JP2004071630A - Phase comparator for josephson circuit and pll circuit using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a phase comparator applicable to a Josephson circuit system and a PLL circuit which is a circuit applied with the same. <P>SOLUTION: A phase comparator 10 is formed of a phase difference detecting loop 11 and a phase difference output SQUID12 which are placed under the ultra-low temperature environment. The phase difference detecting loop 11 is formed of a super-conductor closed loop to which the permanent loop current can be applied, a first input terminal In1 and a second input terminal In2 provided in the super-conductor closed loop, a non-latch type first Josephson switching portion J1 for generating, in a closed loop provided in this closed loop, a permanent loop current which rotates in a first direction, a non-latch type second Josephson switching portion J2 for generating in the closed loop a permanent loop current which rotates in a second direction reversed from the first direction, and inductor portions M1, M2 for magnetic coupling with the phase difference output SQUID12. The phase difference output SQUID12 is a non-latch type DC SQUID, and the inductor portions Ma, Mb provided therein are magnetically coupled with the inductors M1, M2 of the phase difference detecting loop 11. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a phase comparator for a Josephson circuit (superconducting circuit) and a PLL circuit (phase locked loop circuit: phase locked loop circuit) as a typical application example thereof.
[0002]
[Prior art]
In the case of a semiconductor system, various configurations of phase comparators that compare the phase difference between two frequency signals and generate an electrical value output (voltage output or current output) according to the difference have been proposed. However, a circuit whose stable operation has been verified has not yet been proposed for a Josephson circuit. Therefore, as in the semiconductor system, the PLL circuit required to oscillate an extremely stable frequency for stabilizing a clock or to oscillate an arbitrary desired stable frequency as a frequency synthesizer is also required. Also, it has not been proposed yet. The PLL circuit itself has a well-known circuit configuration before the advent of the semiconductor system, and its application range is extremely wide, and it is almost essential for various signal synchronization systems. It also plays an important role in stabilization, and has been expected in Josephson circuits.
[0003]
[Problems to be solved by the invention]
However, although the circuit is already mature in the semiconductor system, the Josephson circuit has a restriction that even if it is a single switching element, it must use a circuit element that performs almost purely digital operation peculiar to this. In addition, because of the principle that one magnetic flux quantum Φo is treated as one bit, the configuration of the phase comparator in the existing semiconductor system cannot be simply used.
[0004]
Accordingly, the present invention proposes a new phase comparator suitable for a Josephson circuit and a PLL circuit which is a typical application circuit using the phase comparator.
[0005]
[Means for Solving the Problems]
In the present invention, first, as a Josephson circuit phase comparator,
Having a phase difference detection loop and a phase difference output SQUID placed in a cryogenic environment;
The phase difference detection loop,
a superconducting closed loop capable of passing a permanent ring current;
b first and second input terminals provided in the superconducting closed loop;
c, a non-latching first Josephson that is provided in a closed loop and switches once when a flux quantum of a first frequency signal is input to a first input terminal, and generates a permanent ring current that rotates in a first direction in the closed loop. . A switching unit;
d is provided in the closed loop, and once the flux quantum of the second frequency signal is input to the second input terminal, the switching is performed once, and the closed loop generates a permanent ring current in the second direction opposite to the first direction. A second latch-type Josephson switching section;
e an inductor section forming one of mutual inductances for magnetic coupling to the phase difference output SQUID and forming an inductance large enough to hold a permanent ring current generated during the closed loop;
Has,
The above phase difference output SQUID is a non-latch type DC SQUID,
f The inductor portion provided in itself is the other inductor portion which is magnetically coupled with the above-mentioned one inductor portion so as to form a mutual inductance, and through this mutual inductance, the permanent ring current in the closed loop of the phase difference detection loop And outputting the output current or the output voltage as a variation of the SQUID voltage applied to itself as a phase difference detection output signal;
We propose a phase comparator for Josephson circuits characterized by the following.
[0006]
Further, in the present invention, as a more practical sub-configuration,
A plurality of the above-mentioned phase difference detection loops are juxtaposed, and a plurality of the phase difference output SQUIDs magnetically coupled to each other through the mutual inductance are also provided with the same number.
