TUNING SYSTEM ON DIELECTRIC SUBSTRATES

Abstract: A tuning system for use in communications of an integral construction and stable in operation comprising a dielectric; a pair of first and second electrodes (29,30) confronting each other with said dielectric interposed therebetween, each of said first and second electrodes having at least one bent portion to present primarily a lumped-constant inductance and having a desired equivalent electrical length; said first and second electrodes having ground or common terminals located out of mutually confronting positions so that voltage signals induced by mutual induction between the electrodes will be opposite in phase with respect to each other, thereby producing a parasitic distributed-constant capacitance due to a potential difference between the electrodes and a dielectric constant of the dielectric; one of said first and second electrodes having a first terminal (27) at a desired position thereof, the electrodes forming a two-terminal circuit net­ work having the first terminal with the ground or common terminals serving as a second terminal, the two terminal circuit network forming an equivalent parallel resonant circuit composed of a lumped-constant inductance formed by said one electrode and a parasitic distributed-constant capacitance between the first and second electrodes; and a feedback amplifier (26) connected to said parallel resonant circuit for generating an inherent voltage vibration to produce a tuning oscillating signal.

Description: BACKGROUND OF THE INVENTION 1. Field of the invention: The present invention relates to a tuning system for use in transmitters and receivers in radio, television, stereo tuners, and personal radio communications, and other commumication equipment. 2. Description of the Prior Art: As more and more waves are used in radio and television broadcasting and communications, tuning systems for selecting the frequencies of waves to be recieved are required to be highly stable and reliable in their performance. There has also been a great demand for the reduction of the cost of manufacture of receivers, transmitters and other communication equipment in which the tuning systems are installed. In particular, the development of a new technology is desired for tuning system circuit components in radio-frequency stages which have been difficult to improve. Conventional tuning systems will be described with reference to the drawings. FIG. 1 of the accompanying drawings shows a basic circuit arrangement of a tuning oscillator device. The tuning oscillator comprises a tuning circuit 4 composed of a tuning inductor 1, a variable capacitor 2, and a fixed capacitor 3. To the tuning circuit 4, there is connected an input terminal or output terminal 6 of a feedback amplifier 5. FIG. 2 illustrates another circuit arrangement of a conventional tuning amplifier device, which comprises a tuning circuit 10 composed of a tuning inductor 7, a variable capacitor 8, and a fixed capacitor 9. A signal applied to an input terminal 11 is amplified by an amplifier 12 which supplies an amplified output through an output terminal 13 to the tuning circuit 10. FIG. 3 shows a circuit arrangement of a conventional mixer device including a tuning circuit 16 comprising a tuning inductor 14 and a fixed capacitor 15. A mixer 9 has a first input terminal 17, a second input terminal 18, and an intermediate-frequency output terminal 20 connected to the tuning circuit 16. FIG. 4 shows conventional components of which the tuning circuits 4, 10, 16 shown in FIGS. 1 through 3 are composed. The components include a tuning inductor 21, a variable capacitor 22, and a fixed capacitor 23 which are interconnected by circuit conductors 24, 25. However, the conventional arrangements have been subjected to the following disadvantages: In the tuning system shown in FIG. 4, the inductor component 21 is larger in size as compared with the other components, and is particularly much larger in height, with the result that the equipment in which the tuning system is incorporated is not rendered smaller in size and lower in profile. A ferrite core inserted in the inductor component is variable in position due to mechanical vibrations, resulting in wide variations in tuning frequencies. The inductance of the inductor component is unstable due to a large degree of temperature-dependency of the magnetic permeability of the ferrite core, a feature which also causes tuning frequencies and also Q of the resonator filter circuit to vary widely. For keeping tuning frequencies stably at their target settings, the components are required to be installed highly accurately in predetermined positions. Where the tuning systems are mass-produced as RF resonator filters, it is difficult to maintain a desired installation accuracy and hence the tuning frequencies tend to differ largely from their target settings and cannot be caused to converge to fixed values. Therefore, there has been difficulty experienced with the mass production of the tuning systems. The tuning system arrangement shown in FIG. 4 has other drawbacks. The inductor and the capacitor are constructed as separate components that are interconnected by long conductors. The long conductors tend to produce unwanted lead inductances and stray capacitances which cause the tuning circuit to operate unstably and make it difficult to achieve an initial design target. Accordingly, no sufficient frequency selectivity can be ensured, and undesired resonant conditions are produced at uncertain frequencies, with the consequence that no tuning systems of desired design can be achieved. This invites abnormal oscillation, oscillation of unwanted signals, increased higher harmonics in oscillated signals, resulting increased distortions, and a narrowed width in which a variable tuning oscillation frequency varies. Furthermore, no sufficient frequency selectivity can be ensured, and undesired resonant conditions are produced at uncertain frequencies, with the consequence that no tuning amplifiers of desired design can be achieved. This invites abnormal oscillation, response of unwanted signals, increased higher harmonics in amplified signals, resulting increased distortions, a narrowed width in which a variable tuning oscillation frequency varies, and deterioration in abilities to remove intermodulation interference and spurious interference. Also, no sufficient frequency selectivity can be ensured, and undesired resonant conditions are produced at uncertain frequencies, with the consequence that no intermediate-frequency tuning circuits of desired design can be