US3805176A

US3805176A - Microwave circuit

Info

Publication number: US3805176A
Application number: US00339882A
Authority: US
Inventors: T Kudo; K Kurachi
Original assignee: Fujitsu Ltd
Current assignee: Fujitsu Ltd
Priority date: 1970-07-22
Filing date: 1973-03-09
Publication date: 1974-04-16
Anticipated expiration: 1991-04-16
Also published as: DE2134250B2; DE2134250A1

Abstract

A microwave circuit comprises a waveguide which prevents the passage of the idler frequency. A coaxial line extends from the waveguide substantially perpendicular to the waveguide. A varactor is mounted in the waveguide at the intersection of the waveguide and the coaxial line. Short-circuit means provides an equivalent short-circuit surface in the waveguide at approximately a quarter wavelength from the diode. An idler circuit comprises the inductance exhibited by the waveguide. Idler circuit adjusting means is provided in the waveguide at more than a quarter wavelength of the idler frequency for adjusting the idler frequency. The idler circuit is a resonant circuit which resonates at a frequency higher than the selfresonant frequency of the diode.

Description

451 Apr. 16, 1974 MICROWAVE CIRCUIT Tatsuo Kudo, Kawasaki; Koichi Kurachi, Yokohama, both of Japan Inventors:

Fujitsu Limited, Kawasaki, Japan Filed: Mar. 9, 1973 Appl. No.: 339,882

Related US. Application Data Continuation-impart of Ser. No. 162,340, July 14, 1971, abandoned.

Assignee:

Foreign Application Priority Data Primary Examiner1-1erman Karl Saalbach Assistant Examiner-Darwin R. Hostetter Attorney, Agent, or Firm--l-Ierbert L. Lerner [57] ABSTRACT A microwave circuit comprises a waveguide which prevents the passage of the idler frequency. A coaxial line extends from the waveguide substantially perpendicular to the waveguide. A varactor is mounted in the waveguide at the intersection of the waveguide and the coaxial line. Short-circuit means provides an equivalent short-circuit surface in the waveguide at approximately a quarter wavelength from the diode. An idler circuit comprises the inductance exhibited by the waveguide. Idler circuit adjusting means is provided in the waveguide at more than a quarter wavelength of the idler frequency for adjusting the idler frequency. The idler circuit is a resonant circuit which resonates at a frequency higher than the self-resonant frequency of the diode.

5 Claims, 21 Drawing Figures CHOKE" PM 752 4 IOLER (7260/7 40.105774 6 Elf/WENT r5 TRANS/02,4452 7 coax/42 Cali meme 6' MICROWAVE CIRCUIT The present application is a continuation-in-part of pending application Ser. No. 162,340, filed July 14, 1971 now abandoned, for microwave circuit, assigned to the assignee hereof.

The invention relates to a microwave circuit. More particularly, the invention relates to a microwave circuit having an idler resonant circuit, which microwave circuit may be utilized in a parametric amplifier, a multiplier, a frequency converter, or the like.

A parametric amplifier is a microwave amplifier having as its basic element an electron tube or solid state device whose reactance can be varied periodically by an AC voltage at a pumping frequency. Operation is at room temperature. The diode amplifier, ferromagnetic amplifier, and up-converter are examples. The parametric amplifier is also called mavar, paramp and reactance amplifier.

In a conventional parametric amplifier, the selfresonance of a variable capacity diode such as, for example, a varactor diode utilized as the amplifying element, is generally utilized as the idler circuit in order to widen the bandwidth of the parametric amplifier. A varactor is a semiconductor device characterized by a voltage-sensitive capacitance which resides in the space-charge region at the surface of a semiconductor bounded by an insulating layer. A varactor may be utilized in automatic frequency control and electronic tuning circuits, and for parametric amplification. A varactor is also called varicap and voltage-variable capacitor.

The self-resonance of a varactor diode may be di' vided into a first self-resonance having a frequency frl which is available when the impedance viewed-from the varactor diode is zero, that is, when the varactor diode is short-circuited, and a second self-resonance having a frequency fr2 available when the outside impedance viewed from the varactor diode is infinite, that is, when the varactor diode is open-circuited. Generally, the frequency fr2 of the second self-resonance is larger than the frequency fr] of the first self-resonance. In the case of a varactor diode utilized in a parametric amplifier for microwaves, the frequency fr2 is approximately 1.4 to 2 times frl. I

When the first self-resonance of the varactor diode is utilized as the idler circuit, the conventional parametric amplifier has a narrow bandwidth, an uneven and complicated amplification characteristic curve, and occasional oscillation, and the capability of the varactor diode relative to the noise characteristic cannot be fully utilized. When the second self-resonance of the varactor diode is utilized as the idler circuit, the conven tional parametric amplifier has a spurious response, produces an uneven and complicated amplification characteristic curve and sometimes oscillates.

