Experimental Verification of Superconductor Digital Circuits

Licentiate thesis, 2005

Superconducting digital technology based on Rapid Single Flux Quantum logic (RSFQ) offers more than 50 times advantages in speed and $100$ times less power consumption than today's mainstream semiconductor electronics. This technology is attractive for many applications and in particular for Digital Signal Processing (DSP) for multi-user detection in a 3G wireless systems being developed at Chalmers University. The being developed DSP is targeting record performance of 30 giga operation per second. It consists of a digital core, an RSFQ Multiply Accumulate unit (MAC), RSFQ fast cache memory, and external semiconductor memory. Such an architecture allows to implement memory fetch at 30 GHz speed. The development of the complex high speed cryogenic digital system requires solution of the several interface, packaging and testing problems: cooling down to $4$ K operational temperature, magnetic shielding, multi channel high bandwidth interfacing of low voltage signals in a hybrid RSFQ-CMOS memory, and implementation of the high speed test setups. The key steps towards the solution of these problems is the subject of this thesis. The main emphasis of the work is on the development of room temperature interfacing equipment for digital superconductor electronics and experimental testing of RSFQ DSP components. The work has involved the following parts: development of the low frequency cryoprobe, design of the full system MCM cryoprobe, low frequency testing of RSFQ components and development of a high speed testbench. A large part of the work is connected to the experimental verification of DSP components and their gradual improvement against functionality. The RSFQ circuits tested were fabricated at advanced multilayered processes available from NGST 8 kA/cm^2, HYPRES 1 kA/cm^2 and HYPRES 4.5 kA/cm^2. A number of different designs where experimentally measured at low speed using the data acquisition system Octopux. In order to investigate the influence of bias-supply lines on RSFQ circuit operation, measurements were performed to characterize the mutual inductance between different layers. For verification of high bandwidth interconnects, passive micro-strip lines with drivers and receivers where tested. From the being developed RSFQ DSP several of the key logic blocks have been experimentally tested at low speed: 4*4, 5*5, a new 4*4 parallel multiplier, 20*5 and 4*15 shift registers memories. For high speed testing a testbench employing RSFQ shift registers for clock injection/capture was successfully implemented and verified up to 33.5 GHz.