Lossy synthesis techniques are used to synthesize high performance filters using resonators with a finite Q. With these techniques a transmission response of a lossless filter with lowest possible roll-off is obtained, except for an overall constant additional insertion loss. The out-of band performance is not affected. These types of filters are useful in receiver architectures where the filter can be placed following an amplifying stage and some increase in insertion loss can be tolerated without affecting the noise figure. Due to the lower resonator Q that can be employed, low cost and compact filters (such as microstrip filters) can be realized. Hence this work presents the synthesis and design of two planar lossy filters. Both filters have been designed using microstrip technology, with center frequencies of 1 GHz and 1.6 GHz, and bandwidth of 36 MHz and 80 MHz, respectively. Both filters exhibit a flat in-band response with an insertion loss of 10 dB and 7.5 dB, respectively. The topologies obtained from the synthesis result in a 4th and a 6th order filters at 1 GHz and 1.6 GHz, respectively. Both topologies present a uniform Q distribution throughout the networks, this is all the resonators of the filter are equal to 200 and 290, respectively. In contrast with conventional filter networks, where all the couplings are reactive, lossy filters require some of the couplings to be resistive. The main difference between the two filters presented is the type of frequency response. While the filter at 1 GHz synthesizes a response with a stopband 10 dB return loss, the filter at 1.6 GHz synthesizes a purely reflective response in the stopband. The presentation will also show details of the software package that has been written to synthesize filters following various forms of pre-distortion, classical (no-loss considered in the synthesis) and lossy filter synthesis. This software obtains the coupling matrix of several network topologies for a given response and allows performing rotations on them to find the desired topology. Additionally, the software allows to evaluate the effect of loss in the networks resulting from the synthesis, even in those cases where the synthesis results in an ideal lossless network (i.e., classical and pre-distortion synthesis).

Peer Reviewed