A first frequency signal is applied in parallel to a first input terminal of each of the plurality of phase difference detection loops, and a second frequency signal is also applied to a second input terminal in parallel;
A plurality of phase difference outputs SQUIDs are connected in series to increase detection sensitivity;
We also propose a phase comparator characterized by
[0007]
In the present invention, as a PLL circuit for a Josephson circuit,
The reference frequency output from the reference frequency oscillator is compared with 1 / N of the oscillation frequency output from the electric value controlled oscillator, where N is an integer of 1 or more, and a phase difference detection output signal based on the difference between the two frequencies is output. A phase comparator, and a loop filter that integrates the output of the phase comparator and outputs a control electric value as a current or voltage output from which an AC component has been removed or reduced. A PLL circuit that receives its control electric value and adjusts its own oscillation frequency in a direction in which the difference between the two frequencies in the phase comparator disappears,
The phase comparator has a phase difference detection loop placed in a cryogenic environment and a phase difference output SQUID;
The phase difference detection loop is
a superconducting closed loop capable of passing a permanent ring current;
b first and second input terminals provided in the superconducting closed loop;
c is provided in a closed loop, and when a flux quantum of a reference frequency as a first frequency signal is input to a first input terminal, the flux is once switched, and a non-latch type second circuit that generates a permanent ring current rotating in a first direction in the closed loop. A Josephson switching unit;
d is provided in the closed loop, and once the flux quantum of the frequency signal of 1 / N of the oscillation frequency as the second frequency signal is input to the second input terminal, once switched, the closed loop is switched to the first direction in the direction opposite to the first direction. A second non-latching Josephson switching section for producing a permanent ring current in two directions;
e an inductor part forming one of mutual inductances for magnetic coupling to the phase difference output SQUID;
Has,
The phase difference output SQUID is a non-latch type DC SQUID,
f The inductor portion provided in itself is the other inductor portion which is magnetically coupled with the above-mentioned one inductor portion so as to form a mutual inductance, and through this mutual inductance, the permanent ring current in the closed loop of the phase difference detection loop And outputting the output current or the output voltage as a variation of the SQUID voltage applied to itself as a phase difference detection output signal;
A PLL circuit for a Josephson circuit is proposed.
[0008]
Also in this PLL circuit, the above-described plural configurations of the phase difference detection loop and the phase difference output SQUID can be used. That is, both the reference frequency signal and the 1 / N frequency signal of the oscillation frequency may be divided and supplied to the plurality of first and second input terminals.
[0009]
The output from the phase comparator is taken out as a control current through a low-pass filter, which is a loop filter. On the other hand, the electric value controlled oscillator is a relaxation oscillator having a DC SQUID configuration and a Josephson oscillator included in the relaxation oscillator. In order to modulate the critical current of the son switching section, a configuration in which the current control oscillator is formed by magnetically coupling to the inductor included in the relaxation oscillator and including the above-described control current flowing inductor is also proposed.
[0010]
Further, in this case, it is preferable that the inductor, which is magnetically coupled to the inductor of the relaxation oscillator and through which the control current flows, be used as the inductor forming the low-pass filter, further simplifying the circuit configuration.
[0011]
BEST MODE FOR CARRYING OUT THE INVENTION
Even when the PLL circuit is realized in a Josephson circuit system, if only the circuit block configuration is used, it is possible to conform to the basic configuration in an existing semiconductor system, for example, as shown in FIG. That is, the reference frequency fs output from the reference frequency oscillator (OSC) 20 and, in general, 1/1 of the oscillation frequency fo of a voltage controlled oscillator (VCO) 30 whose oscillation frequency fo is controlled by a voltage value. N (N is an integer of 1 or more), a phase comparator (PD: Phase Detector) 10 that outputs a phase difference output signal Ec based on the difference between the two frequencies, and integrates the output of the phase comparator 10 And a loop filter 50 for outputting a voltage output Vc from which the AC component has been removed or reduced. The voltage control oscillator 30 receives the voltage value output from the loop filter 50 as the control voltage Vc, Of its own oscillation frequency fo such that the difference between the two frequencies fs and fo / N is eliminated. .
