Feature Upgrade Request: Add option for Reduced Q Elliptic Filters
A common limitation in the filter design in foton are the generically infinite Q elliptic filters (and cheby2 filters). These filters are often way more aggressive than are needed -- specifically at the notch frequencies -- which means that one looses more phase than is necessary when implementing, for example, a low-pass.
It would be awesome if we could implement an additional option to the "elliptic" pop-up menu that was the "Q" of the notches, where the user has control over the Q of the notches.
For example code / math for how to implement this, check out the function @brian.lantz wrote, https://svn.ligo.caltech.edu/svn/seismic/Common/MatlabTools/myellip_z2.m
Below, I use this function to demonstrate the phase benefits of reducing the Q of a 1 Hz corner frequency, 1 dB ripple, 40 dB stop-band suppression, 4th order elliptic filter from "infinite" (5000) to 10. 15 degrees of phase at the corner frequency, essentially for free**!
** (if you don't have some sharp mechanical/electrical/digital feature in your control plant that you're trying to strategically invert with the notch of the infinite Q elliptic in addition to suppressing high frequencies, which is why we should retain the "infinite" Q option -- but this "infinite" Q filter can be replicated by just entering in a Q of > 1000.)