achieved. This invites abnormal oscillation, generation of spurious signals, increased distortions caused by increased higher harmonics in intermediate-frequency signals, and deterioration in abilities to remove intermodulation interference and spurious interference. In an article entitled "A linear voltage controlled oscillator" by J. Dearden in Electronic Engineering, vol. 55, no. 678, June 1983, a design for such an oscillator is described. In the circuit a loose coupling between a primary oscillator and an output amplifier is provided by a pair of coupled inductances, L 1 and L2, which are bent pieces of wire in defined physical relationship to each other. One of the inductances is in a resonant circuit of the primary oscillator, and the frequency of oscillation varies in accordance with an input control voltage. In U.S. patent 3243742 a variable frequency resonator is described. This comprises two flat conductive electrodes which are preferably generally circular and have defined cut out portions and which are placed on either side of a flat dielectric material. Tuning is performed by rotation of one electrode with respect to the other, as the rotation varies the areas of the electrodes which directly face each other across the dielectric. Since known tuning circuits are composed of discrete components having minimum unit functions, there is a limitation on efforts to reduce the number of parts used and improve the manufacturing process. The present invention provides tuning system comprising: a dielectric; a pair of first and second electrodes confronting each other with said dielectric interposed therebetween, each of said first and second electrodes having at least one bent portion to present primarily a lumped-constant inductance and having a desired equivalent electrical length; said first and second electrodes having ground or common terminals located either adjacent mutually opposite ends of the electrodes or at an intermediate position of one electrode and adjacent both ends of the other electrode so that voltage signals induced by mutual induction between the electrodes will be opposite in phase with respect to each other, thereby producing a parasitic distributed-constant capacitance due to a potential difference between the electrodes and a dielectric constant of the dielectric; one of said first and second electrodes having a first terminal at a given position thereof, the electrodes forming a two-terminal circuit network, said ground or common terminals serving as a second terminal, the two-terminal circuit network forming an equivalent parallel resonant circuit composed of a lumped-constant inductance formed by said one electrode and a parasitic distributed-constant capacitance between the first and second electrodes; and one or more of the following items;- a) a feedback amplifier connected to said parallel resonant circuit for generating an inherent voltage vibration to produce a tuning oscillating signal at the resonance frequency of said parallel resonant circuit; b) an amplifier connected to said parallel resonant circuit for generating a tuning amplifying action at the resonance frequency of said parallel resonant circuit to produce a selective tuning amplified signal; c) a mixer connected to said parallel resonant circuit and responsive to frequency signals to be mixed for generating a frequency converting action to produce an intermediate frequency tuning signal, said resonance frequency of said parallel resonant circuit corresponding to the frequency of the signal present at the mixer terminal to which said parallel resonant circuit is connected. An advantage of the present invention is the provision of a tuning circuit which is composed of an integral construction of inductor and capacitor components, and which enables apparatus into which it is incorporated to be low in profile, small in size, stable and highly accurate in operation against variations in ambient conditions such as mechanical vibrations and temperature variations, and stable in operation at high frequencies without suffering from adverse influences by connector leads in the tuning circuit. Examples of apparatus into which such a tuning circuit may be incorporated are a tuning oscillator, a tuning amplifier or a mixer device. A further advantage of the present invention is the provision of a tuning system having a tuning circuit as outlined above which has a reduced number of components so that the tuning system can be manufactured by an improved process, the tuning system being capable of trimming a tuning frequency through a simple adjustment procedure and improving the stability of the trimmed tuning frequency. One or a plurality of such resonant circuits according to this invention may be provided, and the tuning frequency of the or each resonant circuit may be trimmed by cutting off the electrodes of the or each resonant circuit or connecting ground terminals to the electrodes. The above functional circuits may be connected directly or through other circuit components. The above and other objects, features and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which preferred embodiments of the present invention are shown by way of illustrative example. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a circuit diagram of a conventional tuning oscillator device; FIG. 2 is a circuit diagram of a conventional tuning amplifier device; FIG. 3 is a circuit diagram of a conventional mixer device; FIG. 4 is a perspective view of the components of a tuning circuit of a conventional tuning system; FIGS. 5 and 6 are circuit diagrams of tuning oscillator devices according to embodiments of the present invention; FIGS. 7 through 9 are circuit diagrams of tuning amplifier devices according to embodiments of the present invention; FIGS. 10 and 11 are circuit diagrams of mixer devices according to embodiments of the present invention; FIGS. 12 through 14 are block diagrams of tuning oscillator devices of a PLL synthesizer control system according to embodiments of the present invention; FIGS. 15 through 18 are circuit diagrams of balanced tuning oscillator devices according to embodiments of the present invention; FIGs. 19 through 22 are circuit diagrams of balanced tuning amplifier devices according to embodiments of the present invention; FIGS. 23 through 27 are circuit diagrams of balanced mixer devices according to embodiments of the present invention; FIG. 28 is a block diagram of a superheterodyne tuning system according to an embodiment of the present invention; FIGS. 29(a), 29(b), 29(c) through 32(a), 32(b), 32(c) are front, side, and