MATLAB: How to convert from time domain to frequency domain

filteringfrequency domain

I have this filter:

filter_2 = firceqrip(2,0.6,[0.05 0.03]);

I want to convert it to the frequency domain to multiply it by a signal (i.e filter in the frequency domain)

Call fft() or pwelch().

If you want the phase and amplitude for different frequencies then these are just:

angle( res );
real( res );

abs( res );

if you want the magnitude rather than amplitude.

You need to take the Fourier transforms of both signals.