A Tunable Active Phase Shifter Using a Thin Film Ferroelectric Capacitor

Abstract

This paper describes an active phase shifter with a large amount of variable phase. We propose a design that has second-order all-pass network characteristics and that uses a tunable ferroelectric capacitor. The transmitted phase is changed by varying the capacitance of a ferroelectric capacitor. A computer simulation is presented that shows that the network, even with markedly non-ideal transistors, can provide a true all-pass response over the frequency band of interest (100 MHz–400 MHz). These simulated results demonstrate an analog tunability of about 200° with a gain variation of about 3 dB at 300 MHz—when using a Ba0.96Ca0.04Ti0.84Zr0.16O3 (BCTZ) capacitor with a tunability of 2:1. The simulation performed at 300 MHz because the physical layout of the real life circuit will be done mostly with the discrete components. As the self resonance frequency of most of the discrete components lies in the few hundreds of MHz range, our preferred frequency is a practical one to deal with. The simulation also predicts a flat band gain of approximately 10 dB with ± 2 dB of gain ripple.