how to plot a centralize upconverted signal around the carrier frequency - Matlab

Question

I have an upconverted signal that is around the carrier frequency fc. I'm trying to plot this signal's FFT but failed to scale the frequency axis correctly as the signal's FFT is centered around the DC. I tried:

f = 1;
fs = 10*f;
t = 0:1/fs:5;
fc = 100;
x = sin(2*pi*f*t);
x_up = x .* exp(1i*2*pi*fc*t);
f_fc = linspace(-fc,fc,length(t));
plot(f_fc,fftshift(abs(fft(x_up))))

How should I scale it correctly? Thanks.

Hi! In order for your example to be [Minimal and Reproducible](https://stackoverflow.com/help/minimal-reproducible-example), you should provide example values for `fc`, `t` and `srx`. — BillBokeey, Jul 06 '22 at 08:00
it is normal since the upconversion is made by multiplying with complex exponent. you can take the real part of the signal. — A.B.A, Jul 06 '22 at 12:55

score 0 · Answer 1 · answered Jul 06 '22 at 13:16

I found my mistake. The fixes in the code should be:

the sampling frequency should be a multiplication of fc and not f, such as: fs = 2*fc
the scaling of the frequency axis should be: f_fc = linspace(0,2*fc,length(t)) In that way the FFT of the upconversion signal is shown around the carrier frequency fc.

how to plot a centralize upconverted signal around the carrier frequency - Matlab

1 Answers1