How to choose stopband ripple and passband ripple of filter for High pass filter

I am using high pass filter.

Data known:

Cutoff 10Hz. Sampling frequency 50KHz.

How can I choose stopband ripple and passband ripple ? Based on what criteria?

Answer 1

[Mathieu NOE]

hello

why not start with simpler filters like butterworth or bessel filters ?

. If you don't have any specific requirement for stopband ripple and passband ripple , here you can start play with that :

Fs = 50e3;
samples = 5000;
dt = 1/Fs;
t = (0:samples-1)*dt;
y = 3*sign(sin(2*pi*50*t)); % signal - adjust amplitude to your needs

% high pass filter : Butterworth filter
NN = 2; % filter order
fc = 10; % cut off frequency (Hz)
Wn = fc/(Fs/2); % normalized cut off frequency 
[B,A] = butter(NN,Wn,'high');
y_filtered = filter(B,A,y); % signal (after HP filtering)

figure(1)
plot(t,y,t,y_filtered);legend('signal','HP filtered signal');

[632541]

Can you please alter for fir filters with the same requirements

[Mathieu NOE]

You have a very low cut off frequency (10 Hz) compared to the sampling frequency (50 kHz)

this will generate a very long FIR flters (> 5,000 coefficients) wich is not very efficient . Why not stick with an low order IIR filter ?

do you really need a 50 kHz sampling rate ?

[632541]

How does sampling and cutoff related (50KHz is less or more?) ...

What makes the difference if i choose IIR in this case?(though this is related to DSP please clear my doubts)

[Mathieu NOE]

For a FIR filter , the lower the (normalized) frequency , the longer the filter (in this examlple we are talking about coefficients length in thousands ! )

for a IIR filter , the amount of coefficient is much less , but as we have poles , we must be carefull in DSP implementation (instabilities dues to accumulation of round off errors)

the best method is to convert any filter into "biquad" sections - there are plenty of publications about biquad implementation on DSP

[632541]

Thanks Mathieu NOE!Your answer cleared my doubts

[Mathieu NOE]

my pleasure

[632541]

Hi Mathieu NOE,

I know desired cut-off frequency(10Hz).

How do I select the fpass and fstop frequencies?

[Mathieu NOE]

hello

what filter are we talking ? FIR / IIR ?

for a bessel or butterworth IIR filter , there is only 2 parameters (order and cut off frequency)

[632541]

IIR filter,

Order also not known

Only cutoff and sampling frequency are known.

[Mathieu NOE]

ok , so why not use bessel or butterworth IIR filter ?

[632541]

yes, going with butterworth filter design order to be set.

How to choose that

[Mathieu NOE]

well, choosing a filter means you know what portion of the spectrum should be attenuated ... what is the goal here ? are you not able to do your code based on my first suggestion ?

Fs = 50e3;
samples = 5000;
dt = 1/Fs;
t = (0:samples-1)*dt;
y = 3*sign(sin(2*pi*50*t)); % signal - adjust amplitude to your needs
% high pass filter : Butterworth filter
NN = 2; % filter order
fc = 10; % cut off frequency (Hz)
Wn = fc/(Fs/2); % normalized cut off frequency 
[B,A] = butter(NN,Wn,'high');
y_filtered = filter(B,A,y); % signal (after HP filtering)
figure(1)
plot(t,y,t,y_filtered);legend('signal','HP filtered signal');