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Hi Mob, When you look to design a filter, ideally you would want a brick wall type of filter. But then to design a filter with such a characteristic is physically not possible. So you try and design a filter whose curve would fit within the desired characteristics. That is you allow for certain ripple in the passband (which is nothing but the fluctuation in the gain at different frequencies in the spectrum of interest) and also a transition band. say for eg: |______________ |_____A________|__ | | | | | | | |B | | | | | | | | |__|_________________ |______________|__|_____C___________ fp fs f In the above case you specify that you can allow rp amount of deviation in the gain in the passband and want 'As' amount of attenuation relative to passpand in the stop band. Using chebyshev, butterworth, elliptic etc approximations, you fit in a curve that would lie entirely within the three boxes (named A,B and C) in the diagram. Each type of approximation would give you a different type of curve with different order of polynomial (and hence filter) in each case. Now coming to your question of choice of rp and rs, The smaller ones you choose, the order of filter would increase accordingly. So it is more of a tradeoff between your desired response and computational complexity. Hope whatever I have written makes some sense to you. regards, Ravi >Could anyone tell me how to choose ripple for passband and stopband in >fliter design? Any definition please? To me it is not clear. >Great thanks. >Mob. >example: >rp = 0.01; % Passband ripple >rs = 0.1; % Stopband ripple V Ravi Chander Student ID : 200211014 MTech(ICT) DA-IICT, Gandhinagar . http://intranet.da-iict.org |