Design and realization of bandpass filters using composite resonators to obtain transmission zeros

A synthesis theory of bandpass filters (BPFs) with modified Butterworth-Chebyshev or Chebyshev-Chebyshev frequency response is proposed. Instead of using cross-couplings between non-adjacent resonators, composite resonators are employed to produce transmission zeros in order to get sharp attenuations of the filter. The filter has as many transmission zeros in both its lower and upper stopbands as the number of resonators it used. The theory is developed based on the well-know lumped-element equivalent circuit of a BPF. Therefore, the classical design equations remain valid provided a few new formulas are supplemented. An easy realization method of the filter using microstrip lines is described, and 2-degree and 3-degree example filters with modified Chebyshev-Chebyshev response are designed. The measured filter characteristics agree excellently with the theoretical predictions.