FABRICATION AND CHARACTERIZATION OF YBCO MICROSTRIP DELAY-LINES

We have fabricated and characterized microstrip delay lines with YBCO films as the material for the strip and polished bulk copper for the ground plane. The film-carrying LaAlO3 substrate is flipped over the copper ground plane and separated from it by a polyimide laminate that acts as the microstrip dielectric. Linewidths are varied from 100 to 600-mu-m, total length from 10 to 65 cm. Two winding shapes are evaluated, concentric circular and serpentine. For a total geometric length of 60 cm, a TDR-measured delay of 5 nanoseconds is obtained with a line impedance of 50-OMEGA. These delay lines require an area of high quality YBCO films of one square inch. The directly-measured transmission loss (S21) up to 10 GHz is ten times lower at 77 K than for identical lines made with gold films and comparable at 4.2 K to the loss of identical lines made with niobium films. These are to our knowledge the longest, directly-coupled YBCO delay lines operating at 77 K, fabricated and measured to date. We discuss the design constraints, fabrication, and properties of these lines and the issues involved in obtaining longer delays.