rear elevational views of tuning circuits for use in the tuning system of the present invention; FIGS. 33(a) and 33(b) are side and front elevational views of a tuning circuit for use in the tuning system of the invention; FIGS. 34(a), 34(b), 34(c) through 35(a), 35(b), 35(c) are front, side, and rear elevational views of tuning circuits for use in the tuning system of the present invention; FIGS. 36(a) through 36(e), 37(a) and 37(b), and 38 are diagrams explanatory of the principles of operation of the tuning circuit of the invention; FIG. 39 is a diagram illustrative of the principles of operation of a conventional tuning circuit; FIGS. 40(a) through 40(c) are front, side, and rear elevational views of a tuning circuit, explanatory of a method of adjusting a tuning frequency of the tuning system of the invention; FIGS. 41(a) and 41(b) are circuit diagrams of equivalent circuits, explanatory of a method of adjusting a tuning frequency of the tuning system of the invention; FIG. 42 is a diagram showing the relationship between the length of a transmission line and the reactance; FIGS. 43 through 46 are illustrative of modes of variable adjustment of a variable capacitor and a variable inductor of tuning circuits; and FIG. 47 is a perspective view of a tuning system according to an embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 5 shows a tuning oscillator device according to an embodiment of the present invention. A feedback amplifier 26 has an input or output terminal 26 connected to a tuning circuit 28. The tuning circuit 28 includes a transmission-line electrode 29 having an inductance composed of a distributed inductance and a lumped inductance produced by bending the transmission line. The tuning circuit 28 also includes a transmission-line electrode 30 disposed in confronting or juxtaposed relation to the transmission-line electrode 29 with a dielectric (not shown) interposed therebetween or on a surface of the dielectric. The transmission-line electrodes 29, 30 jointly constitute a transmission line for producing a negative reactance. The transmission-line electrodes 29, 30 have ground terminals disposed at opposite ends. The tuning circuit 28 has an input terminal 27 (which is the same as the input or output terminal 27 of the feedback amplifier 26) that is connected to an open terminal of the transmission-line electrode 29. FIG. 6 illustrates a tuning oscillator device according to another embodiment of the present invention. A feedback amplifier 31 has an input or output terminal 32 connected to a tuning circuit 33. The tuning circuit 33 includes a transmission-line electrode 34 having an inductance composed of a distributed inductance and a lumped inductance produced by bending the transmission line. The tuning circuit 33 also includes a transmission-line electrode 35 disposed in confronting or juxtaposed relation to the transmission-line electrode 34 with a dielectric (not shown) interposed therebetween or on a surface of the dielectric. The transmission-line electrodes 34, 35 jointly constitute a transmission line for producing a negative reactance. The transmission-line electrodes 34, 35 have ground terminals disposed at opposite ends. The tuning circuit 33 has an input terminal 32 (which is the same as the input or output terminal 32 of the feedback amplifier 21) that is coupled to an open terminal of the transmission-line electrode 34. To the input terminal 32 connected to the open terminal of the transmission-line electrode 34, there are connected voltage-variable capacitance diodes 36, 37 as variable reactance elements. Designated at 38 is a control voltage input terminal for the voltage-variable capacitance diodes 36, 37. In the embodiments shown in FIGS. 5 and 6, the terminals of the tuning circuits 28, 33 which are shown as ground terminals may not be connected to ground, but may serve as common terminals in the tuning circuits 28, 33, which may be connected to the other circuits including the respective feedback amplifiers. Furthermore, the input terminals 27, 32 in the tuning circuits 28, 33 are not limited in position to the ends of the transmission-line electrodes 29, 34, but may be coupled to desired positions thereon which have desired impedances. The voltage-variable capacitance diodes 36, 37 may not be connected to the prescribed position on the transmission-line electrode 34, but may be connected to a desired position on the transmission-line electrode 34. Where it is necessary to adjust the tuning frequencies of the tuning circuits 23, 33 in the embodiments of FIGS. 5 and 6, the transmission-line electrodes 30, 35 may be cut off at desired positions, or the ground terminals of the transmission-line electrodes 29, 30, 34, 35 may be connected to desired positions thereby to change the distributed capacitance and inductance. The arrangement shown in FIG. 5 can provide a tuning oscillator device of a simple construction having a single oscillation frequency, and the arrangement of FIG. 6 can provide a tuning oscillator device capable of varying an oscillation frequency as desired. The feedback amplifiers in the above tuning oscillator devices may be constructed of semiconductor devices such as transistors, field-effect transistors, or ICs, or vacuum tubes. As described above, the tuning circuit comprises electrodes disposed in confronting relation with a thin dielectric interposed therebetween or in juxtaposed relation on a surface of a dielectric, the tuning circuit being connected to an input or output terminal of a feedback amplifier. Therefore, the tuning circuit has the following advantages: (1) A tuning circuit for use in a tuning oscillator device can provide a resonant circuit without any connector lead between an inductor and a capacitor, and can perform a tuning function. This arrangement eliminates a lead inductance and a stray capacitance which would be generated in the tuning circuit. Accordingly, no unexpected resonance other than resonance at a target tuning frequency will be present in a wide frequency range. As a result, stable frequency selectivity characteristics can be ensured to sufficiently increase the level of a fundamental of a signal to be oscillated and reduce the levels of harmonics. Any distortion of the oscillated signal can therefore be greatly stabilized and reduced. The stable frequency selectivity characteristics can sufficiently solve the problem of emitting a spurious interference signal. (2) Since a tuning oscillator device has a tuning circuit which can be fabricated as a module, the constants of the inductance and capacitance of the tuning