phase measurement of the signal

[Rajasekaran08]

Dear all,

I am trying to measureme the phase of the sine waveform.

I generated sine waveform and give to "spectral measurement" express VI to measure the magnitude and phase. The spectral measurement block gives magnitude and phase spectrum array with frequency difference of 1hz.

The input signal is 10hz sine waveform. so i checked the 10th element in the magnitude and phase array. I am getting the correct magnitude. but i am getting different values for phase measurement

For example, If the input signal having the phase angle of 0 degree, the spectral measurement gives as -90 degree. If input signal have the phase shift of 20 degree, the measurement blocks gives as -70 degree.

I cant able to find that how this -90 degree phase shift occurs.

Kindly help to solve it. Thank in advance.

with regards,

Rajasekaran V

[johnsold · Accepted solution]

Rajasekaran V,

The Spectral Measurement VI uses the FFT internally. In the detailed help for the FFT the following definition appears:

Note that exp(-j*2*pi*theta) = cos(2*pi*theta) - i*sin(2*pi*theta). Thus phase shift is defined with respect to cosine rather than sine, explaining the 90 degree shift.

Lynn

[Rajasekaran08]

Dear Lynn,

Thanks for your reply.

I got the answer. I will add +90 degree the phase spectrum 1D array and find the actual phase of the waveform at all the frequency.

Thank you.