# High performance Band-Pass Filter

Question asked by Jean.Debarros on Aug 29, 2016
Latest reply on Sep 2, 2016 by Matt.Duff

Hi, I need to design a #high performance band-pass filter with these characteristics:

* Low corner-frequency (f_cL) = 5 Hz

* High corner-frequency (f_cH) = 35 Hz

The challenges is that for both corner-frequencies, I need basically a 1 Hz roll-off. I mean, frequencies < 4 Hz and > 36 Hz should be ideally removed!

My first problem is to define what could be the order of such filter; I know that it must be greater than a 10th order. Also, I am planning to cascade actually two filters to build the band-pass filter:

* A High-Pass Filter (HPF) at 5 Hz

* A Low-Pass Filter (LPF) at 35 Hz

And both the HPF and LPF can be themselves done by cascading several sub-filters; for instance, if the order of the LPF is 14, then I'm thinking cascading a 8th-order LPF followed by a 5th-order LPF both centered at the same f_cL of 35 Hz; the same approach of the HPF.

So, my questions:

1) How to determine the order of the filters without invoking too much math: rule-of thumb, calculator or formula...

2) Best engineering way to achieve a high-order filter with flat gain response (Butterworth I'm thinking) with a good accuracy of the delay introduce so I could compensate for it with an All-Pass Filter in case this is an issue later on.

Thanks for any help.

Dr Jean Debarros