An object of the invention is to provide a microwave circuit which eliminates the disadvantages of the conventional parametric amplifiers.

Another object of our invention is to provide a microwave circuit which facilitates the adjustment of the idler circuit of a parametric amplifier.

Another object of the invention is to provide a microwave circuit which provides a wide bandwidth of the parametric amplifier, eliminates the spurious response of the parametric amplifier, produces an even and uncomplicated amplification characteristic curve, eliminates oscillations, and permits the full utilization of the capability of the varactor diode in relation to the noise characteristic.

Still another object of our invention is to provide a microwave circuit having an idler resonant circuit which may be readily adjusted, with less deterioration of the frequency band characteristic and with an excellent noise characteristic.

Still a further object of the invention is to provide a microwave circuit which permits the operation of the parametric amplifier at the point at which the noise characteristic of the varactor diode is the most suitable by utilizing an idler frequency higher than the selfresonant frequency of the varactor diode, confining the energy of the idler frequency in an area very close to the varactor diode and widening the amplification bandwidth, and adjusting the idler circuit with facility and rapidity.

Still another object of our invention is to provide a microwave circuit which enables a parametric amplifier operated at room temperature to produce a sufficiently wide bandwidth and reduces the noise temperature at the ends of the band to a minimum.

A further object of the invention is to provide a mi crowave circuit which permits a parametric amplifier to function with efficiency, effectiveness and reliability.

In accordance with the invention, a microwave circuit comprises a waveguide which prevents the passage of the idler frequency/The waveguide exhibits an inductance. A coaxial line extends from the waveguide substantially perpendicular to the waveguide. A varac tor is mounted in the waveguide at the intersection of the waveguide and the coaxial line. Short-circuit means provides an equivalent short-circuit surface in the waveguide at approximately a quarter wavelength from the diode. An idler circuit comprises the inductance exhibited by the waveguide. Idler circuit adjusting means is provided in the waveguide at more than a quarter wavelength of the idler frequency for adjusting the idler frequency.

The waveguide exhibits an inductance in the area of the varactor diode. The coaxial line exhibits a reactance in the area of the diode. The diode has a selfresonant frequency. The idler circuit comprises the inductance exhibited by the waveguide in the area of the diode and the reactance exhibited by the coaxial line in the area of the diode. The idler circuit is a resonant circuit which resonates at a frequency higher than the self-resonant frequency of the diode.

The idler circuit adjusting means may comprise a screw inserted into the waveguide, or a pair of spaced screws inserted into the waveguide.

The idler circuit adjusting means may comprise a first screw inserted into the waveguide at a first specific distance from the diode and extending into the waveguide to a first variable depth and a second screw inserted into the waveguide at a second specific distance from the diode and extending into the waveguide to a second variable depth.

A radial choke is provided between the waveguide and the coaxial line for choking the idler electric power. Fine adjusting means for the idler circuit comprises a disc having a concave groove formed therein adjacent the coaxial line between the radial choke and the varactor diode. The fine adjusting is determined by the depth of the groove. The disc varies the electrical length between the choke and the diode.

A Iowpass filter between the waveguide and the coaxial line has a low pass filter element for preventing the leakage of electric power to the coaxial line. Fine adjusting means for the idler circuit comprises a concave groove formed in the filter element. The fine adjusting is determined by the depth of the groove.

Support means supports the varactor diode in the waveguide. A low pass filter is provided adjacent the diode between the waveguide and the coaxial line. Fine adjusting means for the idler circuit comprises a concave groove formed in the support means in the waveguide adjacent the diode. The fine adjusting is determined by the depth of the groove.

In order that the invention may be readily carried into effect, it will now be described with reference to the accompanying drawings, wherein:

FIG. la is a sectional view of an embodiment of a conventional parametric amplifier;

FIG. lb is the equivalent circuit of the idler circuit of the embodiment of the parametric amplifier of FIG. 1a;

FIG. 2a is a sectional view of another embodiment of a conventional parametric amplifier;

FIG. 2b is the equivalent circuit of the idler circuit of the embodiment of the parametric amplifier of FIG. 2a;

FIG. 3a is a sectional view of an embodiment of a parametric amplifier utilizing the microwave circuit of the invention;

FIG. 3b is the equivalent circuit of the idler circuit of the embodiment of the parametric amplifier of FIG. 3a;

