# "Plot Spectrum" question

Greetings. My understanding of FFT is that it results in both real and imaginary parts. Those parts can be expressed as a magnitude + phase, or as Re and Im. 2 questions:

1. what is plotted by “Plot Spectrum” ?

2. if the magnitude is plotted, is there a way to see the corresponding phase.

Thanks very much.

I don’t know how to get the phase and I’m not sure what it means… Phase generally has to be relative to something.

You might have to use something like MATLAB.

The amplitude to frequency spectrum never shows the phase, since it is plotted on a “flat plane”, and phases would represent the angles of the bars about the frequency axis.
As in this case only arithmetically real data is used as input on the Fourier transform, I wouldn’t expect any phase different from zero on the output.
However I might be wrong with this concept, I can do Fourier discrete transforms in CAS (like MATLAB), but never tried to force an arithmetically complex output.

Hi DVDdoug,

Yes, I’ve used Matlab for FFT signal analysis + simulation. That’s where I got some exposure to phase.

“Phase” refers to where to start the sinusoids.

Going further, for each frequency, you get the amplitude (A) and phase (P). To reconstruct a signal, at each frequency (f), you use y=Asin(ft+P). (might be cos(ft+P). It’s been a while . . .). Of course, you have to sort out degrees, radians, etc.

Hi lvalogic,

What is “CAS” ?

Thanks very much

Hi Trebor,

Looks pretty cool !

CAS means Computer Algebra System.
MATLAB is an example of it, but I use maxima for my algebraic tasks.