Re: filter order

On Apr 14, 9:53 am, ggher...@xxxxxxxxx wrote:
On Apr 14, 5:19 am, Candide Voltaire <candideguev...@xxxxxxxxx> wrote:

I was wondering if the following statement is always true

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

Can anyone here confirm this or provide a counterexample

regards,
Candide

In this 'best of all possible worlds' you are correct.  (As long as
you count the stray inductance and capacitance in the circuit.)

George Herold

As long as the filter is an "all pole" filter, the statement is
correct. However, some filters, such as the elliptic (Cauer) also
have real zeros, which require more Ls and Cs. By definition, the
order of the filter is the order of the denominator polynomial of its
transfer function. Consider, for example, a 3 pole elliptic filter.
This filter has 3 poles (because the order of the denominator
polynomial is 3) and 2 zeros. The total number of reactive
components (inductors and capicitors) is 4, even though the order of
the filter is 3. The same problem exists with inverse Chebyshev
filters, since these filters also have real zeros, as well as complex
poles, so the number of reactive components exceeds the order of these
filters. For the other classical filter types (Butterworth,
Chebyshev, Legendre, Bessel) the number of reactive components equals
the order of the filter. Pardon my long-windeness :-)
Regards,
Jon
.

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