Planar lossy filters for satellite transponders

Abstract

Several designs of lossy filters for receivers in satellite transponders have been investigated in order to improve the performance in terms of flatness compared to a classical hairpin filter design. Flatness and insertion loss performances for all solutions are summarized. The reference design is a microstrip filter made of coupled hairpin resonators. Two approaches have investigated for improving the flatness of the reference design. Lossy filter designs have been compared to the reference, considering the same specifications and the same technology. An in-line network with resistive cross-couplings and a transversal network with heterogeneous Q resonators have been designed and fabricated first. Theoretically, the transversal network leads to better performances in terms of flatness, but its implementation is generally difficult, especially considering the input/output junctions between multiple paths, which are naturally dispersive, causing spurious transmissions in the stopband. Moreover, a Monte Carlo analysis has been performed, showing higher sensitivity of this later solution with respect to manufacturing tolerances. Consequently, considering measured performances, in-line networks with resistive cross-couplings appear to be the best solution for implementing our receiver filter. In particular, the version with two RCCs appears as a good compromise between flatness and insertion loss. Finally, the in-line network has been transformed introducing non-resonant nodes and additional resistive cross-couplings in order to design absorptive lossy filters. The resulting filters allow attenuating the reflected wave substantially with a reduced impact on the absolute level of losses.