Chebyshec and Butterworth

[Psytrox]

Hello,

How do I find the ordenans (n) when making a chebyshev or butterworth filter. I know so far that I have to look at the graphs. But I don't understand what to look at/how to look at it.

- Psytrox

 

[Roff]

I'm not sure I understand your question.
A filter is used to pass some frequencies while rejecting others. The required filter order is determined by the amount of out-of-band rejection required by the application.

 

[crutschow]

Don't know what "ordenans" is, but if you are referring to the filter order, then that is determined by the rolloff outside the passband. It's 20dB/decade (6dB/octave) rolloff per order, e.g. 1st order gives 20dB/decade, 2nd order gives 40dB/decade, etc.

 

[MrAl]

Hello,

How do I find the ordenans (n) when making a chebyshev or butterworth filter. I know so far that I have to look at the graphs. But I don't understand what to look at/how to look at it.

- Psytrox

Hi,


Are you asking about the coefficients?

 

[RCinFLA]

Verb
'ordenan' (infinitive ordenar)

1.Third-person plural (ellos, ellas, also used with ustedes?) present indicative form of ordenar.
2.(used formally in Spain) Second-person plural present indicative form of ordenar.

Never heard it used as a noun. Maybe it is 'ordinates', a.k.a. vertical axis. Still does not help clarify the question.

If you are referring to the different plots on a graph that represents the family of curves for a given number of poles then the different plots represent the degree of ripple allowed for the design, starting with Butterworth of zero ripple, then subsequent Chebishev designs with progressively more ripple in filter passband.

The greater the allowed ripple for a given number poles, the greater the skirt selectivity. At far out frequency all the designs eventually continue at 6 db per octave per pole rolloff (unless limited by physical implementation resulting in stray coupling around filter).

Hint, on practical side of filter design. Bessel (linear phase response) is an undercoupled filter. Butterworth (maximally flat) is critically coupled filter. Chebishev filters of more and more ripple are overcoupled filters.

 

[Psytrox]

Well, I'm not sure exactly what its called.

Lets say I want to make a filter that meets a certain requirement:

Max passband "reduction" : 0,5 dB
Min passband "reduction" : 60 dB
Passband frequency: 200Hz
Stopband frequency: 900Hz

and then question one is, "Decide the filters "orden" (I'm not sure what this is in english)

But if you look at the file that I've attached n = "orden",

(I don't know exactly what the english terms are, so please be patient - I'll try and explain it as best i can.)


Then back to my main questions, how exactly can i look at the graph that I've attached and find out what n is supposed to be? In this case it is supposed to be 4, but I only know that because I've looked at the answer.

 

[crutschow]

So apparently "orden" is the filter order.

Your requirements are somewhat ambiguous. Do you mean the Min stopband "reduction" is 60dB rather thant the Min passband "reduction"? If so then you just look on the graph for where a difference in frequency ratio of 900 to 200 = 4.5 gives 60dB minimum. Ω is apparently the stopband to passband frequency ratio so you look at where 4.5Ω and 60dB intersect. That is just below the 4th order curve, so you need a minimum of a 4th order filter.

 

[MrAl]

Hi there Carl,


I would tend to agree with that because when i use my own formulas for calculating the minimum required N for a stopband gain of -60db at ws=4.5*wc and max passband ripple of 0.5db (as per that pdf graph) i get a value of 4 for N also, which again is the minimum value of N that would meet the requirements of the filter.