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 building blocks of an HTS counting ADC, feature a directly coupled readout junction stack similar to those used in LTS RSFQ circuits. The directly coupled circuit delivers higher readout voltage, hence potentially higher operating speed than magnetically coupled alternatives. The circuits were fabricated using two multilayer HTS SNS junction integrated circuit technologies (one at Northrop Grumman and the other at TRW) featuring three YBCO depositions. 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 I/sub C/R/sub N/ product, and as high as 100 /spl mu/V at 65K. Test results are compared with predicted fabrication process parameter requirements. To increase margins and tolerance to process parameter variations, improvements to the design have been made using JSpice and MALT.