Phase modification of a waveform which consists of multiple frequencies.

[by]

How Can I modify/change the phase of a waveform which consists of many frequncies? Thanks a lot.

[LocalDSP]

You need to be more specific on what you are trying to do. If your waveform consists of several different tones (frequencies), each tone will have its own 'phase' so there is not a unique phase for your waveform.

Are you trying to delay the entire waveform? phase shift a specific tone? Please explain your application and intentions.

[by]

First thank you very much for your answer.
I want to change the phase of each tone (frequecy) through my DSP network. the application is that the group delay of this composite waveform is linear after this waveform passes through my DSP network.

[LocalDSP]

If I understand you right you want to re-construct your signal with the same tone frequencies and amplitudes but force the phase of each tone to, for example, zero (independantly of its actual phase).

If you are in LabVIEW you can for example use the Extract Single Tone Information high-level VI to detect the exact frequency and amplitude of each tone and then re-construct the waveform in the time domain using the detected freq. and ampl. and the phase of your choice.

Hint: If you want to extract the information for multiple tones, use the VI in a loop and export the residual signal after each run to be used as input signal for the following run.

[by]

Thanks LocalDSP.
Your answer is very helpful. by the way I have another question.
What if the waveform is a frequecy sweep siganl such as chirp signal?

[LocalDSP]

Honnestly it sounds to me like what you try to do is to equalize the phase of your 'channel', that is correct the phase characteristic of the output signal to match the input signal.

If this is the case, you need a different approach. You'll have to measure the transfer function (frequency response) between output and input and then correct your output signal for the measured phase characteristic. This can be done either using an allpass filter or using some FFT-Inverse FFT based techniques. None of this is trivial, so refer to literature for the details.

I hope this helped.

[by]

I plan to DFT input first, then manipulate its amplitude and phase in frequency domain according to the Transfer function of my network, finally Inverse DFT modified data and output them.
Your suggestion is great and valuable.
Thank you very much.