The effect of frequency dependent couplings on filter response

The paper shows that coupled resonator filters with the frequency-dependent coupling coefficients which vary linearly with the normalized lowpass frequency can be transformed into equivalent circuits with frequency-independent coupling coefficients while maintaining the filter response. In particular, the topology and strength of the frequency-dependent coupling can be determined beforehand according to the actual situation. It is convenient to build a circuit model to simulate the filter response under the actual frequency dependent situation. A filter example is designed and the simulation results are in good agreement with the theory which verifies the correctness of the theory.