I have a question about real and complex numbers. I want to plot the phase and magnitude of a signal that will chirp in frequency from 10MHz to 30MHz. I really want to do more than this, but I think this can easily explain my problem. This can be done in Matlab with no problem, using a complex signal. However in the real world I don't have a complex value, just a real value sampled from an A/D. Is there anyway I can create a complex value from this signal by oversampling? I know there are techniques to do this if I sample at four times the input frequency, but the input frequency is changing so I don't think I can use those techniques. I have also tried mixing the real signal with a complex signal created in Matlab and this gets me close, but is not quite what I am looking for. Thanks in advance for any help you could give me.

# Real vs. Complex

Started by ●February 26, 2007

Reply by ●February 26, 20072007-02-26

birdforsale@gmail.com wrote:> I have a question about real and complex numbers. I want to plot the > phase and magnitude of a signal that will chirp in frequency from > 10MHz to 30MHz. I really want to do more than this, but I think this > can easily explain my problem. This can be done in Matlab with no > problem, using a complex signal. However in the real world I don't > have a complex value, just a real value sampled from an A/D. Is there > anyway I can create a complex value from this signal by oversampling? > I know there are techniques to do this if I sample at four times the > input frequency, but the input frequency is changing so I don't think > I can use those techniques. I have also tried mixing the real signal > with a complex signal created in Matlab and this gets me close, but is > not quite what I am looking for. Thanks in advance for any help you > could give me. >Look up Hilbert transform. Sample twice as fast as you would with a complex signal, and use a Hilbert transform to create a quadrature signal. Now you have a complex signal at twice the sampling rate of your original complex signal. Complex decimation to get exactly back to your naturally complex signal is possible. Steve

Reply by ●February 27, 20072007-02-27

> I have a question about real and complex numbers. I want to plot the > phase and magnitude of a signal that will chirp in frequency from > 10MHz to 30MHz....> Is there anyway I can create a complex value from this signal ... ?You can try the Matlab 'hilbert' function to create a complex signal. This contains the 'instantaneous' magnitude and phase of your signal. To look at the phase, use the 'unwrap' function. Regards, Andor