aligning s parameters with fft with arbitrary frequency domain

Background:

I am working to dembed an arbitrary time domain signal with s2p files.

Problem Statement:

I am getting incorrect time domain signal when getting the de-embedded signal in the time domain, which I believe is a misalligned frequncy domain.

Best Attempt:

If I have sparameters that are over a different frequency range than the fft of a signal is interpl the best way to allign the S12 before multiplying?

%%setup of frequency variables
dt = time(2)-time(1);              % Sampling period       
df = 1/dt;                   % Sampling frequency         
L_of_t = length(time);         % Length of signal
t = (0:L_of_t-1)*dt;        % Time vector
n = 2^nextpow2(L_of_t);
%f = dFs*(0:(n/2))/n;
f = df*(0:(L_of_t/2)-1)/L_of_t;    %frequency range
f_length = length(f);   %length of the frequency range

%% allignment of S parameters
alligned_S12 = interp1(s_param_inv.Frequencies(k),s_param_inv.Parameters(2,1,k),f);
parfor k=1:lengthofmatritime %can be a parforloop
    fd_final_sig(k) = s_param_inv.Parameters(2,1,k) * fd_sparam_simcircuit(k);%fd_sparam_simcircuit(k);
end
%% converting the dembedding frequency domain signal back to the time domain
td_final_sig = ifft(fd_final_sig,'symmetric');