Quote:Since the frequency response of such an structure depends on the phase of input
I believe you are misunderstanding the nature of N-path filters. The frequency response does not depend on the phase of the input. The filter would have to be nonlinear for that to be true. N-path filters are linear with respect to the input. However N-path filters are also time-varying, which may be what you are thinking.
Since the filter is linear with respect to its input, it is easiest to consider the input as being asynchronous from the clock. In that case the phase is constantly shifting and has no meaning.
To simulate an N-path filter with SpectreRF you need to include all the paths, run a PAC analysis, and you would sweep the input over a range of frequencies.. You should leave the PACphase at 0. Generally the only time you set PACphase is when you have two sources that are generating a differential signal. In that case one would be set to 0 and the other set to 180.
-Ken