| SUNY RSFQ Cell Library |
... one does not need to have light for having the speed of light.
K. Likharev
All parameters are in PSCAN dimensionless units .
These two elements are essentially Josephson Transmission Line segments with parameters chosen to make one end match a "standard" JTL and another end to have an impedance matched with a passive superconductive microstrip transmission line (MSL).
While active JTLs are perfect to pass an RSFQ pulse over a short distance, for longer signal paths they require too many Josephson junctions and large chip area, they also have large propagation delay and introduce too much timing uncertainty due to technological spread and thermal noise in Josephson junctions. For any realistic circuit (except ones which have purely systolic structure) it is beneficial to make long-distance signal transfers using matched superconductive MSLs.
Such lines, with their very low attenuation and dispersion, allow one to pass picosecond waveforms for distances well exceeding the typical chip size, with a low crosstalk. As a consequence, ultrafast digital signals can be passed along the chips ballistically (rather than diffusively) with a propagation speed c approaching that of light [in the media].
K.Likharev, V. Semenov, "RSFQ Logic/Memory Family..."
Ballistic propagation of pulses in superconductive MSL means that a large number of pulses can coexist (and propagate) through the line.
Timing uncertainty introduced even by a long MSL is much smaller than that of a long JTL since MSL does not contain any Josephson junctions which are subject to relatively large technological variations and thermal noise.
This particular pair of driver/receiver cells were optimized by V. Semenov to match MSLs with impedance 2.3 Ohm which is convenient for present-day HYPRES technology. The extremely small resistor in superconductive path (large green rectangle on receiver layout below) provides decoupling between driver and receiver cells.
An MSL with this driver/receiver pair was experimentally tested to have margins of +/- 30%.
View online the files necessary to simulate the circuit with PSCAN or download the whole compressed directory.
These cells were optimized for interconnectivity to be a part of cell library.
In this simulation two pulses were injected in an MSL (simulated as 16 LC segments). The top plot shows pulse propagation through the MSL driver, the middle one through the MSL receiver. The bottom plot shows voltages on three capacitors within the MSL.
The physics of superconductive transmission lines was analysed well before RSFQ:
R. L. Kautz, "Picosecond pulses on superconductive striplines", J. Appl. Phys. 49(1), pp. 308-314, January 1978.
R. L. Kautz, "Miniaturization of normal-state and superconductive microstriplines", J. Res. NBS, vol. 84, pp. 247-259, February 1979.
The possibility of their use in RSFQ circuitry was discussed in:
K. Likharev and V. Semenov, "RSFQ logic/memory family: A new josephson-junction technology for sub-terahertz clock-frequency digital systems", IEEE Trans. Appl. Supercond., vol. 1, pp. 3-28, March 1991.
Experimental verification of RSFQ pulses transfer in MSL over 1 cm length was presented in:
S. Polonsky, V. Semenov, and D. Schneider, "Transmission of Single-Flux-Quantum Pulses along Superconducting Microstrip Lines", Applied Superconductivity, vol. 3, March 1993.
This particular design of driver/receiver pair was used in:
V. Semenov, Y. Polyakov, and A. Ryzhikh, "Decimation filters based on RSFQ logic/memory cells", in 6th Int'l Supercond. Electronics Conf. Extended Abstracts, Berlin, Germany, pp. 344-346, June 1997.
Paul BUNYK
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