FIG. 4a is a sectional view of another embodiment of a parametric amplifier utilizing another embodiment of the microwave circuit of the invention;

FIG. 4b is the equivalent circuit of the idler circuit of the embodiment of the parametric amplifier of FIG. 40;

FIG. 5 is a graphical presentation of the frequency characteristics of the conventional parametric amplifiers and the parametric amplifiers of the invention;

FIGS. 6a, 6b, 6c and 6d are schematic diagrams of the waveguide of the microwave circuit of the invention for explaining the operation of the invention;

FIG. 7 is the equivalent circuit of the idler circuit of another embodiment of the parametric amplifier of the invention;

FIG. 8 is a graphical presentation of the relation between the idler frequencies and the bandwidths and noise temperatures of the idler circuit;

FIG. 9 is a graphical presentation of the relation between the signal frequencies and the gain and noise temperatures of the parametric amplifiers;

FIGS. 10a and 10b are sectional views of an embodiment of the parametric amplifier of the invention;

FIG. 11a is a sectional view of part of another embodi-ment of the parametric amplifier of the invention;

FIG. 11b is a sectional view of part of still another embodiment of the parametric amplifier of the invention; and I FIG. 12 is a graphical presentation for explaining the characteristics of the embodiments of FIGS. 10a, 10b and 11a, 11b.

In the figures, the same components areidentified by the same reference numerals.

The microwave circuit of the present invention is described with reference to parametric amplifiers utilizing microwave circuits.

FIG. la illustrates the parametric amplifier constituted when the first self-resonance of the varactor diode is utilized as the idler circuit. The parametric amplifier of FIG. la comprises a varactor diode 1. An electric power supply pumping waveguide 2 is reduced in its vertical dimension or height in the electric field direction from the normal vertical dimension of a waveguide 3 by a tapered portion. The waveguide 2 is also reduced in its horizontal dimension or width in the direction perpendicular to the electric field direction. The reduction in dimensions permits the transmission of the pumped electric power, but prevents the transmission of the idler frequency through the waveguide 2.

A choke filter 4 is tuned to the idler frequency and is spaced from a wide wall 5 of the waveguide 2 by a distance equal to a quarter wavelength of the idler frequency. The impedance is made sufficiently small in value by the idler frequency at the position of the wide wall 5 of the waveguide 2, when the filter 4 is viewed from the varactor diode 1. A coaxial conductor having an inner conductor 6 is provided perpendicularly to the waveguide 2 and operates to provide the tuning of the reactance of the varactor diode with the signal frequency.

A transformer 7 varies the coupling coefficient of the signal circuit. The coaxial conductor has a signal input and output end 8. The signal input and output end 8 is connected to a circulator (not shown in the FIG.) for separating the input and output. A variable shortcircuit end 9 is provided for the waveguide 2, matching said waveguide and the varactor diode I. The end 9 is an arbitrary distance L2 from the varactor diode 1.

FIG. lb shows the equivalent circuit of the idler resonant circuit of the parametric amplifier of FIG. 1a. The equivalent circuit of FIG. lb comprises an inductance l1 and a series circuit arrangement 12 of a capacitor and an inductance connected in series with the varactor diode 1. The inductance 11 indicates that the waveguide 2 of FIG. 1a, which prevents the passage or propagation of the idler frequency, may be regarded as an inductance. The series circuit arrangement 12 of FIG. lb indicates that the filter 4 of FIG. la, viewed from the wide wall 5 of the waveguide 2, may be regarded as a series resonant circuit.

When the second self-resonance of the varactor diode is utilized as the idler circuit, the parametric amplifier is constituted as shown in FIG. 2a. The embodiment of FIG. 2a is different from that of FIG. 1a in the spacing of the filter 4. In the embodiment of FIG. 2a, the filter 4 is spaced from the wide wall 5 of the waveguide 2 by lessthan one tenth wavelength of the idler frequency. The choke filter 4 is thus closer to the wide wall 5 of the waveguide 2 in FIG.- 2a.

The equivalent circuit of FIG. 2b includes the inductance l1 and a parallel resonant circuit 13 having a capacitor connected in parallel with an inductor. The parallel resonant circuit 13 is connected in series with the varactor diode l. The equivalent circuit of FIG. 2b indicates that the filter 4, viewed from the wide wall 5 of the waveguide 2, may be regarded as the parallel resonant circuit 13, and that the parallel resonant circuit 13 is coupled to the inductance 11, as indicated by a double-headed arrow 14.