[0012]
The frequency of the voltage controlled oscillator 30 is divided by the N frequency divider 40 into 1 / N. Generally, when it is desired to obtain an output frequency fo higher than the reference frequency fs, or by changing the value of N, any desired value can be obtained. In many cases, the oscillation frequency fo of the voltage-controlled oscillator 30 is intentionally increased beforehand in order to obtain the output frequency fo. If the frequency division is not required (when N = 1), of course, , N divider 40 can be omitted. Conversely, if the N frequency divider 40 is configured as a variable frequency divider, it becomes a frequency synthesizer, and an arbitrary output oscillation frequency fo can be obtained at a desired frequency interval according to the number of dividable frequencies. Further, since the loop filter 50 is configured as an integrator for removing an AC component, the loop filter 50 is also substantially a low-pass filter (LPF: Low Pass Filter) 50.
[0013]
However, in the case of a semiconductor system, the output oscillation frequency fo is often obtained by the voltage-controlled oscillator 30 as described above, but in a Josephson circuit system which is an object of the present invention, one embodiment of the present invention described later is used. In some cases, it may be convenient to configure a current controlled oscillator (CCO) 30 instead of voltage control. Therefore, when both are described collectively, they can be called an electric value controlled oscillator (ECO). In the case of the current control oscillator, the control voltage Vc becomes the control current Ic.
[0014]
What has to be newly considered is the phase comparator 10. This is because the other circuit elements cannot be used if the existing circuit elements are left unchanged or slightly modified. For example, the reference frequency oscillator 20 can be satisfied by guiding the reference frequency signal under a cryogenic environment from a stable frequency source configured using a crystal oscillator or the like outside a cryogenic environment. For example, a pulse train of magnetic flux quantum (SFQ: Single-Flux-Quantum) generated at a stable time interval via a converter to a magnetic flux quantum train or the like can be generated.
[0015]
The voltage controlled oscillator 30 can also be configured using an existing Josephson relaxation oscillator whose oscillation frequency changes according to the bias voltage, and the low-pass filter 50 can be configured with a passive circuit such as an inductor and a resistor or a resistor and a capacitor. However, a specific proposal for the phase comparator 10 could not be recognized so far.
[0016]
In view of the above, the present invention provides a phase comparator 10 adapted to a Josephson circuit system, as can be seen in the embodiment described here. In recent Josephson circuit systems, the so-called RSFQ circuit (Rapid Single-Flux-Quantum circuit: high-speed one-way circuit) of an asynchronous system is used for signal propagation in consideration of high-speed operation instead of the signal line configuration of the previous synchronous system. It often conforms to a method called a magnetic flux quantum circuit, and it is necessary to conform to this method. In short, a non-latching type junction is applied to the Josephson junction interposed in the signal propagation path, that is, even if the state once transitions to the voltage state with the arrival of the signal (magnetic flux quantum SFQ), it automatically returns to the original state as the signal decreases or disappears. Which returns to the zero voltage state.
[0017]
Now, when considering the phase difference, as shown in FIG. 1 (B), it usually corresponds to the angle difference Δθ between two signals usually represented by a sine wave. However, in the Josephson circuit, as shown in FIG. 1B, a first SFQ pulse train having a period τ that occurs or arrives at a certain time tn and sets a time difference τ between the next occurrence time and the arrival time tn ′. The phase difference Δθ can be represented by the time difference Δτ from the second SFQ pulse train generated (or arriving) at a time difference Δτ from the SFQ pulse train. That is,
Δθ = 2πΔτ / τ. . . . . (1)
In a relationship.
[0018]
To find the phase difference, it is only necessary to find the difference in the period.Because one pulse train arrives and the next pulse train arrives, a circuit with a different electrical state can operate in a cryogenic environment. If it can be realized by a circuit system, and if its electrically different state can be output as an electric signal to a subsequent processing circuit, the intended phase comparator 10 can be configured. The present invention has provided a phase comparator 10 whose principle configuration is shown in FIG. 1C, for example.
