Receive shaping filter manufacture

An 8-pole Elliptic Butterworth shaping filter for QPSK regenerative uplinks was made from invar and silver plated using TE111 dual-mode circular cavity technique. Tuning is acceptable, though not satisfactory. Low weight and very low loss cavities are achieved. The transmit filter is square root raised cosine type with a 40% rolloff factor. The measured transfer function of the prototype was simulated to produce a 0.72 dB degradation at a bit error rate = 0.0001. Since the computed response of the filter produces a degradation of 0.39 dB, a 0.33 dB implementation margin can be inferred. Filter complexity is such as to require a long and difficult adjustment phase and the final filter response is likely to suffer from a not negligible implementation margin that brings the actual performance away from its theoretical capability. A six pole filter may be closer to theory.