Multiple solutions for the synthesis of microwave filters with predistorted transfer functions .
Résumé
This paper examines the problem of multiple solutions for the synthesis of predistorted transfer functions of microwave narrow-band bandpass filters. The losses in narrow-band bandpass filters affect the flatness of the in-band transfer function and the selectivity sharpness. The predistortion is a synthesis technique used to recover acceptable flatness and selectivity of the transfer function knowing the level of losses. Applying this technique, multiple solutions are available, leading to different circuit realizations and, as a consequence, to different electrical behaviors in presence of losses. In particular, the return loss level in the passband depends deeply on the selected solution. The return loss is generally not a problem in case of IMUX filters but it has generally to be considered in other applications. Simple rules are given in this article in order to select the more appropriate solution in the late case. The theoretical demonstration is illustrated by an example of a 5 pole filter with a predistorted transfer function. The multiple solutions are determined and the effects of losses in term of minimum in-band return loss level are compared for each solution. An experimental model is then used to validate these theoretical results.