Re: filter order

On Apr 14, 6:09 pm, Phil Hobbs
<pcdhSpamMeSensel...@xxxxxxxxxxxxxxxxxx> wrote:
jd_l...@xxxxxxxxx wrote:
By definition, the
order of the filter is the order of the denominator polynomial of its
transfer function.  

Really?  We physics types define the order of a rational function as the
sum of the orders of the numerator and denominator polynomials.

Cheers

Phil Hobbs

Bzzzzz! wrong answer. The order of the denominator is the order of the
filter.

BTW, the original statement holds true even for eliptic filters. Look
at it again:

"The order of a passive filter consisting of only Ls,Rs and Cs can
never be higher than the number of Ls and Cs "

If you have some transmission zeros, it is true they add extra
capacitors, but that isn't the issue. Even with the extra caps, the
order of the filter won't be higher than the number of Ls and Cs. Now
the sum of the Ls and Cs will not be the order, but that isn't what
the theorem says.

Here is another theorem of sorts. The transfer function of any point
along the ladder filter will have the same denominator. I think this
falls out of Mason's rule.

Not that anyone does SCF these days, but if you did a SCF leapfrog
filter, you generally got a better filter if you added transmission
zeros. [This assumes you have filter software to build arbitrary
responses.] The zeros didn't add any more op amps, so there was no
noise penalty. Generally with any active filter, the attenuation of
the input signal reaches the point where the signal falls in the noise
floor. Often you could reduce the order of the filter using
transmission zeros.

.

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