How come my chebyshev order keeps changing once I change my Ws and Wp?

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Im trying to make an 4th Order Chebyshev, with these settings. Wp = 0.9999;
Ws = .0250;
Rp = 1;
Rs = 50;
[n,Wp] = cheb1ord(Wp,Ws,Rp,Rs)
n = 4
[b,a] = cheby1(n,Rp,Wp);
freqz(b,a,500,200)
title('n = 4th Order Chebyshev Type I ECG Lowpass Filter')
I want my cutoff frequency to be 40Hz by using Wp/Ws. (I believe thats the cutofff frequency for the cheby?) And once those variables are saved and I enter [n,Wp] = cheb1ord(Wp,Ws,Rp,Rs), my n changes to 1. Why is that?

Accepted Answer

Sai Sri Pathuri
Sai Sri Pathuri on 4 May 2020
The cheb1ord function calculates the minimum order of a Chebyshev Type I filter required to meet a set of filter design specifications. The lowest order of filter that can be designed with your specifications might be 1. Hence, cheb1ord function is returning the order as 1.
You may refer the documentation of cheb1ord for more details

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