I am attempting to understand how to manipulate portions of a cascaded series of biquad notch filters in a control system I am working with.
As part of determining the composite response of all the cascaded filters, I am starting with looking at the individual biquad filters. I have taken the existing control system coefficients that I have for the filters and have saved the various biquad filters in the Filter design tool. When I use the MATLAB/Simulink filter design tool, I obtain an unexpected response as well as an unstable root/pole plot. I have excerpted an image below of one of the filters from the control system.
My specific questions are:
Is this response still consistent with a notch filter?
If this is still applicable as a notch filter, why would the filter start with a negative magnitude and then return to a positive magnitude? For context, I am used to the analog realm and a notch filter starting at 0 db and ending at 0db.
If I needed to attenuate a specific frequency as viewed from power spectra of the feedback in the control system, would it be appropriate to modify a particular notch filter of my cascaded series to focus in on that frequency or since they are cascaded, would I have to modify all terms slightly?
Thanks for any insight.