A compact semi-lumped tunable complex-impedance transformer

This article describes the design and performance of a compact tunable impedance transformer. The structure is based on a transmission line loaded by varactor diodes. Using only two pairs of diodes, the circuit is very small with a total length of only λ/10. Both the frequency range and the load impedance can be tuned by varying the varactor bias voltages. Our design provides a tunable operating frequency range of ± 40% and an impedance match ranging from 20 Ω to 90 Ω at 0.8 GHz and from 30 Ω to 170 Ω at 1.5 GHz. In addition, a new approach that considers losses for the simulation and measurement of this impedance transformer was investigated. The measured performance of a 1 GHz prototype design confirmed the validity of this new approach.