Phase instability?

[happybird]

Hellow!

I am trying to measure the phase angel of a simple sine wave from a osilloscope,i use "extract single tone information.VI" which give me the phase information of the signal,but to my disappointed,the phase result is always shifting.I am sure that the signal is stable,because it is from a function generator.so are there any reason that the phase is shifting so much? i notice that the the other two out put from "extract single tone information.VI " is much more stable.

Thanks!

[altenbach]

Well, we need to know if the phase of your analog signal is changing (likely) or if the "extract single tone information.VI" is faulty (unlikely).

What happens if you display your signal on a graph indicator. Does it seem to scroll or does it look locked? Phase is quite meaningless without a reference point.

Have you tried testing your code with the "simulate signal" express VI to eliminate the external hardware?

[happybird]

Hellow!

The signal on the graph looks locked,how to relate the phase to a fix point? i dont see this option in:"extract single tone information"

attached is a simplifed Code of mine,it seems to work well.

[johnsold]

If you dig down into the Extract Single Tone Information.vi you find (guessing about a password protected diagram) that the data is apparently derived from a complex FFT. It is too early in the morning to work through all the mathematics but I think the phase you get from an FFT is related to the reference function. I quote from Chapter 4 of the LV Analysis Manual:

"The phase is relative to the start of the time record or relative to a
single-cycle cosine wave starting at the beginning of the time record.
Single-channel phase measurements are stable only if the input signal is
triggered. Dual-channel phase measurements compute phase differences
between channels so if the channels are sampled simultaneously, triggering
usually is not necessary.
The phase is the arctangent of the ratio of the imaginary and real parts and
is usually between π and –π radians, or 180 and –180 degrees."

Lynn

[happybird]

So are there any specific VI which can handle the difference of phase from two channel?