circuit are prevented from being varied due to mechanical vibrations, with the result that oscillation tuning characteristics are highly stable. By constructing the dielectric in the tuning circuit of a material having a dielectric constant less dependent on temperature, any variations in the capacitance due to changes in ambient temperature can greatly be reduced, thus making tuning characteristics highly stable. Consequently, the oscillation frequency characteristics and unwanted-interference-signal elimination characteristics of the tuning oscillator device are not dependent on variations in ambient conditions, remain stable not only during an initial period, but over a prolonged period of time. (3) The tuning oscillator device has a tuning circuit of a simple unitary construction and is of a highly simple arrangement. Thus, there can be achieved a tuning oscillator device which is extremely low in profile and small in size, with the result that the amount of unwanted radiation of an oscillated signal from the tuning circuit can be minimized. This makes the oscillation of the tuning oscillator device stable and prevents the tuning oscillator device from interfering with other oscillator systems. (4) By using the dielectric in the tuning circuit as a circuit board or substrate for the tuning oscillator device, the mounting arrangement of the tuning oscillator device is improved, and the number of components constituting the tuning circuit is greatly reduced. The tuning oscillator device therefore lends itself to mass production and can be fabricated at a reduced cost. FIG. 7 shows a tuning amplifier device according to an embodiment of the present invention. A signal applied to an input terminal 39 is amplified by an amplifier 40, and an amplified output signal therefrom is supplied through an output terminal 41 to an input terminal 41 (which is the same as the output terminal 41 of the amplifier 40) of a tuning circuit 42. The tuning circuit 42 includes a transmission-line electrode 43 having an inductance composed of a distributed inductance and a lumped inductance produced by bending the transmission line. The tuning circuit 42 also includes a transmission-line electrode 44 disposed in confronting or juxtaposed relation to the transmission-line electrode 43 with a dielectric (not shown) interposed therebetween or on a surface of the dielectric. The transmission-line electrodes 43, 44 jointly constitute a transmission line for producing a negative reactance. The transmission-line electrodes 43, 44 have ground terminals disposed at opposite ends. FIG. 8 shows a tuning amplifier device according to another embodiment of the present invention. A signal applied to an input terminal 45 is amplified by an amplifier 46, and an amplified output signal therefrom is supplied through an output terminal 47 to an input terminal 47 (which is the same as the output terminal 47 of the amplifier 46) of a tuning circuit 48. The tuning circuit 48 includes a transmission-line electrode 49 having an inductance composed of a distributed inductance and a lumped inductance produced by bending the transmission line. The tuning circuit 48 also includes a transmission-line electrode 50 disposed in confronting or juxtaposed relation to the transmission-line electrode 49 with a dielectric (not shown) interposed therebetween or on a surface of the dielectric. The transmission-line electrodes 49, 50 jointly constitute a transmission line for producing a negative reactance. The transmission-line electrodes 49, 50 have ground terminals disposed at opposite ends. To the input terminal 47 which is coupled to an open terminal of the transmission-line electrode 49, there are connected voltage-variable capacitance diodes 51 as variable reactance elements. Designated at 51 is a control voltage input terminal for the voltage-variable capacitance diodes 51. FIG. 9 illustrates a tuning amplifier device according to still another embodiment of the present invention. A signal applied to an input terminal 53 is tuned and selected by an input tuning circuit 54 and supplied to an input terminal 56 of an amplifier 55. An amplified output signal from the amplifier 55 is delivered through an output terminal 57 to an output tuning circuit 58, by which the signal is tuned and selected and issued to an output terminal 59. The input tuning circuit 54 has transmission-line electrodes 60, 61, a voltage-variable capacitance diode 62, and a control voltage input terminal 63, and the output tuning circuit 58 has transmission-line electrodes 64, 65, a voltage-variable capacitance diode 66, and a control voltage input terminal 67. The input and output tuning circuits 54, 58 are of the same construction as that of the tuning circuit 48 described with reference to FIG. 8. The input terminal 53 and the output terminal 59 are connted to desired positions on the transmission-line electrodes 60, 65 which have required impedances. In the embodiments shown in FIGS. 7 through 9, the terminals of the tuning circuits 42, 48, 54, 58 which are shown as ground terminals may not be connected to ground, but may serve as common terminals in the tuning circuits 42, 48, 54, 58, which may be connected to the other circuits including the respective amplifiers 40, 46, 55. Furthermore, the input terminals 41, 47, 57, and the output terminal 56 in the tuning circuits 42, 48, 54, 58 are not limited in position to the ends of the transmission-line electrodes 43, 49, 61, 64, but may be coupled to desired positions thereon which have desired impedances. The voltage-variable capacitance diodes 51, 62, 66 may not be connected to the prescribed positions on the transmission-line electrodes 43, 49, 61, 64, but may be connected to a desired positions on the transmission-line electrodes 44, 50, 60, 65. Where it is necessary to adjust the tuning frequencies of the tuning circuits 42, 48, 54, 58 in the embodiments of FIGS. 7 through 9, the transmission-line electrodes 44, 50, 60, 65 may be cut off at desired positions, or the ground terminals of the transmission-line electrodes 43, 44, 49, 50, 60, 61, 64, 65 may be connected to desired positions thereby to change the distributed capacitance and inductance. The arrangement shown in FIG. 7 can provide a tuning amplifier device of a simple construction having a single tuning frequency, the arrangement of FIG. 8 can provide a tuning amplifier device capable of varying a tuning frequency as desired, and the arrangement of FIG. 9 can provide a tuning amplifier device having higher frequency selectivity characteristics and capable of varying a tuning frequency as