Lowpass doesn't work as expected

Hi, I try to filter my signal- in the blue color using lowpass filter and after filtering it I get the orange one which is clearly not what I wanted. Do you have any Idea why?

The code:

Fs = 120e9; % Sampling frequency

T = 1/Fs; % Sampling period

L = 2^20; % Length of signal

t = (0:(L-1))*T; % Time vector

f = Fs*(0:(L/2))/L; % Frequency vector

A = 1; % coeficient of the sin(w*t)

omega=2*pi*1.144409179687500e+09; % ~1GHz frequency

phaseNoise=0.001*ones(1,length(t)); %pure constant phase noise

sine_signal_with_PN=sin(omega*t+phaseNoise); %signal with PN

signal_with_PN=cos(omega*t+phaseNoise); %shifted signal with PN

NF=sin(omega*t+phaseNoise)-A*sin(omega*t); %the notch filter

y=signal_with_PN.*NF; %the signal after processing

%creating the lowpass filter

lpFilt = designfilt('lowpassfir','PassbandFrequency',0.005, ... %2*f/Fs=NormFreq

'StopbandFrequency',0.01,'PassbandRipple',0.5, ...

'StopbandAttenuation',100,'DesignMethod','kaiserwin');

yBeforeLpf=2*y/A;

PN_after_LPF =filter(lpFilt,yBeforeLpf);

filteredPN=filter(lpFilt,phaseNoise);

%% Graph of signal before LPF

figure();

y_ft=db(fft(yBeforeLpf));%db

plot(f,y_ft(1:(L/2+1)));

hold on;

PN_FT=db(fft(PN_after_LPF));%db

plot(f,PN_FT(1:(L/2+1)));

xlabel('f[Hz]');

xlim([0 3e9]);

ylabel('|Y^F|[dB]');

grid minor;

title('FFT Of Signal After Scheme With PN Before LPF');

Answer 1

[Mathieu NOE]

hi

modofied a bit your code - I did not use designfilt but otherwise your code seems to work

just replaced your FIR filter with a more traditionnal butterworth IIR filter

clc

Fs = 120e9;             % Sampling frequency                    
T = 1/Fs;               % Sampling period  
L = 2^20;               % Length of signal
t = (0:(L-1))*T;          % Time vector
f = Fs*(0:(L/2))/L;     % Frequency vector
A = 1;                % coeficient of the sin(w*t)
omega=2*pi*1.144409179687500e+09;  % ~1GHz frequency 
phaseNoise=0.001*ones(1,length(t)); %pure constant phase noise
sine_signal_with_PN=sin(omega*t+phaseNoise); %signal with PN
signal_with_PN=cos(omega*t+phaseNoise); %shifted signal with PN
NF=sin(omega*t+phaseNoise)-A*sin(omega*t); %the notch filter
y=signal_with_PN.*NF; %the signal after processing
%creating the lowpass filter
% lpFilt = designfilt('lowpassfir','PassbandFrequency',0.005, ... %2*f/Fs=NormFreq
%          'StopbandFrequency',0.01,'PassbandRipple',0.5, ...
%          'StopbandAttenuation',100,'DesignMethod','kaiserwin');

[BlpFilt,AlpFilt] = BUTTER(2,1e-4);
     
     
yBeforeLpf=2*y/A;

% PN_after_LPF =filter(lpFilt,yBeforeLpf);
PN_after_LPF =filter(BlpFilt,AlpFilt,yBeforeLpf);

% filteredPN=filter(lpFilt,phaseNoise);
filteredPN=filter(BlpFilt,AlpFilt,phaseNoise);

% %% Graph of signal before LPF
% figure(1);
% y_ft=20*log10(abs(fft(yBeforeLpf)));%db
% plot(f,y_ft(1:(L/2+1)));
% hold on;
% PN_FT=20*log10(abs(fft(PN_after_LPF)));%db
% plot(f,PN_FT(1:(L/2+1)));
% xlabel('f[Hz]');
% xlim([0 3e9]);
% ylabel('|Y^F|[dB]');
% grid minor;
% title('FFT Of Signal After Scheme With PN Before LPF');

%% Graph of signal before LPF
figure(1);
y_ft=20*log10(abs(fft(yBeforeLpf)));%db

PN_FT=20*log10(abs(fft(PN_after_LPF)));%db

semilogx(f,y_ft(1:(L/2+1)),'b',f,PN_FT(1:(L/2+1)),'r');
xlabel('f[Hz]');
xlim([0 3e9]);
ylabel('|Y^F|[dB]');
grid minor;
title('FFT Of Signal After Scheme With PN Before LPF');

[Liad Gerstman]

I copied it to my MATLAB and it doesn't seem to work (changed BUTTER to butter). The output is the following:

I want to filter delta in 2.28e9 and leave the dc delta unchanged.

Thank's for trying

[Mathieu NOE]

OK

II changed quite a bit your code , instead of doing one fft on the entire length of the signal, I have done a periodogram using pwelch function. The results look to me better

you may have to play with NFFT

clc

% Fs = 120e9;             % Sampling frequency                    
Fs = 120e9;             % Sampling frequency                    
T = 1/Fs;               % Sampling period  
L = 2^20;               % Length of signal
t = (0:(L-1))*T;          % Time vector
f = Fs*(0:(L/2))/L;     % Frequency vector
A = 1;                % coeficient of the sin(w*t)
omega=2*pi*1.144409179687500e+09;  % ~1GHz frequency 
phaseNoise=0.001*ones(1,length(t)); %pure constant phase noise
sine_signal_with_PN=sin(omega*t+phaseNoise); %signal with PN
signal_with_PN=cos(omega*t+phaseNoise); %shifted signal with PN
NF=sin(omega*t+phaseNoise)-A*sin(omega*t); %the notch filter
y=signal_with_PN.*NF; %the signal after processing
%creating the lowpass filter
% lpFilt = designfilt('lowpassfir','PassbandFrequency',0.005, ... %2*f/Fs=NormFreq
%          'StopbandFrequency',0.01,'PassbandRipple',0.5, ...
%          'StopbandAttenuation',100,'DesignMethod','kaiserwin');

[BlpFilt,AlpFilt] = BUTTER(2,1e-4);
     
yBeforeLpf=2*y/A;

PN_after_LPF =filter(BlpFilt,AlpFilt,yBeforeLpf);

%% Graph of signal before LPF

NFFT = 1024*2;    % to have highest frequency resolution 
NOVERLAP = round(0.75*NFFT);

 [y_ft, freq]=pwelch(yBeforeLpf,hamming(NFFT),NOVERLAP,NFFT,Fs);
 
 [PN_FT, freq]=pwelch(PN_after_LPF,hamming(NFFT),NOVERLAP,NFFT,Fs);

figure(1);
semilogx(freq,20*log10(y_ft),'-+b',freq,20*log10(PN_FT),'-+r');
xlabel('f[Hz]');
xlim([0 3e9]);
ylabel('|Y^F|[dB]');
grid minor;
title('FFT Of Signal After Scheme With PN Before LPF');