Filter Object Not Filtering Data?
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I have been trying to low pass filter some data without any luck. Everytime i run the data through my filter object, I get a 1:1 output regardless of whether I am well above the cutoff frequency or not! I don't care exactly how it is done (I can make the filter using FIR, IIR, etc). I found the fdatool GUI and it outputs m-file code for various filter types, I can see the Bode plot, so I know it is correct (see filter (2) below)... I also tried a much simpler method using fdesign.lowpass (see filter (1) below)...
When I plot the sine wave, it comes out at the right frequency (10kHz) and with a magnitude of '1'. When I plot the filtered sine wave "y", it also comes out at 10kHz with a magnitude of '1'. There is no phase shift or magnitude difference at all. I have tried really large frequencies and really small frequencies to make sure I wasn't having issues with 'normalized filters' or anything like that... Please Help!!!
Here are different filters I have tried (1):
d=fdesign.lowpass('N,Fc',10,1000,100000); %Designs a low pass filter specification "d" with "Order, Frequency, Sampling Rate"
designmethods(d) %Returns the available filter methods for the designed filter (Window, Hamming, Hanning...)
Hd = design(d); %Uses the default design method returned from designmethods -- "Hd" is your filter object
fvtool(Hd);
Here are different filters I have tried (2):
%From the FDATool Filter Design For Butterworth
Fs = 48000; % Sampling Frequency
Fpass = 1000; % Passband Frequency
Fstop = 4000; % Stopband Frequency
Apass = 1; % Passband Ripple (dB)
Astop = 20; % Stopband Attenuation (dB)
%Calculate the order from the parameters using BUTTORD.
[N,Fc] = buttord(Fpass/(Fs/2), Fstop/(Fs/2), Apass, Astop);
%Calculate the zpk values using the BUTTER function.
[z,p,k] = butter(N, Fc);
%To avoid round-off errors, do not use the transfer function. Instead get the zpk representation and convert it to second-order sections.
[sos_var,g] = zp2sos(z, p, k);
Hd = dfilt.df2sos(sos_var, g);
fvtool(Hd);
Here is how i am applying the filter:
% generate a sine wave
frequencysine = 10000;
x = 0 : 1/frequencysine*10/1000000 : 1/frequencysine*10;
figure(2)
sine=sin(frequencysine*2*pi*x);
plot(x,sine)
figure(3)
y = filter(Hd,sine);
plot(x,y)
1 个评论
Stephen
2012-6-26
Weird, I get the same thing when I try it. Don't know what is wrong though. HD looks fine so maybe it's in how you implement it.
回答（1 个）
Wayne King
2012-6-26
You are not designing a filter with the same sampling frequency as your input signal. The sampling rate of your sine wave is 1 gigahertz, but you design your filter for data sampled at 100,000 Hz. That is the problem
d = fdesign.lowpass('N,F3dB',10,1000,1e9);
Hd = design(d,'butter');
y = filter(Hd,sine);
2 个评论
Jeff Alves
2012-6-26
Wayne King
2012-6-26
Did you try the example I gave you? it removes the 10,000 Hz component. You should design the filter with the sample frequency of the data.
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2012-6-26
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