desired. The amplifiers in the above tuning amplifier devices may be constructed of semiconductor devices such as transistors, field-effect transistors, or ICs, or vacuum tubes. As described above, the tuning circuit comprises electrodes disposed in confronting relation with a thin dielectric interposed therebetween or in juxtaposed relation on a surface of a dielectric, the tuning circuit being connected to an input and/or output terminal of an amplifier. Therefore, the tuning circuit has the following advantages: (1) A tuning circuit for use in a tuning amplifier device can provide a resonant circuit without any connector lead between an inductor and a capacitor, and can perform a tuning function. This arrangement eliminates a lead inductance and a stray capacitance which would be generated in the tuning circuit. Accordingly, no unexpected resonance other than resonance at a target tuning frequency will be present in a wide frequency range. As a result, stable frequency selectivity characteristics can be ensured to sufficiently increase the level of a fundamental of a signal to be amplified and reduce the levels of harmonics. Any distortion of the amplified signal can therefore be greatly stabilized and reduced. The stable frequency selectivity characteristics can sufficiently reduce the problem of intermodulation interference and spurious interference which would be produced in simultaneously amplifying a number of signals. (2) Since a tuning amplifier device has a tuning circuit which can be fabricated as a module, the constants of the inductance and capacitance of the tuning circuit are prevented from being varied due to mechanical vibrations, with the result that amplification tuning characteristics are highly stable. By constructing the dielectric in the tuning circuit of a material having a dielectric constant less dependent on temperature, any variations in the capacitance due to changes in ambient temperature can greatly be reduced, thus making tuning characteristics highly stable. Consequently, the amplification gain characteristics and unwanted-interference-signal elimination characteristics of the tuning amplifier device are not dependent on variations in ambient conditions, but remain stable not only during an initial period, but over a prolonged period of time. (3) The tuning amplifier device has a tuning circuit of a simple unitary construction and is of a highly simple arrangement. Thus, there can be achieved a tuning amplifier device which is extremely low in profile and small in size, with the result that the amount of unwanted radiation of an amplified signal from the tuning circuit can be minimized. This makes the amplification of the tuning amplifier device stable and prevents the tuning amplifier device from interfering with other amplifier systems. (4) By using the dielectric in the tuning circuit as a circuit board or substrate for the tuning amplifier device, the mounting arrangement of the tuning amplifier device is improved, and the number of components constituting the tuning circuit is greatly reduced. The tuning amplifier device therefore lends itself to mass production and can be fabricated at a reduced cost. FIG. 10 shows a mixer device according to an embodiment of the present invention. A mixer 68 serves to mix input signals applied to input terminals 69, 70 and issues an intermediate-frequency output signal to an output terminal 71 connected to a tuning circuit 72. The tuning circuit 72 includes a transmission-line electrode 73 having an inductance composed of a distributed inductance and a lumped inductance produced by bending the transmission line. The tuning circuit 72 also includes a transmission-line electrode 74 disposed in confronting or juxtaposed relation to the transmission-line electrode 73 with a dielectric (not shown) interposed therebetween or on a surface of the dielectric. The transmission-line electrodes 73, 74 have ground terminals disposed at opposite ends. The tuning circuit 72 has an input terminal 71 (which is the same as the output terminal 71 of the mixer 68) that is connected to an open terminal of the transmission-line electrode 73. FIG. 11 shows a mixer device according to another embodiment of the present invention. A mixer 75 serves to mix input signals applied to input terminals 76, 77 and issues an intermediate-frequency output signal to an output terminal 78 connected to a tuning circuit 79. The input terminals 76, 77 are connected respectively to the tuning circuits 80, 81. The tuning circuits 79, 80, 81 include transmission-line electrodes 82, 83, 84 each having an inductance composed of a distributed inductance and a lumped inductance produced by bending the transmission line. The tuning circuits 79, 80, 81 also include transmission-line electrodes 85, 86, 87 disposed in confronting or juxtaposed relation to the transmission-line electrodes 82, 83, 84 with dielectrics (not shown) interposed therebetween or on surfaces of the dielectrics. The transmission-line electrodes 82, 85; 83, 86; and 84, 87 have ground terminals disposed it opposite ends. The tuning circuit 79 has an input terminal 78 (which is the same as the output terminal 78 of the mixer 80) that is connected to an open terminal of the transmission-line electrode 82. The input terminals 76, 77 of the mixer 75 (which are the same as the output terminals 76, 77 of the mixers 80, 81) are connected to open terminals of the transmission-line electrodes 83, 84. To the tuning circuits 80, 81, there are connected voltage-variable capacitance diodes 88, 89, respectively. Thus, variable tuning circuits are connected respectively to the input terminals 76, 77 of the mixer 75. More specifically, where an amplifier is connected to an input terminal 90 coupled to the transmission-line electrode 86 in the tuning circuit 80, there is provided as a pre-stage a variable selecting and amplifying function, and where a feedback amplifier is connected to an input terminal 91 coupled to the transmission-line electrode 87 in the tuning circuit 81, there is provided as a pre-stage a variable oscillating function. Variable control in the variable selecting and amplifying function and the variable oscillating function is dependent on contrtol voltages applied to control terminals 92, 83 of the voltage-variable capacitance diodes 88, 89. The input signals applied to the input terminals 76, 77 are mixed by the mixer 75, and the intermediate-frequency signal produced by mixing the input signals is supplied to a selective load circuit formed by the tuning circuit 79 and issued from an output terminal 94 coupled to the