[0019]
The phase comparator 10 according to the present invention has a phase difference detection loop 11 and a phase difference output SQUID12. "SQUID" is a flux quantum interference device (skid) well known in the art. First, to explain the former, two non-latch type Josephson switching units J1 and J2 and one in principle may be used, but due to the magnetic coupling via mutual inductance with a phase difference output SQUID12 described later. In terms of circuit configuration, there is a superconducting closed loop that has inductor portions M1 and M2 shown as two in series and can pass a permanent ring current.
[0020]
The non-latch type Josephson switching units J1 and J2 will not be described in detail because of a known technique. However, for example, a weak type having no hysteresis due to its own structural factors such as a so-called microbridge type and edge junction type. There are a coupling element and a non-latching type in which the hysteresis characteristic is intentionally lost by attaching an over-damping resistor in parallel to a latch-type Josephson junction having hysteresis. Since the latch type joining is more mature in terms of material, structure and manufacturing method, the latter configuration is generally adopted. Of course, any one can be used for the present invention, and the Josephson switching unit itself may have a non-latch type SQUID configuration. Hereinafter, the operation is performed in the RSFQ mode, unless otherwise specified. Therefore, each of the switching units or switching circuit systems described herein is of a non-latch type.
[0021]
The inductance Lm of the inductors M1 and M2 inserted between one ends of the two Josephson switching units J1 and J2 is, with the critical current of the two Josephson switching units Im being one flux quantum Φo. ,
Lm × Im> Φo. . . . . (2)
The size is selected to satisfy the following relationship.
[0022]
With such a value, the condition for holding the magnetic flux in the superconducting closed loop is satisfied, and the connection between one Josephson switching unit J1 and one inductor M1 is used as the first input terminal In1, and the other Josephson switching unit is used. Assuming that the connection between J2 and the other inductor M2 is a second input terminal In2, one magnetic flux quantum + Φo with a “+” symbol is provided for convenience through the signal line 21 shown on the left side in the figure. When the first Josephson switching section J1 once transitions to the voltage state and then closes, the single flux quantum Φo is captured and reaches the first input terminal In1. Of the permanent ring current Iloop flows during the closed loop.
[0023]
Similarly, through the signal line 31 shown on the right side in the figure, for convenience, one magnetic flux quantum -Φo with a “-” sign travels from right to left in the figure and goes to the second input terminal In2. When it arrives, the second Josephson switching section J2 once transitions to the voltage state and then closes, the one flux quantum Φo is captured, and the permanent ring current Iloop of the corresponding magnitude again flows in the closed loop.
[0024]
However, what is characteristic here is that the direction of circulation of the permanent ring current Iloop in the superconducting closed loop caused by the arrival of one flux quantum + Φo at the first input terminal In1 and one flux at the second input terminal In2. This means that the circulation direction of the permanent ring current Iloop in the loop caused by the arrival of the quantum -Φo is opposite. The phase comparator 10 of the present invention effectively utilizes this fact.
[0025]
That is, returning to FIG. 1B, the time tn, tn '. . . . Pulse sequence periodically arrives at the first input terminal In1 as + Φo, causing a permanent ring current Iloop in the first direction in the superconducting closed loop of the phase difference detection loop 11, and after a time difference Δτ. Times tn + a, tn + a '. . . . When the pulse train (one magnetic flux quantum) -Φo which periodically occurs at the second input terminal In2 generates a permanent ring current Iloop in the opposite direction, the previously generated permanent ring current is set to + Iloop. Then, since the former is canceled by -Iloop of the same size but in the opposite direction, the permanent ring current substantially disappears there.
[0026]
From this, the phase difference detection loop 11 can generate a permanent ring current that flows only for a time corresponding to the phase difference between the two incoming frequency signals. After all, if one of the signals representing + Φo is a pulse train signal having the reference frequency fs from the reference frequency oscillator 20 and the other −Φo is a pulse train having the oscillation frequency fo from the voltage controlled oscillator 30 or a frequency of 1 / N thereof. , The phase difference between them can be detected. Although the N frequency divider 40 is omitted in FIG. 1 (C), it is needless to say that, when used, the N frequency divider 40 is connected between the voltage controlled oscillator 30 and the second input terminal In2 of the phase difference detection loop in this case. This is interposed.
