Problems with a Band Pass Filter

Question

David Jones 2020-11-18

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/651973-problems-with-a-band-pass-filter

Capture_1.JPG

Hi

I am having a problem with bandpass filtering can somebody please assist me.

Sampling Rate 50

Number samples 3000

Sig Gen 0.2 Hertz

Amplitude 100mV

Filter

function y = doFilter(x)

%DOFILTER Filters input x and returns output y.

% MATLAB Code

% Generated by MATLAB(R) 9.8 and DSP System Toolbox 9.10.

% Generated on: 18-Nov-2020 10:12:16

persistent Hd;

if isempty(Hd)

Fstop1 = 0.05; % First Stopband Frequency

Fpass1 = 0.15; % First Passband Frequency

Fpass2 = 0.35; % Second Passband Frequency

Fstop2 = 0.45; % Second Stopband Frequency

Astop1 = 60; % First Stopband Attenuation (dB)

Apass = 1; % Passband Ripple (dB)

Astop2 = 60; % Second Stopband Attenuation (dB)

Fs = 50; % Sampling Frequency

h = fdesign.bandpass('fst1,fp1,fp2,fst2,ast1,ap,ast2', Fstop1, Fpass1, ...

Fpass2, Fstop2, Astop1, Apass, Astop2, Fs);

Hd = design(h, 'butter', ...

'MatchExactly', 'stopband', ...

'SOSScaleNorm', 'Linf');

set(Hd,'PersistentMemory',true);

end

y = filter(Hd,x);

I have attached a jpeg of my results from Matlab. The first trace is the signal after being sampled, the second is the magnitude fft, and the last trace is the filter results but as you can see I have a strange wobble just after the star and when it recovers the amplitude varies. Can you please tell me where I am going wrong.

Thank you

David