Superconducting analog to digital converter

A superconducting analog to digital converter comprises a plurality of comparators, each of which includes a quantum flux parametron having a superconducting loop with two Josephson devices and exciting inductors, a first load inductor connected to the superconducting loop, and means for supplying exciting current to inductors inductively coupled with said exciting inductors and an rf-SQUID comprising a superconducting loop with a second load inductor and a Josephson device, whereby an input signal is converted to a positive or negative signal by the rf-SQUID for each unit change of the input signal by the amount of the magnetic flux quantum and then the converted signal is sampled and amplified by the quantum flux parametron.

FIELD OF THE INVENTION 
This invention relates to superconducting switching circuits which operate 
at very low temperatures and particularly relates to analog to digital 
converters using a quantum flux parametron which is a parametron-type 
switching circuit involving Josephson devices. 
DESCRIPTION OF THE PRIOR ART 
The quantum flux parametron is a parametron-type superconducting switching 
circuit which uses Josephson devices exhibiting the Josephson effects. In 
the quantum flux parametron, a dc magnetic flux quantum is used as a 
signal medium and the operation is based on a new concept. The quantum 
flux parametron is extremely adequate for computer elements because the 
quantum flux parametron operates at a very high speed, consuming less 
power. The quantum flux parametron also has very good qualities as analog 
circuits such as magnetic flux sensors because the quantum flux parametron 
can amplify very weak magnetic flux in a high gain. The basic operation of 
the quantum flux parametron is disclosed in U.S. application Ser. No. 
146,160, filed on Jan. 20, 1988. 
On the other hand, an analog to digital converter using the Josephson 
devices has been already realized. For example, in C. A. Hamilton et al. 
"Superconduncting A/D Converter Using Latching Comparators" IEEE Trans. 
Magn., Vol. MAG-21, No. 2, pp. 197-199, March 1985 there is disclosed an 
analog to digital converter comprising quantum interference circuits using 
the Josephson devices. This analog to digital converter operates more than 
ten times faster than semiconductor conventional converters. The prior art 
circuit using the Josephson devices utilizes latching operation of the 
Josephson devices. The critical current of the Josephson device can be 
controlled by flowing a control current near the Josephson device. 
Therefore, the Josephson device can be easily transmitted from a 
superconducting state to a voltage state by applying the control current 
to the Josephson device. However, the Josephson devices do not cause 
transition from the voltage to the superconducting state only by removing 
the control current. Thus, in the conventional Josephson devices, a 
special procedure should be used to cause the Josephson device to do 
transition from the voltage state to the superconducting state. The 
latching operation comprises a step of cutting the Josephson device off 
from a power supply to reverse the voltage stage of the Josephson device 
to the superconducting state. 
In order to operate the analog to digital converter at a high speed, a high 
frequency alternating current power supply is needed. But, the latching 
operation restricts the performance of the analog to digital converter as 
described below. 
(1) In order to prevent input data from varying during sampling, very 
narrow width pulses should be supplied from the high frequency alternating 
current power supply. 
(2) Acceptable region of voltage or current of the alternating current 
power supply is very narrow to keep the Josephson device in a 
predetermined biassed condition. Consequently, overshooting of the 
waveform of the high frequency alternating current power supply should be 
severely restricted. 
In order to be released from these restrictions, several improvements have 
been atemptted. For example, in D. A. Petersen et al. "A High Speed 
Analog-to-Digital Converter Using Josephson Self-Gate-AND Comparators" 
IEEE Trans. Magn., Vol. MAG-21, No. 2 pp. 200-203, March 1985, there has 
been provided a comparator for sampling input data on rising edges of 
clock pulses from the power supply. This comparator is called a 
self-gate-and (SGA) circuit and it is effective to avoid the above 
restriction (1). However, the circuit arrangement is complicated and the 
operational margin of the circuit is narrow. Further, this compartor is 
not effective for the above restriction (2). The quantum flux parametron 
is a switching circuit without causing the latching operation since the 
voltage state is not used as one of the switching states. In the quantum 
flux parametron, input signals are sampled on rising edges of exciting 
current. The above restrictions are therefore easily removed. However, a 
high-speed analog to digital converter using the quantum flux parametrons 
has not been proposed. 