transmission-line electrode 85. In the embodiments shown in FIGS. 10 and 11, the terminals of the tuning circuits 72, 79, 80, 81 which are shown as ground terminals may not be connected to ground, but may serve as common terminals in the tuning circuits 72, 79, 80, 81, which may be connected to the other circuits including the respective mixers. Furthermore, the input terminals 71, 78 and the output terminals 76, 77 in the tuning circuits 72, 79, 80, 81 are not limited in position to the ends of the transmission-line electrodes 73, 82, 83, 84, but may be coupled to desired positions thereon which have desired impedances. The voltage-variable capacitance diodes 83, 84 may not be connected to the prescribed positions on the transmission-line electrodes 83, 84, but may be connected to a desired positions on the transmission-line electrodes 83, 84. Where it is necessary to adjust the tuning frequencies of the tuning circuits 72, 79, 80, 81 in the embodiments of FIGS. 10 and 11, the transmission-line electrodes 74, 85, 86, 87 may be cut off at desired positions, or the ground terminals of the transmission-line electrodes 73, 74, 82, 85, 83, 86, 84, 87 may be connected to desired positions thereby to change the distributed capacitance and inductance. The mixers in the above mixer devices may be constructed of semiconductor devices such as transistors, field-effect transistors, or ICs, or vacuum tubes. As described above, the tuning circuit comprises electrodes disposed in confronting relation with a thin dielectric interposed therebetween or in juxtaposed relation on a surface of a dielectric, the tuning circuit being connected to an input and/or output terminal of an amplifier. Therefore, the tuning circuit has the following advantages: (1) A tuning circuit for use in a mixer device can provide a resonant circuit without any connector lead between an inductor and a capacitor, and can perform a tuning function. This arrangement eliminates a lead inductance and a stray capacitance which would be generated in the tuning circuit. Accordingly, no unexpected resonance other than resonance at a target tuning frequency will be present in a wide frequency range. As a result, stable frequency selectivity characteristics can be ensured to sufficiently increase the levels of fundamentals of signals to be mixed and reduce the levels of harmonics. Any distortion of the mixed signal can therefore be greatly stabilized and reduced. The stable frequency selectivity characteristics can sufficiently reduce the problem of intermodulation interference and spurious interference which would be produced in simultaneously mixing a number of signals. (2) Since a mixer device has a tuning circuit which can be fabricated as a module, the constants of the inductance and capacitance of the tuning circuit are prevented from being varied due to mechanical vibrations, with the result that mixing tuning characteristics are highly stable. By constructing the dielectric in the tuning circuit of a material having a dielectric constant less dependent on temperature, any variations in the capacitance due to changes in ambient temperature can greatly be reduced, thus making tuning characteristics highly stable. Consequently, the conversion gain characteristics and unwanted-interference-signal elimination characteristics of the mixer device are not dependent on variations in ambient conditions, but remain stable not only during an initial period, but over a prolonged period of time. (3) The mixer device has a tuning circuit of a simple unitary construction and is of a highly simple arrangement. Thus, there can be achieved a mixer device which is extremely low in profile and small in size, with the result that the amount of unwanted radiation of an amplified signal from the tuning circuit can be minimized. This makes the mixing operation of the mixer device stable and prevents the mixer device from interfering with other mixer systems. (4) By using the dielectric in the tuning circuit as a circuit board or substrate for the mixer device, the mounting arrangement of the mixer device is improved, and the number of components constituting the tuning circuit is greatly reduced. The mixer device therefore lends itself to mass production and can be fabricated at a reduced cost. FIG. 12 illustrates a tuning oscillator device according to an embodiment of the present invention. A tuning circuit 100 comprises a transmission-line electrode 95 having a distributed inductance, and a transmission-line electrode 96 disposed in confronting relation to the transmission-line electrode 95 with a dielectric (not shown) interposed therebetween, the transmission-line electrode 96 having a distributed capacitance. The transmission-line electrodes 95, 96 have ground terminals disposed at opposite ends, and jointly form a parallel-connected circuit 97 of an inductor and a capacitor. To an open terminal 98 of the transmission-line electrode 96, there is connected a voltage-variable capacitance diode 99 to which is applied an output voltage from a low-pass filter 108 in a PLL circuit 102 through a AC-signal blocking resistor 101. The open terminal 98 of the transmission-line electrode 96 is connected to a feedback amplifier 104, thus constituting an oscillator circuit. A portion of an oscillated signal output from the feedback amplifier 104 is supplied to a fixed frequency divider 105 and frequency-divided thereby. A frequency-divided output from the fixed frequency divider 105 is supplied to a variable frequency divider 106 in which it is frequency-divided into a variable frequency according to a digital code supplied to an input terminal 107 for a digital tuning control signal. A frequency-divided output from the variable frequency divider 106 is compared in phase with an oscillated output from a quartz oscillator 108 by a phase comparator, which supplies a phase detection signal to a low-pass filter 103. FIG. 13 shows a tuning oscillator device according to another embodiment of the present invention. A tuning circuit 115 is of the same construction as that of the tuning circuit 100 shown in FIG. 12, and a PLL circuit 117 and a feedback amplifier 119 are of the same construction as that of the PLL circuit 102 and the feedback amplifier 104 shown in FIG. 12. To an input terminal 122 of a variable frequency divider for a digital tuning control signal, there is supplied an output from a digital signal processor 125 composed of a latch, RAM or ROM and serving to store a tuning control digital signal code applied to an input terminal 126 or convert such a signal into another digital signal code. FIG. 14 