Question on using Power Spectrum

[SumLee]

Hello, Labview users

I have a question on using Power Spectrum.

Attached a program which used power spectrum to measure amplitude of sin wave.

But if input of frequency changes and input of
amplitude does not changes, the output measured
amplitude always changing.

What is the reason ?

Thanks a lot!!

[ErikMan]

I think you are seeing this because of the "granularity" of the generated wave. If you slowly modify the frequency from one value to another, the amplitude will pulse up and down.

If you change your sampling rate and your number of samples in waveform to 10k, the generated wave will be smoother, and this phenomenon goes away.

Hope this helps!

-Erik

[LocalDSP]

The variation in the maximum value of you spectrum is caused by the discrete nature of the process. To reduce this variation, use the Flat Top window instead of the Hanning window you are using now. The flat top window is especially designed for that purpose.

Alain