Synthesis of Cross-Coupled Triple-Passband Filters Based on Frequency Transformation

A synthesis method is presented for triple-passband filters. The proposed method is based on a frequency transformation from the normalized frequency domain to the actual frequency domain. A novel triple-passband filter topology with cross coupling is obtained, which consists of admittance inverters and parallel resonators. The coupling coefficients and the external quality factors can be calculated analytically. To validate the theory, a triple-passband filter is designed and fabricated using microstrip structure. The theoretical response meets the specifications very well, and the measured result shows a good agreement with the simulated one.