[sdiy] 8th order?

[flightofharmony at comcast.net]

James Dunn wrote:
> Magnus Danielson wrote:
>> From: James Dunn <james at 4thharmonic.com>
>> Subject: [sdiy] 8th order?
>> Date: Tue, 05 Jun 2007 10:40:03 +0100
>> Message-ID: <46652F73.3050707 at 4thharmonic.com>
>>
>>  
>>> What does 8th order mean? Is it simply another way of describing the 
>>> number of poles in a filter or db/oct?
>>>     
>>
>> It relates back to the theoretical background of linear diffrential 
>> system
>> and describes the degree of the system. The order of the system also 
>> descibes
>> how many poles and how many zeros the system can have. The placement 
>> of the
>> zeros gives the db/Oct number, but the school-book examples with all 
>> zeroes in
>> a bunch over at infinity for a low-pass makes the db/Oct being -6.02 
>> db/Oct
>> times the order. An n-pole system is of the n:th degree.
>>
>> You require at least n reactive components (capacitors, inductors) to 
>> acheive
>> a n:th order system, but it is not always that you get that order, 
>> you can get
>> less. For example will a dual-PI notch filter with RC links have 3 
>> capacitors
>> but only show up as a 2:nd degree system. The db/oct measure does not 
>> give a
>> good measure here.
>>
>> I hope you see the difference and also apprechiate why we talk about 
>> 8th order
>> rather than 48 db/Oct systems.
>>
>>   
> er, I'm afraid not! Thanks very much for your in-depth answer though - 
> I think I need to spend some more time with the Arts of Electronics.. 
> :) I also think I have a good enough understanding for what I need for 
> the time being though.
>
> Thanks again
>
> James
So, in short form, the order of the filter refers to the number of poles 
and zeroes. The dB performance is based on filter type. The 
characteristic equation specifies the performance ( cutoff frequency, 
etc.) of the circuit, then you build a circuit to match that 
specification. To build an 8th order filter you cascade four 2nd order 
stages.

~flight