Design of ferroelectric phase shifters for minimum performance variation over temperature

Abstract

This paper investigates design techniques for continuously variable ferroelectric phase shifters to minimize their performance variations over temperature. Various compositions of barium strontium titanate (BST) thin films on sapphire substrates were characterized in the temperature range of 0/spl deg/C-80/spl deg/C to assess their change in capacitance and tenability. It was found that a minimum capacitance variation (C/sub max//C/sub min/) of around 1.17 could be obtained using a Ba/Sr ratio of 1.2 and (Ba+Sr)/Ti ratio of 0.8. Based on this composition, all-pass network phase shifters were designed using 2 /spl mu/m- and 4 /spl mu/m-spaced interdigital capacitors (IDCs). In the case of the phase shifter with 2 /spl mu/m-spaced IDC, a phase shift of more than 100/spl deg/ was obtained over a 40 V bias range at 2.4 GHz. The loss figure-of-merit (FOM) of the phase shifter was held to 62.5/spl deg//dB /spl plusmn/ 2.5/spl deg//dB from 10/spl deg/C to 75/spl deg/C. The variation of a phase shift was held to only 4/spl deg/ in the temperature range of 10/spl deg/C-50/spl deg/C.