The effects of component Q distribution on microwave filters

The effects of lossy components on the passband response of a bandpass filter are studied in this paper. It is found that the resonator Q has pronounced effects on the insertion losses, while its effects on the group delays are rather minor. The first-order approximation is used to estimate the deviations of insertion loss and group delay from the ideal. The effects of a lossy resonator in each stage are individually analyzed to evaluate which components are more critical and should be paid more attention. Filters designed by the predistortion technique are also discussed with emphasis on the effects of component Q distribution on the transmission level. When resonators with different Q are used in a filter, a bell-shaped Q distribution is proposed to achieve the optimal passband response. Finally, three filters with different component Q distributions were designed by combining the dielectric and microstrip-line resonators, and the measured results agreed well with the theoretical predictions.