The equivalent inductance ll of the waveguide 2, through which the idler frequency cannot be passed in the conventional parametric amplifier of FIGS. 1a and 2a, is substantially unnecessary and must therefore be made as small as possible. It has, however, been difficult to make the inductance ll sufficiently small without a disadvantageous influence on the signal circuit or pump circuit. The inductance ll narrows the bandwidth of the parametric amplifier of FIG. 1a and causes a spurious response of the parametric amplifier of FIG. 2a. Furthermore, the inductance 11 produces an uneven and complicated amplification characteristic curve and sometimes causes oscillations.

The parametric amplifier of FIG. 1a has an additional disadvantage that the self-resonance of the varactor diode is utilized as the idler circuit. Generally, however, the first self-resonant frequency is lower than the frequency which minimizes the noise factor. This prevents the full utilization of the capability of the varactor diode in relation to the noise characteristic.

FIG. 3a illustrates an embodiment of the microwave circuit of the invention utilized in a parametric amplifier. In the parametric amplifier of FIG. 3a, the second self-resonance of the varactor diode 1 is utilized. The waveguide 2 through which the idler frequency cannot pass is equivalently short-circuited at a point 100 at a distance L0, which is a quarter wavelength of the idler frequency, from the varactor diode 1. In accordance with the invention, the idler circuit adjusting element 15, which may comprise, for example, a screw, is inserted in the waveguide 2 in the direction of the electric field of said waveguide at a distance L1 from the varactor diode l. The distance L1 is greater than one quarter wavelength of the idler frequency. The element may be from one quarter to one half wavelength from the diode 1. The distance L2 is greater than L1. The end 9 is a screw which is capable of adjusting the pumping circuit without affecting the idler circuit.

The idler equivalent circuit shown in the equivalent circuit of FIG. 3b is thus a parallel resonant circuit 16 having a tuning frequency which is continuously variable. The parallel resonant circuit 16 replaces the inductance ll of the equivalent circuits of the prior art of FIGS. lb and 2b. This indicates that the structure of the parametric amplifier of FIG. 3a permits the obtaining of a wideband single-humped amplification characteristic when the signal circuit is single-tuned.

FIG. 4a illustrates another embodiment of the microwave circuit of the invention as utilized in another embodiment of a parametric amplifier. The choke filter 4 is again provided close to the waveguide 2, as in the conventional parametric amplifier shown in FIG. 2a.

The second self-resonance of the varactor diode 1 is utilized. In the idler equivalent circuit of the equivalent circuit of FIG. 4b, the parallel

resonant circuits

13 and 16 are mutually coupled, as shown by the doubleheaded arrow 14. Experimentation has proven that the structure of the embodiment of FIG. 4a may produce a double-humped amplification characteristic, even when the signal circuit is single-tuned.

FIG. 5 illustrates the amplification frequency characteristics of parametric amplifiers exhibited when the signal circuits are single-tuned. In FIG. 5, the abscissa represents the amplified frequency f in gigahertz and the ordinate represents the gain in db. A curve 17 illustrates the characteristic of the parametric amplifier of FIG. 1a. A curve 18 illustrates the characteristic of the parametric amplifier of FIG. 2a. A curve 19 illustrates the characteristic of the parametric amplifier of FIG. 3a. A curve 20 illustrates the characteristic of the parametric amplifier of FIG. 4a.

It is evident that the characteristic curves l9 and 20 of the parametric amplifiers of FIGS. 3a and 4a of the invention are superior to the characteristic curves I7 and 18 of the conventional parametric amplifiers of FIGS. la and 2a. In accordance with conventional theory, the frequency-gain characteristic of the parametric amplifier of FIG. 4a, if the idler has two parallel resonant circuits l3 and 16 of different resonant frequencies (FIG. 4b), should become a double-humped characteristic with an extremely deep valley when a resistance other than the series resistance of the varactor diode 1 is neglected. Actually, however, as shown in the curve 20 of FIG. 5, there is no deep valley, and when the resonant frequencies of the parallel resonant circuits l3 and 16 are made to approach each other, the two resonant frequencies approach each other due to the mutual coupling of said parallel resonant circuits and a double-humped characteristic with a valley of about 1 db may be provided.

The foregoing cannot be satisfactorily explained by the conventional concept of regarding an operation as only either one of the current excitation type, wherein an external circuit viewed from a varactor diode may be regarded as a series resonant circuit, or the voltage excitation type, wherein an external circuit viewed from a varactor diode may be regarded as a parallel res onant circuit. The satisfactory explanation for the fact that the valley of the double-humped amplification fre quency characteristic of the curve 20 of FIG. 5, indicative of the parametric amplifier of FIG. 4a, is not very deep, is that the two humps primarily indicate the current excitation type operation and the valley indicates the combination of the current excitation type operation and some voltage excitation type operation. If the idler circuit of the parametric amplifier of FIG. 40 has a circuit loss, the valley of the double-humped characteristic becomes shallow and becomes, as a whole, a wideband characteristic, and even the amplification frequency characteristic may be obtained. Therefore, in the case where some deterioration of the noise characteristic is allowable, it is possible to realize a wideband characteristic by including a resistor in addition to the idler circuit adjusting element or screw 15.