SUMMARY OF THE INVENTION 
An object of this invention is to realize a very high speed analog to 
digital converter with a wide margin for the power supply variance, 
without using very narrow sampling pulses. 
In order to attain the object, this invention provides an A/D converter 
using comparators each of which comprises an rf-SQUID and a quantum flux 
parametron connected therewith. 
In this construction, the rf-SQUID converts an input signal into a positive 
or negative signal for unit change in the input signal by the amount of 
the magnetic flux quantum (2.07.times.10.sup.-15 Wb), and then the quantum 
flux parametron samples and amplifies the converted signal on the rising 
edge of the exciting current.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 2A shows an example of an rf-SQUID which is used in this invention. 
This rf-SQUID has a superconducting loop 101 which is formed by a 
Josephson device 100, a load inductor 102 and a second winding of an input 
transformer 103. A first winding of the input transformer 103 is connected 
to an input line 104. Magnetic flux, which is generated by current flow 
through the input line 104, passes through the superconducting loop 101. 
Circulating current Ic flowing through the superconducting loop 101 
satisfies the quantization condition of fluxoids and obeys the following 
Eq. (1). 
EQU 2.pi..times.Ls.times.Im.times.sin .theta./.PHI..sub.0 
+.theta.+2.pi..times..PHI.ex/.PHI..sub.0 =2n.pi. (1) 
EQU Ic=Im.times.sin .theta. 
where .PHI..sub.0 is the magnetic flux quantum (2.07.times.10.sup.-15 Wb), 
Ls is a total inductance of the load inductor and the input inductor, Im 
is a critical current of the Josephson device 100, .theta. is an electron 
wave phase of the Josephson device 100 and .PHI.ex is an input magnetic 
flux which is generated by the input current and passes through the 
superconducting loop. 
If Im&lt;.PHI..sub.0 /(2.pi.Ls), a partial sum of Eq. (1) 
F=2.pi..times.Ls.times.Im.times.sin .theta./.PHI..sub.0 +.theta. does not 
have hysteresis as a function of .theta.. Therefore, as shown in FIG. 2B, 
solutions of Eq. (1) behave in such a way that the direction of the 
circulating current Ic is inverted to show a positive or negative signal 
for each unit change in the input signal by the amount of the magnetic 
flux quantum .PHI..sub.0. 
FIG. 1 shows an embodiment of a comparator according to this invention. In 
the embodiment shown in FIG. 1, the circulating current Ic flowing through 
the load inductor 102 of the rf-SQUID of FIG. 2A is picked up by a sense 
inductor 206 which is inductively coupled with the load inductor 102. The 
signal picked up by the sense inductor 206 is introduced into the quantum 
flux parametron as an input signal. The quantum magnetic flux parametron 
has a superconducting loop 210 including two Josephson devices 201 and 202 
and superconducting exciting inductors 203a and 204a. A superconducting 
load inductor 205 is connected to the superconducting loop 210. Inductors 
203b and 204b are inductively coupled with the superconducting inductors 
203a and 204a. An exciting current supply 208 generates exciting current 
which flows through an exciting line 207 and produces magnetic flux. The 
magnetic flux passes through the superconducting loop 210 via inductive 
coupling between the inductors 203b and 204b and the superconducting 
inductors 203a and 204a, and then the magnetic flux excites the quantum 
flux parametron. Message of the input signal for the quantum flux 
parametron is represented by the direction of the input signal current and 
the input signal is sampled on the leading edge of the exciting current. 
Therefore, the circuit of FIG. 1 constructs a comparator in which an input 
signal is converted to a positive or negative signal by the rf-SQUID for 
each unit change in the input signal by the amount of the magnetic flux 
quantum, and the converted signal is sampled on the leading edge of the 
exciting current and then amplified by the quantum flux parametron. 
