Convert a digital filter to transfer function

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Sandi2000

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I have a coefficients of FIR filter ,found it by 'fir1' function, in general, i want to convert a digital filter to transfer function with apply this formula :



The method :

Design a 48th-order FIR filter passband , with a cutoff frequency w=[0.35 0.65]. Compute the coefficients of the equivalent transfer function.

Code: 
   % Coefficients of fir filter
   c = fir1(48,[0.35 0.65]);

   %Filter Transfer Function
    [b,a] = tf(c);

The error found:

Error using tf
Too many output arguments.
Click to expand...

How can i solve this error ? how can i find a transfer function of fir filter with this specifications?
 

Sandi2000 said:
What do you mean by tool's name?can you give me an example ?
Click to expand...
Your questions are issues of specific tool.
What tool do you use ?

Many people don't describe tool's name they use in this forum.
 
Last edited:

pancho_hideboo said:
Your questions are issues of specific tool.
What tool do you use ?

Many people don't describe tool's name they use in this forum.
Click to expand...

Do you mean signal processing toolbox?
 

It is Matlab code.

The wanted transfer function format is for a recursive (IIR) filter.
There is no division in the transfer function for a FIR filter.
You can approximate any filter transfer function with a FIR filter,
but it is difficult to convert an approximation back to the "original" function.
 
std_match said:
It is Matlab code.
Click to expand...
So he uses signal processing toolbox and Control system toolbox in MATLAB.

Code: 
>> c = fir1(48, [0.35 0.65]);

>> tf(c, 1, -1, 'Variable', 'z^-1')
 
Transfer function:
-0.002024 + 3.212e-018 z^-1 + 0.002246 z^-2 - 2.001e-018 z^             
                                                                        
        -3 - 6.37e-018 z^-4 - 7.613e-018 z^-5 - 0.006161 z^             
                                                                        
        -6 - 7.01e-018 z^-7 + 0.01176 z^-8 - 1.203e-017 z^-9            
                                                                        
        - 0.005932 z^-10 - 0.01689 z^-12 - 5.219e-018 z^-13             
                                                                        
        + 0.04207 z^-14 - 2.491e-017 z^-15 - 0.03612 z^-16              
                                                                        
        + 1.712e-017 z^-17 - 0.02845 z^-18 - 1.259e-017 z^-19           
                                                                        
        + 0.1424 z^-20 + 1.007e-017 z^-21 - 0.2542 z^-22 + 1.109e-016 z^
                                                                        
        -23 + 0.3008 z^-24 + 1.109e-016 z^-25 - 0.2542 z^-26            
                                                                        
        + 1.007e-017 z^-27 + 0.1424 z^-28 - 1.259e-017 z^-29            
                                                                        
        - 0.02845 z^-30 + 1.712e-017 z^-31 - 0.03612 z^-32              
                                                                        
        - 2.491e-017 z^-33 + 0.04207 z^-34 - 5.219e-018 z^-35           
                                                                        
        - 0.01689 z^-36 - 0.005932 z^-38 - 1.203e-017 z^-39             
                                                                        
        + 0.01176 z^-40 - 7.01e-018 z^-41 - 0.006161 z^-42              
                                                                        
        - 7.613e-018 z^-43 - 6.37e-018 z^-44 - 2.001e-018 z^-45         
                                                                        
                 + 0.002246 z^-46 + 3.212e-018 z^-47 - 0.002024 z^-48   
                                                                        
 
Sampling time: unspecified
>>
 
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