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

This paper reports on compact tunable true-time delay lines based on ferroelectric (Ba_0.25Sr_0.75TiO₃) 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 ω→0, is proposed. The physical size of the fabricated delay lines is 2.0×0.33 mm², 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 τ_δ(RT, 0 V) ≈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 τ_δ(145 K, 0 V) ≈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 μA.