FIG. 3 shows another comparator circuit according to this invention. In 
this circuit, a series connection of the Josephson device 100 and a second 
winding of the input transformer 103 is directly connected to the quantum 
flux parametron. An rf-SQUID is constructed with the Josephson device 100, 
the input transformer 103 and the quantum flux parametron. In operation, 
an input signal, which is inputted through the input line 104, is 
converted into a positive or negative signal by the rf-SQUID. Next, the 
converted signal is sampled and amplified by the quantum flux parametron. 
FIG. 8 shows a result of a circuit simulation which is carried out by a 
computer to investigate an operation of a comparator according to this 
invention. In this result, input signal current is increased from zero at 
a constant rate, and the input signal is sampled and amplified by exciting 
the quantum flux parametron with a sinusoidal wave of 20 GHz. It is 
confirmed that the predetermined operation is taken place, that is, an 
output signal is changed for each unit change in the input signal by the 
amount of the magnetic flux quantum. 
FIG. 4 shows another rf-SQUID used in an analog to digital converter 
according to this invention. In the rf-SQUID shown in FIG. 4, an input 
signal is directly supplied to the rf-SQUID not through an input 
transformer as shown in FIG. 2A. In the circuit of FIG. 4, an input 
inductor 300 is connected in parallel with a serial connection of the 
Josephson device 100 and the load inductor 102. An input signal is 
directly injected on the connection point 301 through an input line 104. 
The circuit shown in FIG. 4 obeys Eq. (2) which is obtained from the 
quantization condition of fluxoids. 
EQU 2.pi..times.Ls.times.Im.times.sin .theta./.PHI..sub.0 
+.theta.-2.pi..times.Li.times.Ii/.PHI..sub.0 =2n.pi. (2) 
EQU Ii.times.Im.times.sin .theta.=Iex 
where .theta. is an electron wave phase of the Josephson device 100, Im is 
a critical current of the Josephson device, Ls is an inductance of the 
load inductor 102, Li is an inductance of the input inductor 300, Ii is a 
current flowing through the input inductor 300 and Iex is an input signal 
current. 
By modifing Eq. (2) we will obtain Eq. (3). 
EQU 2.pi..times.(Ls+Li).times.Im.times.sin .theta./.PHI..sub.0 +.theta. 
EQU -2.pi..times.Li.times.Iex/.PHI..sub.0 =2n.pi. (3) 
Eq. (3) has the same form as Eq. (1). Particularly, the form of the current 
which flows through the Josephson device is the same as that of the 
circulating current in FIG. 2A. Therefore, concerning the current flowing 
through the Josephson device 100 or the load inductor 102, the circuit 
shown in FIG. 4 operates in the same way as in the rf-SQUID of FIG. 2A. 
FIG. 5 shows another embodiment of a comparator used in an analog to 
digital converter according to this invention. In the circuit shown in 
FIG. 5, the rf-SQUID shown in FIG. 4 is connected with the quantum flux 
parametron through the inductive coupling between the load inductor 102 
and the sense inductor 206. The rf-SQUID converts an input signal into a 
positive or negative signal for each unit change in the input signal by 
the amount of the magnetic flux quantum. 
FIG. 6 shows another embodiment of a comparator using the rf-SQUID of FIG. 
4. In the circuit shown in FIG. 6, the quantum flux parametron corresponds 
to the load inductor 102 in FIG. 4. An input signal is directly injected 
through the line 104 connected between the input inductor 300 and the 
Josephson device 100. The current, which flows through the Josephson 
device 100 of the rf-SQUID, is directly injected into the quantum flux 
parametron. This comparator operates in the same way as in the comparator 
of FIG. 1. 
FIG. 7 shows a construction of a comparator according to this invention, in 
which sampling characteristic of the quantum flux parametron is improved. 
