Digital filter using code

Hi

I am trying to design a simple low pass digital filter (IIR or FIR) with the following characteristics. I would like to plot the filters response and calculate its transfer function.

Order(N) = 2

Band pass frequency (normalized)(wp)= 0.1

Band pass attenuation(ap) = 0dB(or as close as possible)

Stop band frequency (sf) = 0.4

Stop band attenuation (as) = at least -30dB

I have tried several methods but with such a low order I am finding it difficult to achieve these characteristics.

I would like to try and achieve this using Matlab code only, although I have tried using the digital filter design toolbox but still could not achieve the filter characteristics.

Any help would be greatly appreciated.

Thanks

[Wayne King]

Please tell us which version of MATLAB you are using? When you say "digital filter design toolbox" do you mean the old MathWorks' Filter Design Toolbox?

[jimmi1]

Hi Wayne,

I am using Matlab 2013, I first tried to design the filter using matlab code, when that failed I tried using the "Filter design and analysis" toolbox located in the "signal processing and communications" section. As mentioned this also failed to produce the results I needed. I tried all of the low-pass filters available all of which would not produce the result i needed.

example: I tried to create an FIR filter using Equiripple algorithm, this was not possible and said a higher order "3" was needed.

In other algorithms I could not enter all of the specifications of my filter.

Example: if I use a butterworth and I specify the order number I can not specify Apass or Astop. If I let it decide the order number for me and I supply Apass and Astop it builds a filter with a 5th order.

Thanks

Answer 1

[Wayne King]

It's going to be difficult to get a good lowpass filter with such a low order (even IIR)

FIR:

   d = fdesign.lowpass('N,Fp,Ap,Ast',2,0.1,0.5,30);
    Hd = design(d);
    fvtool(Hd)

% requires DSP System Toolbox -- this gets you very close but you have to increase order bit

   d = fdesign.lowpass('N,Fp,Fst,Ast',4,0.1,0.4,30);
   Hd = design(d);

If you enter:

   measure(Hd)

You get

Passband Edge    : 0.1                       
3-dB Point       : 0                         
6-dB Point       : 0                         
Stopband Edge    : 0.4                       
Passband Ripple  : 0.73383 dB                
Stopband Atten.  : 29.9978 dB                
Transition Width : 0.3