is illustrative of a tuning oscillator device according to still another embodiment of the present invention. A tuning circuit 132, a PLL circuit 134, a feedback amplifier 136, and a digital signal processor 142 are of the same construction as those shown in FIG. 13. An input terminal 143 of the digital signal processor 142 is supplied with an output from a code converter 144 which serves to convert a tuning control serial digital signal code into a parallel digital signal code. As described above, the tuning circuit comprises confronting electrodes with a dielectric interposed therebetween, and a control voltage for a voltage-variable reactance element is varied by a digital signal code. The tuning oscillator device has the following advantages: (1) Inductor and capacitor components of the tuning circuit can be of a simple unitary construction. (2) The tuning circuit is of a low profile and a small size. (3) Since the inducator and capacitor of the tuning circuit are connected without a lead, the tuning circuit is not subjected to any lead inductance and stray capacitance, and hence the operation of the tuning oscillator device is highly stabilized and the tuning accuracy is improved. (4) It is possible to reduce the number of parts of the tuning circuit, so that the manufacturing process can be improved and the cost of manufacture can be lowered. (5) A highly stable tuning control voltage can be generated by a tuning control system using a digital signal code and a stable frequency locking capability of the PLL circuit. Therefore, the tuning accuracy of the tuning oscillator device can be largely increased. (6) The tuning oscillator device can be connected to a multiple-function digital control system controlled by a computer. The tuning oscillator device and any device employing the same can be controlled for sophisticated multiple functions. FIG. 15 shows a balanced tuning oscillator device according to an embodiment of the present invention. A balanced feedback amplifier 148 composed of feedback amplifiers 146, 147 has input or output terminals connected to balanced terminals 150, 151 of a balanced tuning circuit 149. The feedback amplifiers 146, 147 are capable of positive-feedback amplification in a mode in which they operate in mutually opposite phases. The balanced tuning circuit 149 has a main electrode 152 having a neutral ground terminal 153 and providing a distributed inductance, the main electrode 152 having balanced terminals 150, 151. An auxiliary electrode 154 is disposed in confronting relation to the main electrode 152 with a dielectric (not shown) interposed therebetween, and has opposite ends serving as ground terminals 155, 156. A hypothetical neutral point may be produced without providing the ground terminal 153 of the main electrode 152, so that an equivalent ground terminal can be achieved. The balanced terminals 150, 151 may be connected to any positions on the main electrode 152 which present desired impedances, rather than at the ends of the main electrode 152. The ground terminals 155, 156 of the auxiliary electrode 154 may also be connected to desired positions on the auxiliary electrode 154. According to the embodiment of FIG. 15, the balanced tuning circuit can be constructed simply of single main and auxiliary electrodes. FIG. 16 illustrates a balanced tuning oscillator device according to another embodiment of the present invention. A balanced feedback amplifier 159 composed of feedback amplifiers 157, 158 has input or output terminals connected to balanced terminals 161, 162 of a balanced tuning circuit 160. The feedback amplifiers 157, 158 are capable of positive-feedback amplification in a mode in which they operate in mutually opposite phases. The balanced tuning circuit 160 has main electrodes 163, 164 having neutral ground terminals 165, 166 and balanced terminals 161, 162. An auxiliary electrode 167 is disposed in confronting relation to the main electrode 163 with a dielectric (not shown) interposed therebetween, and has a ground terminal 168 positioned in substantially confronting relation to the balanced terminals 161, 162. The balanced terminals 161, 162 may be connected to any positions on the main electrodes 163, 164 which present desired impedances, rather than at the ends of the main electrodes 163, 164. With the embodiment of FIG. 16, the balanced terminals 161, 162 may be positioned closely to each other, so that signal input and output leads may be short. FIG. 17 shows a balanced tuning oscillator device according to still another embodiment of the present invention. A balanced feedback amplifier 171 composed of feedback amplifiers 169, 170 has input or output terminals connected to balanced terminals 173, 174 of a balanced tuning circuit 172. The feedback amplifiers 169, 170 are capable of positive-feedback amplification in a mode in which they operate in mutually opposite phases. The balanced tuning circuit 172 has a main electrode 176 having a neutral ground terminal 175 and balanced terminals 173, 174, the main electrode 176 being of the same construction as that of the main electrode shown in FIG. 15. There are auxiliary electrodes 179, 180 having ground terminals 177, 178. The ground terminals 177, 178, 175, and the balanced terminals 173, 174 may be arranged in the same manner as shown in FIG. 15. In the embodiments shown in FIGS. 17 and 16, the auxiliary electrodes 179, 180, and 167 may be cut off at desired positions near open terminals thereof to vary distributed capacitances for variably establishing tuning frequencies and also to adjust balanced conditions. FIG. 18 is illustrative of a balanced tuning oscillator device according to still another embodiment of the present invention. A balanced feedback amplifier 183 composed of feedback amplifiers 181, 182 has input or output terminals connected to balanced terminals 185, 186 of a balanced tuning circuit 184. The feedback amplifiers 181, 182 are capable of positive-feedback amplification in a mode in which they operate in mutually opposite phases. The balanced tuning circuit 184 has a main electrode 176 and an auxiliary electrode 188 which are arranged in the same manner as shown in FIG. 15. Voltage-variable capacitance diodes 189, 190 are connected to the balanced terminals 185, 186. Designated at 191 is a control voltage terminal, and 192 an unbalanced output tap terminal. With this embodiment, it is possible to construct a balanced variable tuning circuit capable of variably controlling a tuning frequency and of being connected to an unbalanced signal circuit system. The voltage-variable capacitance diodes 