[0027]
To be described in advance, as an existing circuit, a toggle flip-flop has already been proposed in a Josephson circuit system, which is a half that generates one magnetic flux quantum pulse with two magnetic flux quantum pulses. Since it functions as a frequency divider, it is easy to configure the N frequency divider 40 by connecting it in a cascade. In addition, since it is possible to configure the switching line so as to arbitrarily bypass the flip-flop of each stage of the N divider connected in cascade by an existing circuit, after all, by selecting the number of lines to be bypassed, , N can be configured as a variable frequency divider.
[0028]
Now, as described above, since the phase difference detection loop 11 can detect the phase difference between the two incoming frequency signals fo / N and fs, the phase difference is taken out as an electrical output to a subsequent circuit. A phase difference output SQUID12 of a non-latch type DC SQUID configuration is provided, and the inductors Ma and Mb included in this circuit are arranged close to the inductors M1 and M2 of the phase difference detection loop 11 so as to form mutual inductance. That is, the inductors Ma and Mb form one inductor part forming a mutual inductance, and the inductors M1 and M2 form the other inductor part. The configuration as the SQUID 12 itself is the same as the existing configuration, and there is no particular need for modification. The SQUID 12 has two non-latching Josephson switching units Ja and Jb in a superconducting closed loop, and the inductor is connected between one end thereof. A bias voltage Vp is applied between the inductors Ma and Mb, and the other ends of the pair of Josephson switching units Ja and Jb are dropped to a common potential (generally a ground potential such as a ground plane potential). ing.
[0029]
With such a configuration, the phase comparator 10 according to the present invention can detect and output a phase difference of 1 / N between the reference frequency fs oscillated by the reference frequency oscillator 20 and the oscillation frequency fo oscillated by the voltage controlled oscillator 30. In order to know whether or not the output voltage Vc as a signal exhibiting a change width corresponding to the phase difference can be actually extracted from the phase difference output SQUID12, a low-pass filter 50 may be added. Is used. As a result of actual verification, the inventors succeeded in obtaining an output voltage Vc having a variation corresponding to the SQUID applied voltage Vp according to the phase difference.
[0030]
However, as shown in FIG. 2, when the SQUID applied voltage (bias voltage) Vp is high, the change width of the output voltage Vc, which is a voltage value fluctuated by the phase difference Δτ, is considerably small, and it is better to lower the bias voltage Vp. The change width of the phase difference output voltage Vc itself was large. In the example taking the characteristics of FIG. 2, when the bias voltage Vp is lowered from 100 mV, a voltage change width (voltage difference) of 0.26 mV, which is the maximum at 48 mV, is obtained. Therefore, the output voltage Vc can be used as the control voltage Vc for controlling the oscillation frequency fo of the voltage controlled oscillation circuit 30.
[0031]
However, in assembling a practical circuit, the change width of the phase difference output voltage Vc is small, and it is undeniable that it is difficult to use. Then, in order to solve this, the structure shown in FIG. 3 can be proposed. That is, the output of the reference frequency oscillator 20 and the output of the voltage controlled oscillator 30 are each divided by m, and are input in parallel to the corresponding first and second input terminals of the m phase difference detection loops 11. As an existing circuit, there is a circuit for dividing a Josephson pulse into two and a pulse splitter. For example, in the case where the pulse is divided into eight, three stages of the pulse splitter may be used on a tree. Each of the two parts is divided into two parts, and further divided into two parts, resulting in eight parts. On the other hand, for these m phase difference detection loops 11, the phase difference outputs SQUID12 magnetically coupled to each other are connected in series with each other, and the phase difference output voltage Vc is obtained as the sum of the output voltages of the series SQUID12. be able to. However, in the configuration illustrated in FIG. 3, the low-pass filter 50 is changed from the above-described CR configuration to the inductor L and the resistor R, and is changed to a circuit mainly suitable for obtaining the phase difference output current Ic. This is because whether or not the phase difference information can be taken out as the current output Ic was tested. Of course, this may be used. In fact, as a result of the experiments by the present applicant, the single-stage configuration shown in FIG. As compared with the case of (1), the output current Ic exhibiting an approximately 8-fold change width when m = 8 was obtained as theoretically. Conversely, it has been confirmed that an output voltage Vc exhibiting a change width of approximately eight times can be obtained by using a CR low-pass filter configuration.