In the circuit of FIG. 7, a Josephson device 400 is connected in parallel 
with the superconducting devices 203a and 204a to form a magnetic flux 
regulator. The operational priciple of this magnetic flux regurator is 
disclosed in U.S. application entitled "SUPERCONDUCTING CIRCUIT" filed on 
Sept. 9, 1988 by E. Goto and Y. Harada. In this construction, if product 
of a critical current of the Josephson device and an inductance of the 
superconducting inductor 203a, 204a is increased, hysteresis appears in 
the characteristic of the magnetic flux regulator. By using this 
hysteresis, the quantum flux parametron can cause transition from the 
unexcited state to the excited state within a very short time, so that the 
sampling time can be extremely shortened. 
In order to construct an analog to digital converter by use of a comparator 
according to this invention, we can use a method of sequentially dividing 
input current in half by using a ladder type resistor network which is 
disclosed in D. A. Petersen et al. "A High-Speed Analog-to-Digital 
Converters Using Josephson Self-Gate-AND Comparators" IEEE Trans. Magn., 
Vol. MAG-21, No. 2, pp. 200-203, March 1985, or a method of varying 
coupling strength of input transformers which is disclosed in C. A. 
Hamilton et al. "Superconducting A/D Converter Using Latching Comparators" 
IEEE Trans. Magn., Vol. MAG-21, No. 2, pp. 197-199, March 1985. 
FIG. 9 shows an embodiment of an analog to digital converter using a ladder 
type circuit. The ladder circuit is constructed by connecting serial 
resistors 501 and parallel resistors 502, alternatively and terminating at 
a resistor 503. The resistance value of the parallel resistors 502 is 
twice as large as that of the serial resistors and the terminating 
resistor. In this circuit construction, the current flowing through each 
parallel resistor 502 is divided in half, iteratively. The comparator 
according to this invention is connected to each parallel resistor 502 and 
the current flowing through each parallel ragistor is converted to a 
positive or negative signal for each unit change in the input signal by 
the amount of the magnetic flux quantum and then amplified. The operating 
principle of analog to digital conversion in this circuit construction 
will be explained with reference to FIG. 10. In order to convert an analog 
signal to a digital signal, templates such as shown in FIG. 10 are 
prepared. The number of the templates is the same as that of conversion 
bits. The cycles of successive templates differ by a factor of 2. In 
operation, it is determined where the input signal is placed on the 
templates. In the example of FIG. 10, when an input signal is (11) an 
output signal is (1011). In the circuit of FIG. 9, instead of using 
different kinds of templates whose cycles are different by twice, the 
input signal current I.sub.input is divided into halves iteratively and 
conversion is carried out by only one kind of template. 
FIG. 11 is an another embodiment of an analog to digital converter which is 
useful for the embodiments of FIGS. 1 and 3. In this embodiment, the input 
signal current I.sub.input is inputted through an input transformer. In 
the embodiment of FIG. 11, an input transformer having inductive couplings 
103A, 103B, 103C and 103D is provided. The coupling coefficients of 
successive inductive couplings are related by a factor of 2. The second 
windings of the input transformer are connected to the respective 
comparators 500 which have same template each other. Therefore, it is 
obvious that this construction operates in the same way as in the analog 
to digital comparator of FIG. 9. 
As described above, according to this invention, sampling is carried out on 
the rising edges of the exciting current. Therefore, there is provided an 
analog to digital convertor with a wide margin for power supply variance 
and insensitive to overshooting of a waveform of the power supply. In 
order to enable an analog to digital converter to operate at a high speed, 
high frequency alternating current power supply is needed. The above 
described improvements of this invention are necessary to operate the 
circuit at a high speed. Therefore, this invention provides important 
means to realize a high speed analog to digital converter. 
While there have been described what are believed to be the preferred 
embodiments of the present invention, those skilled in the art will 
recognize that other and further modifications may be made thereto without 
departing from the spirit of the invention, and it is intended to claim 
all such changes and modifications as fall within the true scope of the 
invention.