DSP code snippets

The two inputs (.25 & 1) to function firrcos represent as you know sampling frequency of filter module and its cutoff.

The term Fs and Fc I use them for different purpose in the main program.

Fs is the final digital IF. Fc is centre frequency for upconversion. So both are unrelated to filter stage.

For firrcos, the minimum practical upsampling is x2 hence we need to cut off at Nyquist relative to Fs of 2 i.e you can enter as .5 & 2 or .25 & 1. The actual physical sampling frequency of filter is normalised here to 1.

The code implies symbol rate = 12.5 Msps and so upcomverted to 25 by filter then to 100 by upsampling stage then frequency shifted to 10MHz.

I edited that snippet accordingly to make this point clearer, thanks for the question.

kaz