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Can you please explain more about why the signal would alias and be corrupted?
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You stated that your carrier frequency was 20MHz and your sampling frequency was 40MHz. If we assume that your signal of interest is centered on 20MHz and it has some non-zero bandwidth, then if you sample this signal at 40MHz, the Nyquist rate is 20MHz, i.e., the upper-sideband component of the signal will alias on top of the lower-sideband component.
This would be much clearer to you if you created an example for yourself using MATLAB. Eg., you could create a signal sampled at 160MHz, and look at its Fourier transform, then "sample" that signal at 80MHz by keeping every second sample, and look at the Fourier transform, and then "sample" at 40MHz by keeping every forth sample.
But this is starting to get way off-topic, since this forum is really about Altera FPGAs :)
If you have no idea how to create a signal in MATLAB, just ask, and I'll create one for you. Give me an idea of what you need to sample, so that I can create a fake signal that looks similar.
Cheers,
Dave
Honored Contributor
