Frequency Response of Low Pass Elliptic Filter 1
The Elliptic Filter has Poles in the passband and zeros in the
stopband resulting in both passband and stopband ripples in the
frequency response. The Elliptic filter is characterised by 4
parameters, As the minimum stopband attenuation,
Ap the passband ripple, n the order of the filter and
the ratio of the lowest frequency where the attenuation reaches
its minimum value divided by the highest frequency where the attenuation
is less than the passband ripple.
The poles can be derived by specifying 3 of the 4 parameters. The 4th
parameter can then be derived from the other 3.
In the applet below the parameters As, Ap, and
are specified and the smallest value of n is calculated.
The relevant equations can be found in :-
C. Britton Rorabaugh - Digital Filter
Designer's Handbook - Publ by Mc Graw Hill.
In the applet, the magnitude response is shown in red and pink, the phase
shift is shown as bright green(for the poles) and darker green(for the zeros), the
group delay is shown in cyan.
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COPYRIGHT © 2005 Cuthbert Nyack.