[0032]
The next consideration was discrimination of the relationship between the advance and delay of the reference frequency signal fs from the reference frequency oscillator 20 and the oscillation frequency signal fo / N from the electrical value control oscillator 30. The phase difference output voltage Vc or the phase difference output current Ic as a result of detecting the phase difference is eventually generated based on the fact that the critical current Io of the phase difference output SQUID12 is modulated. As shown in (A), in the symmetrical DC SQUID, the area used for phase difference detection (the thick line in the figure) is the critical current Io if the absolute value is the same regardless of whether the detected current is positive or negative. Since the degree of the decrease is the same, it is impossible to know which of the reference frequency signal from the reference frequency oscillator 20 and the oscillation frequency signal from the electric quantity control oscillator 30 has arrived first.
[0033]
Therefore, in order to discriminate this, the SQUID is made to be asymmetric, and as shown in FIG. 4 (B), the monotonically changing portion of the change of the critical current Io based on the phase difference detection is changed to the phase difference. It was designed to be a detection area.
[0034]
FIG. 5 shows an example of a circuit that satisfies this. Here, as described with reference to FIG. 3, an example of a circuit for substantially increasing the detection sensitivity by connecting a plurality of phase difference outputs SQUID12 in series is exemplified. Since the same configuration may be used, only one of them will be described. The reference numerals used are the same as the reference numerals assigned to the corresponding components used in the circuits described above. Further, the phase difference detection loop 11 is shown with only the inductors M1 and M2 extracted therefrom for simplification of the drawing, but may have the same configuration as described above.
[0035]
On the other hand, a bias current Id flowing in one direction is applied to the phase difference output SQUID12 via a resistor Rd. Thereby, naturally, the magnitude of the current flowing through the mutual inductance in the phase difference output SQUID12 changes depending on the direction of the permanent ring current flowing in the phase difference detection loop 11, and the asymmetry shown in FIG. Is obtained. Further, if the line impedance differs between the side receiving the reference frequency fs and the side receiving the oscillation frequency fo from the electric value controlled oscillator 30, a greater effect can be obtained. An additional inductor M3 was provided in the line connected to the inductor M2.
[0036]
With such a contrivance, in a specific experimental example, a current change width of 0.126 mA was obtained in the line passing through the low-pass filter 50 in the phase difference (flux quantum pulse arrival time difference) range of +90 ps to -90 ps. . Although it did not become an ideal linear change, a current output which was large enough to feed back to the electric value controlled oscillator and which could correspond to the phase difference change was obtained. In FIG. 4, the vertical axis represents the critical current, but when this is expressed on the SQUID voltage axis, the characteristic curve becomes line-symmetric with respect to the magnetic flux axis, and the peaks and valleys are reversed.
[0037]
As the voltage-controlled oscillator 30 whose oscillation frequency is controlled by the control voltage Vc generated from the phase comparator 10 via the low-pass filter 50, an existing relaxation oscillator in a Josephson circuit system can be used. The configuration is such that, in FIG. 6A, which will be described when a current control oscillator to be described later is configured, signal lines including inductors L1 and L2 and a resistor R in series with them are omitted, and Has two non-latching Josephson switching units Jo1 and Jo2 in a superconducting closed loop in the figure and inductors Mo1 and Mo2 provided between one ends thereof, and applies a bias voltage to the closed loop. A DC SQUID part to which the accompanying bias current Ib1 is applied is replaced with a single Josephson switching unit, and a relaxation oscillation low-pass filter (including a resistor Rf and an inductor Lf) is connected in parallel to this unit. Then, a flux quantum pulse train is oscillated at a frequency corresponding to the magnitude of the bias voltage. That is, the oscillation frequency can be varied by varying the bias voltage. The oscillated pulse is transmitted to a subsequent circuit via a Josephson transmission line (JTL) also having a known configuration. As shown in the figure, the non-latch type Josephson switching unit Jt, which has transmitted the magnetic flux quantum pulse via the inductor Lt, temporarily transitions to the voltage state because the bias Ib2 is given in advance, and then returns to zero. The configuration is such that the pulse is transmitted by returning to the voltage state, and since this is connected in cascade, the flux quantum pulses are successively propagated like a bucket relay. As described above, in the PLL circuit of the present invention, the JTL configuration can be arbitrarily adopted for a required line portion.
