HTS SFQ T-flip flop with directly coupled readout

The authors report the design, fabrication, and test results of single flux quantum (SFQ) toggle flip flop circuits. These circuits, investigated as b uilding blocks of an HTS counting ADC, feature a directly coupled readout j unction stack similar to those used in LTS RSFQ circuits. The directly coup led circuit delivers higher readout voltage, hence potentially higher opera ting speed than magnetically coupled alternatives. The circuits were fabric ated using two multilayer HTS SNS junction integrated circuit technologies (one at Northrop Grumman and the other at TRW) featuring three YBCO deposit ions. Initial testing demonstrated toggling of a T-flip flop in response to SFQ pulses generated by integrated dc/SFQ converters. The readout voltage is shown to approach 40% of the junction ICRN product, and as high as 100 m u V at 65K. Test results are compared with predicted fabrication process pa rameter requirements. To increase margins and tolerance to process paramete r variations, improvements to the design have been made using JSpice and MA LT.