Using FFT Mag-Phase to display magnitude with respect to frequency

[MarsianaM]

Thanks for the idea to negating the signal.. 

can you explain more about subtract A-B in the time domain? maybe i can use it as alternative.. 

[CoastalMaineBird]

Just use a SUBTRACT node.

if you wire an array (A) into the top terminal, and an array (B) into the bottom terminal, you'll get an array (A-B) from the output terminal.

i.e., it will do one subtraction for each element.

Out[0] = A[0] - B[0]

Out[1] = A[1] - B[1]

Out[2] = A[2] - B[2]

etc.

[MarsianaM]

i tried to negating the signal (by multiply it with -1) and performing fft mag-phase, but nothing came up.. Do you have any idea why? 
actually i need the shifted phase and frequency data to forming a new signal for controlling motor DC speed.. have any idea what should i do?