clc;clear;
fs=400e3;R=920e3;C=10e-12;T=300;K=1.38e-23;duty=0.5;
N=duty/fs/(R*C);
syms f;
X(f)=2/fs * K*T/C * (1-exp(-2*N)) / (1-2*exp(-N)*cos(2*pi*f/fs)+exp(-2*N));
fplot(X(f),[1,10*fs/2])
set(gca,'xscale','log')