Filter Object Not Filtering Data?

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)

[Stephen]

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.

Answer 1

[Wayne King]

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);

[Jeff Alves]

Thanks Wayne, I definitely considered this, but my sine wave is 10kHz and the sampling rate of my filter is 100kHz in the first filter and 48kHz in the second filter

frequencysine = 10000;

I even plotted it to check.

[Wayne King]

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.