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IEEE Transactions on Electron Devices

Margins of a 16-ps/bit interferometer shift register

H. Beha, W. Jutzi, G. Mischke

Published Oct 1, 1980• 15 citations
Publisher

Abstract

Static and dynamic properties of an interferometer with five equal Josephson junctions are investigated as a single-flux quanta shift-register model. A flux quantum is stored in any loop without a bias current or magnetic field. A flux quantum is shifted from one interferometer loop to the adjacent one by applying a magnetic-field control pulse only to one in terferometer loop at a time. Shift current margins determined statically and dynamically are larger than about ±30 percent; the write current margins are ±25 percent. Within these tolerances, and for a maximum Josephson current density of 10 kA/ cm2, delays of 16 ps/bit, including shift and write, have been simulated.

DOI
10.1109/t-ed.1980.20125

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The talk is based on an overview which was published recently (1). For this reason, only an abstract is given here, which includes Later results not contained in the aforementioned overview. The presently-known Josephson memory cells store information as persistent currents in superconducting loops, or what is equivalent as a magnetic flux in the area of the ring. This method of storage is nonvolatile and has no power consumption in the storing state. In a superconducting ring system, the magnetic flux is quantized, and cells have been built which operate either with only one flux quantum, or with many of them. For the Latter cells with many flux quanta (ring cells), quite a number of designs exist. They have one or two Josephson junctions in the ring, which allow writing. An external junction which is controlled by the ring current serves for non-destructive reading of the information. The smallest cell realized had an area of 900 ?m2 with a 2 ?m minimum line width. Switching speed was ? 80 ps. Cells which store information as a single-flux quantum (single flux-quantum cells = SFQ cells) in the ring, usually contain two Josephson junctions used for reading and writing. Read-out in most of these cells is destructive, but methods of reading non-destructively have also been proposed. The smallest device made had an area of ? 150 ?m2 with a minimum line width of 2 ?m. The read signal had a rise time ? 100 ps.

European Solid-State Circuits Conference (ESSCIRC)•Sep 1, 1978•0 citations
Static and dynamic properties of an interferometer with five equal Josephson junctions are investigated as a single-flux quanta shift-register model. A flux quantum is stored in any loop without a bias current or magnetic field. A flux quantum is shifted from one interferometer loop to the adjacent one by applying a magnetic-field control pulse only to one in terferometer loop at a time. Shift current margins determined statically and dynamically are larger than about ±30 percent; the write current margins are ±25 percent. Within these tolerances, and for a maximum Josephson current density of 10 kA/ cm2, delays of 16 ps/bit, including shift and write, have been simulated.
A shift register is described which employs as information bits the flux quantum vortices occurring in Josephson junctions of extended dimensions. The positions of the vortices can be controlled and manipulated by an appropriately designed circuit geometry and by application of currents and magnetic fields. The time required for switching of the position of a vortex can approach the period of the Josephson plasma oscillation (∼10 ps) and the intrinsic energy dissipation is typically ≲ 10-18J/shift.
Timing jitter is an important factor in determining the limit of speed and scale of single-flux-quantum circuits. We estimated the timing jitter by a simple method using 2-bit shift registers. It was estimated by observing the transition between 1- and 2-mode operations while changing the bias current to the clock line. A sharp transition was observed in the shift register with a smaller number of Josephson junctions in the clock and data lines, suggesting the existence of a finite timing jitter. The timing jitter per Josephson junction was estimated to be 0.09 ps at a designed bias current. The temperature dependence supported that the timing jitter comes from thermal fluctuation.
We designed several single flux quantum (SFQ) flip-flops and logic gates composed of Josephson junctions (JJs) and π-shifted JJs (π-JJs) to quantitatively evaluate effectiveness of introduction of π-JJs into the SFQ logic circuit. One-output flip-flops and logic gates were designed on the basis of the circuit design methodology we built for the SFQ circuit containing π-JJs. The designed flip-flops and logic gates have wide operating margins, the dc-bias margins of larger than ±30%, and device parameter margins of ±18%, though the static power consumption are reduced compared to conventional ones composed of JJs. We found that the difference in the critical current density between JJs and π-JJs does not affect the operating margins of the SFQ flip-flop composed of JJs and π-JJs. We devised a circuit structure of the delay flip-flop with complementary outputs composed of JJs and π-JJs (π-DFFC). The analog circuit simulation shows the dc-bias margin of the π-DFFC is larger than ±33%. These results indicate that the large-scale SFQ logic circuit system can be implemented using the flip-flops and logic gates containing π-JJs.
The talk is based on an overview which was published recently (1). For this reason, only an abstract is given here, which includes Later results not contained in the aforementioned overview. The presently-known Josephson memory cells store information as persistent currents in superconducting loops, or what is equivalent as a magnetic flux in the area of the ring. This method of storage is nonvolatile and has no power consumption in the storing state. In a superconducting ring system, the magnetic flux is quantized, and cells have been built which operate either with only one flux quantum, or with many of them. For the Latter cells with many flux quanta (ring cells), quite a number of designs exist. They have one or two Josephson junctions in the ring, which allow writing. An external junction which is controlled by the ring current serves for non-destructive reading of the information. The smallest cell realized had an area of 900 ?m2 with a 2 ?m minimum line width. Switching speed was ? 80 ps. Cells which store information as a single-flux quantum (single flux-quantum cells = SFQ cells) in the ring, usually contain two Josephson junctions used for reading and writing. Read-out in most of these cells is destructive, but methods of reading non-destructively have also been proposed. The smallest device made had an area of ? 150 ?m2 with a minimum line width of 2 ?m. The read signal had a rise time ? 100 ps.