In the case of a cooled parametric amplifier, which is required for an extremely low noise characteristic, even if a wideband and a level amplification frequency characteristic is provided by the use of a single-tuned idler circuit and a double-tuned signal circuit, the noise characteristic is deteriorated at the two ends of the band due to the quality factor Q of the idler resonant circuit. In this case, it is also possible to make the entire amplification frequency characteristic level and lower the maximum equivalent noise temperature within the band by the series connection of parametric amplifiers of the embodiment of FIG. 4a in which the idler circuit is made double-tuned and adjustment is made so that the noise characteristic may be improved at the two ends of the amplified band.

FIGS. 6a, 6b, 6c and 6d explain the principle of operation of the microwave circuit of the invention. FIG. 6a is a top sectional view and FIG. 6b is a side sectional view, taken along the lines VlBVIB of FIG. 6a, of a rectangular waveguide constructed to investigate to what extent the microwave energy enters the rectangular waveguide through which the idler frequency cannot propagate. The rectangular waveguide of FIGS. 6a and 6b is also for investigating how the equivalent short-circuit surface of the waveguide is varied when the frequency approaches the cutoff frequency of said waveguide.

The rectangular waveguide is, as shown in FIG. 6a,

reduced in its horizontal dimensions in the electric field direction from the dimension of a normal waveguide 21 by indentations or steps and is reduced in its vertical dimension by a taper, as shown in FIG. 6b. The portion of the wide wall on the left side of a plane 22 vertical to the axis VlBVIB of the waveguide is reduced symmetrically about said axis, as shown in FIG. 6a. A pair of idler circuit adjusting elements or

screws

23 and 24 are inserted in a waveguide 25 of reduced dimensions extending from the waveguide 21. The idler

circuit adjusting elements

23 and 24 are spaced from each other and are inserted in the direction of the electric field.

The idler circuit adjusting element 23 is spaced from the plane 22 by a distance L3 and the idler circuit adjusting elem ent 24 is spaced from the plane 22 by a distance L4. The idler circuit adjusting element 23 extends into the waveguide 25 a distance d1 and the idler circuit adjusting element 24 extends into the waveguide 25 a distance d2. When microwave frequencies at the cutoff region in the waveguide 25 are applied from the waveguide 21 to the waveguide 25, and the equivalent short-circuit surface is spaced from the plane 22 a distance L, the foremost end of a magnetic field 26 of the microwave arrives at a position in the waveguide 25 spaced from the plane 22 by more than the equivalent short-circuit surface.

FIG. 6c illustrates the variation of a distance L5 (FIG. 6a) between the equivalent short-circuit surface and the plane 22 in the situation where the length (11 and d2 of the inserted portions of the idler

circuit adjusting elements

23 and 24 within the waveguide 25 are both zero and the microwave frequency fis increased and equal to the cutoff frequency fc of the waveguide 25. FIG. 60. shows a curve 27 which indicates the relation between the length d1 of the inserted portion of the idler circuit adjusting element 23 provided at the position of three eighths free space wavelength of the microwave frequency fl) and the distance L5 between the equivalent short-circuit surface and the plane 22.

In FIG. 6d, a curve 28 shows the reflection coefficient. A curve 29 of FIG. 6d shows the relation between the length d2 of the inserted portion of the idler circuit adjusting element 24 at the position of three quarters free space wavelength of the microwave frequency f and the distance L5. As illustrated by the curve 29 of FIG. 6d, series resonance occurs when the length of the inserted portion of the idler circuit adjusting element 23 becomes d0 and the variation of the distance L is rapid around d0 and also around the point of resonance, as seen from the curve 28. Under these conditions, the reflection coefficient 1- also becomes small and the circuit loss increases. It is therefore necessary to avoid the insertion of the idler circuit adjusting element 23 until the point of resonance is approached. The curve 29 of FIG. 6d is essentially a straight line, except for the portion very close to the point of resonance, which indicates that the microwave energy of frequency f0 does not reach the idler circuit adjusting element or screw 24.

