Miscellaneous active electrical nonlinear devices – circuits – and – Gating – Superconductive device
Reexamination Certificate
2001-02-23
2003-04-15
Callahan, Timothy P. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Gating
Superconductive device
C327S528000
Reexamination Certificate
active
06549059
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to Josephson transmission lines, and more particularly, to Josephson transmission lines that employ underdamped Josephson junctions to enhance the speed of signal propagation.
2. Discussion of the Related Art
With recent developments in superconductor technology, superconductor devices based on Josephson Junctions effect are replacing conventional devices based on semiconductor technology for high performance and low power. Digital circuits that employ superconductor technology are often desirable because these devices can simultaneously consume very little power and operate at very high clock speeds as compared to their semiconductor counterparts. Because of low power consumption, it is possible to make systems very compact. Other benefits for signal transmission using superconducting devices include reduced signal attenuation and noise. Digital circuits that employ superconductor devices can operate at clock speeds exceeding 100 GHz.
Josephson transmission lines (JTL) are typically employed in superconductor digital circuits to manipulate and transmit narrow pulse width signals at low power. JTLs employ Josephson junctions at predetermined intervals along the transmission line that regenerate and transmit pulse signals as a single flux quanta (SFQ), or a single quanta of magnetic flux. The Josephson junction functions as a tunneling device that includes two opposing superconductive films, separated by an oxide dielectric layer. A current bias is applied to each Josephson junction. These junctions then switch or flip in response to an incoming transient voltage pulse, regenerating that pulse for the next junction, and returning to their initial state where they are ready to respond to the next pulse. Each Josephson junction generates a voltage pulse when it switches. Typical SFQ pulse signals generated by a Josephson junction are 2-3 ps in width and 1 mV. The time integral of the voltage pulse is equal to a single quanta of magnetic flux &PHgr;
0
=2.07×10
−15
Volt-seconds.
FIG. 1
is a schematic diagram of a standard superconducting Josephson transmission line (JTL)
10
that is representative of the known transmission lines of this type. The JTL
10
is comprised of a sequence of JTL segments
11
. The JTL segment
11
includes an inductor
16
connecting adjacent junctions, a resistively shunted junction circuit
18
connecting one end of inductor
16
to a common ground return, and a biasing resistor connected between the top of the junction
18
and a current source
12
. The isolation inductor
16
, provides inductive isolation between adjacent junctions
11
and allow propagation of the SFQ pulse along the JTL
10
. The biasing resistor
14
is connected in series with a current source
12
which provides an equal amount of current to each of the Josephson junction circuits
18
and
20
.
The Josephson junction circuits
18
and
20
of the JTL
10
are spaced apart at predetermined intervals along the JTL
10
and act to regenerate the SFQ pulses at each stage. Each Josephson junction circuit
18
and
20
is shown as an equivalent circuit of a resistor and Josephson junction in a parallel array. The equivalent elements of the JTL segment
11
and the Josephson junction circuit
18
will be described with the understanding that all of the Josephson junction circuits in the JTL
10
have the same elements. The Josephson junction circuit
18
includes a Josephson junction
22
that is connected in series with a first parasitic inductor
24
. The Josephson junction
22
and the first parasitic inductor
24
are connected in parallel with a damping resistor
26
and a second parasitic inductor
28
. The first and second parasitic inductors
24
and
28
are connected to a reference ground
30
opposite the Josephson junction
22
and the damping resistor
26
. The damping resistor
26
shunts the Josephson junction
22
and helps to define its response to incoming signals. The damping resistor
26
is chosen such that the Stewart-McCumber parameter [W. C. Stewart,
Applied Physics Letters
12, 277 (1968). D. E. McCumber,
Journal of Applied Physics
39, 3113 (1968)], which parameterizes how a Josephson junction is damped, falls between 1 and 2.
When an SFQ pulse impinges the JTL segment
11
, the Josephson junction
22
flips, or increments its internal degree of freedom, or phase by 2&pgr;. When the Josephson junction
22
flips, the Josephson junction
22
regenerates and transmits an SFQ pulse to the next junction. When the next junction receives the SFQ pulse, it recreates and propagates the SFQ pulse to the following junction. Typically, the travel time of the SFQ pulse from one Josephson junction to the other ranges between 2.5-4 picoseconds(ps) depending on the degree of damping (Stewart-McCumber parameter) and the current bias. At any given time, at least two junctions are in the process of advancing their phase.
When the junction flips, the Josephson junction
22
regenerates a voltage pulse having a fixed time integral &PHgr;
0
. In cases where the junction carries current that is less than a predetermined threshold, the Josephson junction does not flip in response to the input pulse and fails to regenerate and retransmit the voltage pulse to the next junction. On the other hand, when the junction carries current that exceeds a predetermined threshold, the Josephson junction goes into a voltage state where it emits a pulse train, or multiple voltage pulses in rapid succession although only one pulse is expected, and leads to erroneous results of a circuit. Damping resistor
26
helps to prevent JTL
10
from going into the voltage state. Conventionally the magnitude of the inductance of the isolation inductor
16
is such that the product of its inductance and the Josephson junction's critical current (the L-I
c
product) is in the range 0.7-1.0 milliAmp-picoHenrys. In addition, each junction
11
is current biased with about 60%-80% of the critical current of the Josephson junction.
Furthermore, as the SFQ pulse is transmitted down the JTL
10
, the damping resistor
26
generates Johnson noise, or current noise that effects the junction
22
. Because the speed of propagation depends upon the applied current bias, the current noise results in timing jitter, or uncertainty in the time of flight of the SFQ pulse. Additionally, because the Johnson noise applied to each junction
22
is independent with respect to the other junction
11
, the timing jitter of each junction
22
is also independent with respect to the other junction. The accumulation of this timing jitter in the JTL
10
increases in proportion to the square root of the number of JTL segments
11
in JTL
10
. The damping provided by the damping resistor
26
and the first and second parasitic inductors
24
and
28
increases the propagation delay and results in a slower speed of signal propagation on a chip as could otherwise be realized. The propagation speed of an SFQ pulse along JTL
10
is about one tenth of the propagation speed along a passive microstrip transmission line fabricated in the same superconducting integrated circuit (IC) technology. Therefore, the standard JTL
10
available for SFQ based devices is slow, and adds timing jitter or timing uncertainty to the signal it carries. This timing uncertainty ultimately limits the maximum clock speed of the clocked logic circuits and the performance of superconducting devices that employ Josephson junction transmission lines.
What is needed is a superconductor Josephson transmission line that provides the transmission and distribution of the SFQ pulses on a superconducting integrated circuit without suffering from the drawbacks discussed above. It is therefore an object of the present invention to provide a superconducting Josephson transmission line that transmits the SFQ pulses at faster speeds with reduced timing jitter.
SUMMARY OF THE INVENTION
In accordance with the teachings of the present invention, a Josephson t
Harness & Dickey & Pierce P.L.C.
Nguyen Hai L.
TRW Inc.
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