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This paper describes a synthesis method for quadruple-passband microwave filters. A frequency transformation method is developed for finding the locations of poles and zeros of the filter response. An eight-pole microstrip quadruple-passband filter is synthesized for validation, with designed pass bands at 1.85-1.88, 1.943-1.97, 2.03-2.057, and 2.12-2.15 GHz. The filter is realized in microstrip and the measured results show good agreement with the theoretical ones.