In FIG. 60, the abscissa represents the frequency in gigahertz and the ordinate represents the distance in mm. In FIG. 6d, the abscissa represents the distance d1 and d2 in mm and the ordinate represents the distance in mm and the reflection coefficient 7. FIG. 6c

illustrates a curve 31 which is intersected at a point 32 by a distance L0. In FIG. 6d, the curve 27 is intersected at a point 33 by the distance L0.

It has been found by actual measurement that the distance L between the equivalent short-circuit surface and the plane 22 (FIGS. 6a and 6b) in the waveguide 25 (FIGS. 6a and 6b) of the same dimensions as the waveguide 2 of FIGS. 3a and 4a under the idler frequency with which the parametric amplifiers of FIGS. 3a and 4a operate, is about a quarter wavelength of the free space wavelength. This illustrates that the idler electromagnetic field distribution within the waveguide 2 of FIGS. 3a and 4a is similar to the electromagnetic field distribution within the waveguide 25 of FIGS. 6a and 6b in which the varactor diode (not shown in FIG. 6a or 6b) is provided in the plane 22.

When the distance L1 (FIGS. 3a and 4a) is greater than the distance L4 (FIG. 6a) the pump circuit is adjustable by the movement of the variable short-circuit end 9 (FIGS. 3a and 4a) without affecting the idler circuit. The idler circuit and the pump circuit may then be adjusted with great facility by adjusting the idler circuit by the idler circuit adjusting element or screw 15 (FIGS. 3a and 4a) and then adjusting the pump circuit by the variable short-circuit end 9 (FIGS. 3a and 4a). The idler circuit of a parametric amplifier may therefore be designed by determining the range within which the distance L5 between the equivalent short-circuit surface and the plane 22 (FIG. 6a) is varied by the idler circuit adjusting element or screw 15 (FIGS. 3a and 4a) in view of the curve 27 of FIG. 6d, determining a distance L0 under the second self-resonant frequency of the varactor diode at a magnitude equal to one quarter free space wavelength, selecting Afin FIG. 60 at a suitable magnitude smaller than that of the signal frequency in order to determine the cutoff frequency fc, and determining the dimensions of the waveguide 2 (FIGS. 3a and 4a).

In the aforedescribed embodiments of the invention illustrated in FIGS. 3a and 4a, the idler circuit adjusting element or screw 15 is provided on the side of the varactor diode 1 facing the variable shortcircuit end 9. Exactly the same adjustment effect may be provided by inserting the idler circuit adjusting element or screw 15 on the side of the varactor diode 1 facing the waveguide 3. In such case, the screw 15 is placed a distance L2 from the varactor diode 1. Two idler circuit adjusting elements or screws may be provided, one on each side of the varactor diode 1. The provision of the idler equivalent short-circuit surface at the position of the waveguide 2 in FIG. 3a spaced from the varactor diode 1 less than a quarter wavelength of the idler frequency is equivalent to the connection of an inductance as the idler external circuit of said varactor diode. In such case, the idler frequency may be made higher than the second self-resonant frequency of the varactor diode 1.

of the parametric amplifier of the invention with an inductance connected to the idler circuit as an external circuit of said idler circuit. In the equivalent circuit, an inductance 34 is constituted by a length L6 which is smaller than a quarter wavelength. A minute reactance 35 of the equivalent circuit is constituted by a length L7 approximating a half wavelength or less. At an eighth wavelength the sum of the inductance 34 and the minute reactance 35 is positive, the external circuit of the varactor diode 1 is inductive and the resonant frequency of the entire circuit is greater than fr2.

FIG. 8 illustrates a curve 36 which shows the relation between the idler frequency of the idler circuits of FIGS. 2a and 7 and the bandwidth of the idler circuit of the parametric amplifier. In FIG. 8, the abscissa represents the idler frequency in gigahertz and the ordinate represents the bandwidth of the idler circuit in megahertz and the noise temperature in degrees. Kelvin. In FIG. 8, curves 37, 38 and 39 illustrate the relation between the idler frequency and the noise temperature at the ends of specific bands. The relation is avail able when the signal circuit is double-tuned and the gain characteristic is level within the bands. The amplified bandwidths of

the'curves

37, 38 and 39 are widened in that order and the idler frequency at whichthe noise temperature becomes a maximum is small.

