Ultrawide-band tunable true-time delay lines using ferroelectric varactors

This paper reports on compact tunable true-time delay lines based on ferroelectric (Ba0.25Sr0.7.5TiO3) varactors integrated on high-resistivity silicon. The delay lines are based on lumped elements, physically implemented as synthetic coplanar-strip lines. An approximate analytical design procedure, exactly valid for omega -> 0, is proposed. The physical size of the fabricated delay lines is 2.0 x 0.33 mm(2), including bias pads. Measurements are performed from room temperature (RT) down to 80 K. The measurements reveal ultrawide-band characteristics for both group delay and insertion loss. At RT, the absolute group delay is tau(delta)(RT, 0 V) approximate to 70 ps with tunability of 20 % under 20-V dc bias, the insertion less than 3.5 dB, and the reflection loss better than 12 dB below 20 GHz. At 145 K, the absolute group delay is increased to tau(delta)(145 K, 0 V); approximate to 100 ps with a tunability of 50% under 20-V applied bias. At 7 GHz, the insertion loss is 3 dB, resulting in figures-of-merit of 0.03 dB/ps and 50 ps/mm. The leakage current at RT is less than 0.1 mu A.