A surrogate model is proposed to model a previously published second-order tunable filter. The surrogate model can be represented by a coupling matrix in a doublet configuration, and a simple method based on the even-odd mode bisection is given to extract the coupling matrix. From the extracted parameters, one can decide how to design a filter with prescribed quasi-elliptic responses and how to adjust varactors to reconfigure the response. Using the proposed methods, the second-order filter can be configured to have a finite transmission zero on either the lower side or the upper side of the passband in a prescribed location even when the center frequency of the filter is tuned. Both simulated and measured results of an example are presented.