In FIG. 9, the abscissa represents the single frequency in gigahertz and the ordinate represents the noise temperature in degrees Kelvin and the gain in db. FIG. 9 illustrates the gain-frequency characteristics and the noise temperature-frequency characteristics at idler frequenciesfil,fi andfi2 (FIG. 8) corresponding to points 41, 42 and 43 of the curve 39 of FIG. 8. In FIG. 9, a curve 44 shows the gain characteristics, and curves 45, 46 and 47 show the noise temperature characteristics corresponding to the

points

41, 42 and 43 in FIG. 8. The noise temperatures at the ends fsl and fs2 (FIG. 9) of the band become a minimum when the idler frequency is fi0. By determining the gain bandwidth from these curves, it is possible to select the idler frequency at the band ends at which the noise temperatures become a minimum.

FIG. a is a vertical sectional view of a practical embodiment of the parametric amplifier of the invention having an idler circuitto which an inductance is connected as an external circuit of the idler circuit. FIG. 10b is a cross-sectional view taken along the lines XB-XB of FIG. 10a. In FIGS. 10a and 10b, the parametric amplifier comprises a varactor diode 48. A pumping electric power supply waveguide 49 has a tapered portion 51 extending into a waveguide 52 through which the pump frequency may be passed, but which prevents the passage of the idler frequency. A variable short-circuit end 53 is provided for matching the pump electric power.

In the parametric amplifier of FIGS. 10a and 10b, an impedance transformer 54 (FIG. 10a) is connected in the signal frequency system. The transformer 54 varies the coupling coefficient of the signal circuit. A radial choke or choke filter 55 (FIG. 10a) chokes the pump electric power and a radial choke 56 chokes the idler electric power. A disc 57 (FIG. 10a) provides an elec' trical length L8 (FIG. 10a) from the varactor diode 48 to the radial choke 56. The disc 57 has a concave portion 58 formed therein. The electrical length L8 is variable in accordance with the variation of the concave portion 58 of the disc 57 even when the thickness of said disc is constant.

A radial choke may be regarded as a quarter wave length line with a short-circuited end. Therefore, when the length L8 is about a quarter wavelength, the coaxial line between the varactor diode 48 and the radial choke 56, viewed at the idler frequency, is equivalent to the length L7 of the half wavelength in FIG. 7. The waveguide 52, through which the idler frequency cannot pass, may be regarded as a line of a specific length with a short-circuited end and said waveguide is viewed from the varactor diode 48. The waveguide 52 therefore becomes equivalent to the length L6 of FIG. 7. In this case, the distance L6 from the equivalent short-circuit surface to the varactor diode 48 is varied by the width W of the waveguide 52.

The width W of the waveguide 52 is therefore selected so that the idler circuit of FIG. 4a may substan tially resonate at the optimum idler frequency fi0 provided by FIG. 8 and the fine adjustment may be provided by the variation of the depth of the concave por tion 58 of the disc 57. This may be achieved with facility and rapidity by exchangeably utilizing a plurality of discs 57 having concave portions 58 of different depths.

FIG. 11a shows another embodiment of the parametric amplifier of the invention utilizing a low pass filter 59 for preventing the leakage of the pump electric power and the idler electric power to the coaxial line side of the waveguide 52. Fine adjustment may be provided in the low pass filter 59 by varying the depth L9 of the concave portion 61 of the low pass filter element 62 of the low pass filter 59.

FIG. 1 lb shows still another embodiment of the parametric amplifier of the invention. In the embodiment of FIG. 11b, a holding stand or support 63 supports or holds the varactor diode 48. The support 63 has a concave portion 64 formed therein. Fine adjustment is provided by varying the depth L10 of the concave portion 64 of the support 63. The low pass filter 59 has a standard low pass filter element 65.

FIG. 12 illustrates the relation between the width W of the waveguide and the external reactance. In FIG. 12, the abscissa represents the width W of the waveguide in mm and the ordinate represents the external reactance XT in ohms. The external reactance equals QL34 XC35 (FIG. 7).

Curves

66, 67 and 68 of FIG. 12 show the relation between the entire external reactance XT of the idler circuit and the width W of the waveguide in a parametric amplifier in which the length L8 of FIG. or the depth L9 of FIG. 11a and L10 of FIG. 11b have a specific constant value. The inductance L34 (FIG. 7) required for the optimum idler frequency is determined by the width W of the waveguide determined from FIG. 12. Fine adjustment may be pro vided by the variation of the reactance XC35 (FIG. 7). It should be noted that the width W of the waveguide is limited to a range WP to Wi, because when said width is less than WP, the pump frequency cannot pass through the waveguide, and when said width exceeds Wi, the idler frequency is passed through the waveguide.

As hereinbefore described, the microwave circuit of the invention utilizes a pump supply waveguide through which the idler frequency cannot be passed and which has an equivalent short-circuit surface, located at a position spaced from. the varactor diode by a quarter wavelength of the idler frequency. An idler circuit adjusting element such as, for example, a screw, is provided at a position spaced from the varactor diode by a quarter wavelength of the idler frequency. The foregoing features result in a simple structure of the microwave circuit, facility and rapidity of adjustment and electrical characteristics which are superior to those of the conventional circuits.

