Packaged Tunable Bandpass Filters Based on Varactor-Loaded Spiral Combline Topology

This article describes the winning work at the student packaged broadband tunable bandpass filters design competition at the 2012 IEEE MTT-S International Microwave Symposium (IMS2012), Montreal, Canada. According to the competition design rules, the broadband tunable bandpass filter circuitry must be planar on a printed circuit board and securely mounted in the metal package with a copper tape lid. The lumped surface-mount components, including resistors, capacitors, and varactors, are allowed and must be tightly mounted on the printed circuit board. The evaluation board will be an unenclosed printed circuit board with 50-X traces connecting the RF ports of the package to the standard 50-X female subminiature version A (SMA) connectors. The filter performance was evaluated based on the measurement performance between the SMA connectors. Figure 1 illustrates the evaluation criteria of tunable bandpass filters. During the in-situ competition evaluation, the voltage range limited between 0 and 20 V was provided for the designers to realize two passbands. The first band specifications are: 1) the passband 965-1035 MHz, 2) the passband insertion loss less than 4.5 dB, 3) the passband return loss more than 15 dB, an 4) the insertion loss for the frequency lower than bandpass filter architecture was designed, which employed the varactor-loaded spiral combline resonator topology with stepped-impedance coupled lines at the input and output ports. The design considerations and measurement results are described.