Practical evaluation of GaN switches in tunable RF filter banks

Abstract

This thesis evaluates the use of Gallium Nitride (GaN) RF switches in a highpower tunable band-pass filter bank covering 225–520 MHz. The work includes filter synthesis, ADS optimization, EM simulation of critical PCB structures, prototype fabrication, VNA measurements, troubleshooting, and RF power testing. The simulated second-order design covered the intended 225–520 MHz frequency range with fractional bandwidths close to 5% and insertion losses of approximately 3.6–5.5 dB. Measurements of the GaN switch evaluation board showed generally similar behavior to the supplied engineering model, although differences increased at higher frequencies. The fabricated prototype could not be fully validated as a complete tunable filter bank because several switches failed during assembly and troubleshooting. The final measurements were therefore performed on a partially reworked circuit. The measured prototype showed the same general resonant behavior as the modified simulation, but with an approximately 10% downward frequency shift and worse matching. Troubleshooting and fault modeling indicated that incomplete RF grounding, rework-related parasitic capacitance, component faults, and assembly effects contributed to the difference between simulation and measurement. The maximum RF power handling and P1dB of the prototype could not be determined with the available measurement setup. EM simulation showed that the switch-node structure had an impedance of approximately 200 Ω, which increased the expected RF voltage stress across the switched capacitors. Capacitor voltage rating was therefore identified as a potential limitation for future high-power testing, but the limiting mechanism was not experimentally isolated. The results show that GaN RF switches remain a possible alternative for high-power tunable filter applications, but the surrounding passive network, grounding, PCB parasitics, and measurement setup are important parts of the complete implementation. Future designs should use improved RF grounding, higher-voltage capacitors, lower switch-node impedance, and simpler staged prototypes before full high-power validation.