Furthermore, in accordance with our invention, the parametric amplifier may be operated at a point at which the noise characteristic of the varactor diode is the most suitable or excellent by utilizing an idler frequency higher than the self-resonant frequency of the varactor diode. The energy of the idler frequency may be confined to an area very close to the varactor diode and the amplification bandwidth may be widened. The idler circuit may be adjusted with facility and rapidity. A special feature of the invention is that a considerable suitable effect may be provided by the application of the invention to a parametric amplifier operated at room temperature in order to provide a sufficiently wide bandwidth and to reduce the noise temperature at the ends of the band to a minimum.

The aforedescribed idler circuit of the invention is also applicable to a varactor diode multiplier having an idler resonant circuit and a choke filter of the image frequency of a frequency converter comprising semiconductor elements, and the like.

While the invention has been described by means of specific examples and in specific embodiments, we do not wish to be limited thereto, for obvious modifications will occur to those skilled in the art without departing from the spirit and scope of the invention.

We claim:

1. A microwave circuit, comprising a waveguide;

a coaxial line having an axis perpendicular to the waveguide;

a diode mounted in the waveguide at the intersection of the coaxial line and the waveguide, thereby providing the waveguide with a region which prevents the passage of the idler frequency, the waveguide having dimensions determined to provide an equiv alent short-circuit surface in the waveguide at a distance of about a quarter wavelength from the diode, the waveguide exhibiting an inductance preventing the passage of the idler frequency;

an idler circuit including the inductance of the waveguide; and

a screw for adjusting the idler frequency provided in the waveguide at a position spaced from the diode by more than a quarter wavelength of the idler frequency.

2. A microwave circuit as claimed in claim 1, wherein the idler circuit comprises the inductance of the waveguide preventing the passage of the idler frequency and a reactance exhibited by the coaxial line.

3. A microwave circuit as claimed in claim 1, wherein the resonant frequency of the idler circuit is selected to be higher than the self-resonant frequency of the diode.

4. A microwave circuit, comprising a waveguide which prevents the passage of the idler frequency, the waveguide exhibiting an inductance;

a coaxial line extending from the waveguide substantially perpendicular to the waveguide;

a diode mounted in the waveguide at the intersection of the waveguide and the coaxial line;

short-circuit means providing an equivalent shortcircuit surface in the waveguide at approximately a quarter wavelength from the diode;

an idler circuit comprising the inductance exhibited by the waveguide; idler circuit adjusting means in the waveguide at more than a quarter wavelength of the idler frequency for adjusting the idler frequency; support means supporting the diode in the waveguide and a low pass filter adjacent the diode between the waveguide and the coaxial line; and

fine adjusting means for the idler circuit comprising a concave groove formed in the support means in the waveguide adjacent the diode, the fine adjusting being determined by the depth of the groove.

5. A microwave circuit as claimed in claim 4, wherein the diode is a varactor diode.

Claims

1. A microwave circuit, comprising a waveguide; a coaxial line having an axis perpendicular to the waveguide; a diode mounted in the waveguide at the intersection of the coaxial line and the waveguide, thereby providing the waveguide with a region which prevents the passage of the idler frequency, the waveguide having dimensions determined to provide an equivalent short-circuit surface in the waveguide at a distance of about a quarter wavelength from the diode, the waveguide exhibiting an inductance preventing the passage of the idler frequency; an idler circuit including the inductance of the waveguide; and a screw for adjusting the idler frequency provided in the waveguide at a position spaced from the diode by more than a quarter wavelength of the idler frequency.

4. A microwave circuit, comprising a waveguide which prevents the passage of the idler frequency, the waveguide exhibiting an inductance; a coaxial line extending from the waveguide substantially perpendicular to the waveguide; a diode mounted in the waveguide at tHe intersection of the waveguide and the coaxial line; short-circuit means providing an equivalent short-circuit surface in the waveguide at approximately a quarter wavelength from the diode; an idler circuit comprising the inductance exhibited by the waveguide; idler circuit adjusting means in the waveguide at more than a quarter wavelength of the idler frequency for adjusting the idler frequency; support means supporting the diode in the waveguide and a low pass filter adjacent the diode between the waveguide and the coaxial line; and fine adjusting means for the idler circuit comprising a concave groove formed in the support means in the waveguide adjacent the diode, the fine adjusting being determined by the depth of the groove.