[0038]
However, the relaxation oscillator can ultimately vary the oscillation frequency by varying its bias current, and furthermore, modulates the critical current of the DC SQUID composed of the Josephson switching units Jo1, Jo2 and the inductors Mo1, Mo2. However, the oscillation frequency can be varied. Therefore, the output of the phase comparator 10 according to the present invention may be converted into a voltage by the low-pass filter 50, and this may be superimposed on the bias voltage of the relaxation oscillator as the control voltage Vc. Magnetic coupling to the oscillator simplifies it. That is, when the electric value controlled oscillator 30 is configured as the current controlled oscillator 30, the converter stage for converting the current into the voltage can be omitted.
[0039]
Therefore, in the circuit shown in FIG. 6A, as a desirable configuration, the current output Ic from the low-pass filter 50 described above is applied to the DC SQUID section of the relaxation oscillator via magnetic coupling, and the Josephson switching section Jo1 is applied. , And Jo2 are variably controlled by modulating the critical current. In this case, the inductors L1 and L2 magnetically coupled to the inductors Mo1 and Mo2 of the DC SQUID are not separately provided as independent circuit components. It is reasonable to divert the inductor L of the low-pass filter 50 described above, and the circuit configuration is further simplified. That is, the inductors L1 and L2 and the resistor R in series with them constitute the loop filter (low-pass filter) 50 described with reference to FIGS.
[0040]
In fact, according to the numerical experiment example of the present inventor, as shown in FIG. 6B, when the bias voltage to the relaxation oscillator is 15 mV, the control current Ic is zero. By changing it within a small range from 0.07 mA to 0.07 mA, a considerably large oscillation cycle change (oscillation frequency change width) could be obtained. As shown in the figure, an appropriate bias voltage value differs depending on circuit parameters. In this experimental example, a very effective result is not obtained at 20 mV. Conversely, it may be necessary to search for an appropriate bias voltage value or the like for each circuit parameter, but a design capable of obtaining a satisfactory oscillation frequency change width by varying the control current Ic is sufficiently and reliably possible. .
[0041]
【The invention's effect】
As described above, according to the present invention, a phase comparator that operates effectively in a Josephson circuit system can be provided for the first time. Although there are a wide variety of application examples, it is a great advantage that a PLL circuit as one of the most effective application examples can be provided. Since the applications of the PLL circuit itself are extremely wide, there is a great deal where the present invention ultimately contributes to this kind of technical field.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram of a circuit configuration example of a phase comparator and a PLL circuit according to the present invention.
FIG. 2 is an explanatory diagram of a characteristic example obtained by a phase comparator configured according to the present invention.
FIG. 3 is an explanatory diagram of an example of a circuit configuration for increasing the sensitivity of a phase comparator.
FIG. 4 is an explanatory diagram for explaining an aspect of phase difference detection with reference to a characteristic diagram.
FIG. 5 is an explanatory diagram of a circuit example in a case where the phase comparator has an asymmetric detection characteristic.
FIG. 6 is an explanatory diagram of a preferred circuit configuration example when configuring a current controlled oscillator.
