Lowpass filter with own cutoff frequency and y axis in dB.

I wnat to design a Lowpass filter in which I can choose my own cutoff frequency and y axis in dB. I have a signal in time domain.

[Star Strider]

lowpass

[Reji G]

It doesn't works as expected.

Answer 1

[Bhavana Ravirala]

Hi Reji,

I understand that you are trying to implement a low pass filter. Using the "lowpass" command in MATLAB output signal from a lowpass filter can be obtained.

Example:

fs = 0.1; % sampling frequency of input signal
t= 0 : 1/fs : 1;
x= sin(2*pi*50*t); % input signal
lowpass(x,3, fs); % setting the higher cutoff frequency as 3Hz

For more information refer to the below documentation:

Lowpass-filter signals - MATLAB lowpass (mathworks.com)Hope this helps!!

[Reji G]

No. This doesn't works as expected.

[Bhavana Ravirala]

Hi Reji,

May you describe us the feature that you are trying to implement, so that we can assist you better.

[Reji G]

I have a mixed signal which contains signals from 50Hz to 10^7Hz. From this I want to extract the frequency contents less than 450Hz. Also I want this filtered plot in Frequency Vs Magnitude(in dB) so that I can know freq. corresponds to the -3dB.

[Bhavana Ravirala]

Hi Reji

From your explanation, I understand that you are trying to implement lowpass filtering to a signal having mixed frequencies.

Example:

Fs = 5*10^7;
t = 0:1/Fs:0.001;
signal = [10*sin(2*pi*50*t(1:10000)) 2*cos(2*pi*2000*t(10001:20000)) 12*sin(2*pi*60000*t(20001:30000)) 3*cos(2*pi*(10^7)*t(30001:end))]; % signal with mixed frequencies

%Now you may try out using multiple cutoff frequencies:

 lowpass(signal, 20000, Fs);
 lowpass(signal, 200000, Fs);
 lowpass(signal, 20000000, Fs);

The power spectrum generation is as the expected outcome.

[Reji G]

Thank you.

I need x axis in frequency and y axis in dB.

My requireement is:

If my cutoff frequency is 100Hz then, the value of y axis corresponds to 100 Hz(in x axis) should be "-3dB". How to plot the graph in this way ?