second before completing. The 50 ms Wait will run in parallel, so it has no effect on the timing in this case.
2. Continuous AO is possible, depending on the hardware you have. However, I have no experience with implementing it. Since your frequency is such that you do not end the AO data segment at the end of a signal cycle, you need to be careful to avoid discontinuities in the signal sent to the AO.
3. There is no data dependency between the AI and the AO. It is possible that the AO could run after the AI in any given iteration of the loop.
4. Your phase reference should be the excitation signal. The best method of evaluating the response of the beam would be to use two sensors, one at the shaker and one at the tip. Then measure the phase shift of the signal at the tip with respect to the shaker signal. If two sensors cannot be used, either measure the voltage sent to the shaker or use the simulated signal that you send to the AO as the reference. In either case you would need to compensate for the response of the shaker.
5. Consider the phase shift in the filter. The steady state phase shift is about 14 degrees. The initial transient lasts about 5 cycles of the input waveform.
6. Your simulation sampling rate is 1000 samples/second. While this satisfies the Nyquist criterion for a 379 Hz signal, it does not give you much data to work with for the phase information. If the hardware will handle it, I would go to 10000 samples/second.
Lynn