[Explanation of symbols]
10. Phase comparator of the present invention
11 Phase difference detection loop
12 Phase difference output SQUID
20 Reference frequency oscillator
30 Electric value controlled oscillator
40 N frequency divider
50 Loop filter

Claims (6)

A phase comparator for a Josephson circuit;
Having a phase difference detection loop and a phase difference output SQUID placed in a cryogenic environment,
The phase difference detection loop,
a superconducting closed loop capable of passing a permanent ring current;
b first and second input terminals provided in the superconducting closed loop;
c A non-latch type first circuit which is provided in the closed loop and switches once when a magnetic flux quantum of a first frequency signal is input to the first input terminal to generate a permanent ring current rotating in the first direction in the closed loop. Josephson switching department;
d is provided in the closed loop, once the flux quantum of the second frequency signal is input to the second input terminal, once switched, the permanent loop current in the second direction opposite to the first direction in the closed loop A second non-latching Josephson switching section that produces
e, an inductor part forming one of mutual inductances for magnetic coupling to the phase difference output SQUID and forming an inductance large enough to hold a permanent ring current generated in the closed loop;
Has,
The phase difference output SQUID is a non-latch type DC SQUID,
f The other inductor part provided in itself is magnetically coupled to the one inductor part so as to form a mutual inductance, and a permanent ring in the closed loop of the phase difference detection loop via the mutual inductance. Extracting the presence of the current and outputting the output current or the output voltage as a change in the SQUID voltage applied to itself as a phase difference detection output signal;
A phase comparator for a Josephson circuit. The phase comparator according to claim 1, wherein:
A plurality of the phase difference detection loops are juxtaposed, and the same number of the phase difference output SQUIDs magnetically coupled to each other through the mutual inductance are also provided;
Applying the first frequency signal in parallel to the first input terminal of each of the plurality of phase difference detection loops and applying the second frequency signal in parallel to the second input terminal;
The plurality of phase difference outputs SQUIDs are connected in series to increase the detection sensitivity;
A phase comparator characterized by the above-mentioned. The reference frequency output from the reference frequency oscillator is compared with 1 / N of the oscillation frequency output from the electric value controlled oscillator, where N is an integer of 1 or more, and a phase difference detection output signal based on the difference between the two frequencies is output. A phase comparator that integrates the output of the phase comparator and outputs a control electric value as a current or voltage output from which an AC component has been removed or reduced. A PLL circuit for a Josephson circuit, which adjusts its own oscillation frequency in a direction in which the difference between the two frequencies in the phase comparator is eliminated by receiving the control electric value output by the phase comparator;
The phase comparator has a phase difference detection loop placed in a cryogenic environment, and a phase difference output SQUID,
The phase difference detection loop includes:
a superconducting closed loop capable of passing a permanent ring current;
b first and second input terminals provided in the superconducting closed loop;
c is provided in the closed loop, and when the magnetic flux quantum of the reference frequency as the first frequency signal is input to the first input terminal, the magnetic flux is once switched to generate a permanent ring current rotating in the first direction in the closed loop. A first Josephson switching unit of the latch type;
d is provided in the closed loop, once the flux quantum of the 1 / N frequency signal of the oscillation frequency as the second frequency signal is input to the second input terminal, once switched, the closed loop and the first direction A second non-latching Josephson switching section for producing a permanent ring current in the opposite second direction;
e an inductor part forming one of mutual inductances for magnetic coupling to the phase difference output SQUID;
Has,
The phase difference output SQUID is a non-latch type DC SQUID,
f The other inductor part provided in itself is magnetically coupled to the one inductor part so as to form a mutual inductance, and a permanent ring in the closed loop of the phase difference detection loop via the mutual inductance. Extracting the presence of the current and outputting the output current or the output voltage as a change in the SQUID voltage applied to itself as the phase difference detection output signal;
A PLL circuit for a Josephson circuit. The PLL circuit according to claim 3, wherein:
A plurality of the phase difference detection loops are juxtaposed, and the same number of the phase difference output SQUIDs magnetically coupled to each other through the mutual inductance are also provided;
Applying the first frequency signal in parallel to the first input terminal of each of the plurality of phase difference detection loops and applying the second frequency signal in parallel to the second input terminal;
The plurality of phase difference outputs SQUIDs are connected in series to increase the detection sensitivity;
A PLL circuit characterized by the above-mentioned. The PLL circuit according to claim 3, wherein:
Taking an output from the phase comparator as a control current through a low-pass filter as the loop filter;
The electric value control oscillator is magnetically coupled to a relaxation oscillator having a DC SQUID configuration and an inductor included in the relaxation oscillator in relation to modulating a critical current of a Josephson switching unit included in the relaxation oscillator. A current-controlled oscillator composed of an inductor through which current flows;
A PLL circuit characterized by the above-mentioned. The PLL circuit according to claim 5, wherein:
The inductor that is magnetically coupled to the inductor of the relaxation oscillator and through which the control current flows is diverted to the inductor forming the low-pass filter;
A PLL circuit characterized by the above-mentioned.

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