189, 190 may be replaced with variable air capacitors. The balanced tuning circuit 184 employed in the tuning oscillator device shown in FIG. 18 may be replaced with one shown in FIG. 16 or 17, other than one shown in FIG. 15. Where the balanced tuning circuit 184 is replaced wth one illustrated in FIG. 16 or 17, the auxiliary electrode 167 or the auxiliary electrodes 179, 180 may cut off at desired positions to select a tuning frequency as desired and adjust a balanced condition. The balanced feedback amplifier in each of the above embodiments may comprise a differential feedback amplifier composed of bipolar transistors, FETS, or their ICs, or a push-pull feedback amplifier. As described above, the balanced tuning circuit comprises electrodes disposed in confronting relation with a thin dielectric interposed therebetween or in juxtaposed relation on a surface of a dielectric, the balanced tuning circuit being connected between balanced output terminals or balanced input terminals of a balanced feedback amplifier. Therefore, the balanced tuning circuit has the following advantages: (1) A balanced tuning circuit is composed of a simple unitary construction of an inductor component and a capacitor component, so that a balanced oscillator device is of a highly simple arrangement. (2) Since the balanced tuning circuit is of a low profile and a small size, the balanced oscillator device can also be of a low profile and a small size, and the amount of unwanted radiation from the balanced oscillator device can be greatly reduced. Therefore, a stalbe oscillator system can be achieved. (3) Since the inducator and capacitor of the balanced tuning circuit are connected without a lead, the balanced tuning circuit is free from the influence of any lead inductance and stray capacitance, and hence the operation of the tuning circuit is highly stabilized, the tuning accuracy is improved, and a completely balanced condition can be maintained. (4) As the balanced tuning circuit can be fabricated as a module, the constants of the inducator and capacitor are not varied when subjected to mechanical vibrations, and the tuning frequency and the balanced condition are quite stable. (5) By constructing the dielectric substrate of a material which is less temperature-dependent, the balanced tuning circuit has a tuning frequency and a balanced condition that are highly stable against variations in the ambient temperature. (6) Because of the advantages (3), (4), and (5), the balanced condition of the balanced tuning oscillator device is maintained not only during an initial period but also over an extended period of time. Therefore, even-numbered harmonics of the oscillated signal can ideally be cancelled out to stably reduce the distortion rate of the oscillated signal. (7) It is possible to reduce the number of parts of the balanced tuning circuit, so that the manufacturing process for the balanced tuning circuit can be improved and the cost of manufacture can be lowered. (8) By using the dielectric in the balanced tuning circuit as a circuit board or substrate for the feedback amplifier, the mounting arrangement of the balanced tuning amplifier device is improved. FIG. 19 shows a tuning amplifier device according to an embodiment of the present invention. An unbalanced signal applied to an input terminal 193 is converted into balanced signals by an unbalanced-to-balanced mode converter 194, and the balanced signals are applied to and amplified by amplifiers 195, 196 in an opposite-phase mode. Amplified outputs are supplied to balanced terminals 198, 199 of a balanced tuning circuit 197. The balanced tuning circuit 197 has a main electrode 200 having a neutral ground terminal 201 and providing a distributed inductance, the main electrode 200 having balanced terminals 198, 199. An auxiliary electrode 202 is disposed in confronting relation to the main electrode 200 with a dielectric (not shown) interposed therebetween, and has opposite ends serving as ground terminals 203, 204. A hypothetical neutral point may be produced without providing the ground terminal 201 of the main electrode 200, so that an equivalent ground terminal can be achieved. he balanced terminals 198, 199 may be connected to any positions on the main electrode 200 which present desired impedances, rather than at the ends of the main electrode 200. The ground terminals 203, 204 of the auxiliary electrode 202 may also be connected to desired positions on the auxiliary electrode 202. According to the embodiment of FIG. 19, the balanced tuning circuit can be constructed simply of single main and auxiliary electrodes. FIG. 20 illustrates a tuning amplifier device according to another embodiment of the present invention. An unbalanced signal applied to an input terminal 205 is converted into balanced signals by an unbalanced-to-balanced mode converter 206, and the balanced signals are applied to and amplified by amplifiers 207, 208 in an opposite-phase mode. Amplified outputs are supplied to balanced terminals 210, 211 of a balanced tuning circuit 209. The balanced tuning circuit 209 has main electrodes 212, 213 having neutral ground terminals 214, 215 and balanced terminals 210, 211. An auxiliary electrode 216 is disposed in confronting relation to the main electrodes 212, 213 with a dielectric (not shown) interposed therebetween, and has a ground terminal 217 positioned in substantially confronting relation to the balanced terminals 210, 211. The balanced terminals 210, 211 may be connected to any positions on the main electrodes 212, 213 which present desired impedances, rather than at the ends of the main electrodes 212, 213. With the embodiment of FIG. 20, the balanced terminals 210, 211 may be positioned closely to each other, so that signal input and output leads may be short. FIG. 21 shows a tuning amplifier device according to still another embodiment of the present invention. An unbalanced signal applied to an input terminal 218 is converted into balanced signals by an unbalanced-to-balanced mode converter 219, and the balanced signals are applied to and amplified by amplifiers 220, 221 in an opposite-phase mode. Amplified outputs are supplied to balanced terminals 223, 224 of a balanced tuning circuit 222. The balanced tuning circuit 222 has a main electrode 226 having a neutral ground terminal 225 and balanced terminals 223, 224, the main electrode 226 being of the same construction as that of the main electrode shown in FIG. 19. There are auxiliary electrodes 229, 230 having ground terminals 227, 228. The ground terminals 225, 227, 228, and the balanced terminals 223, 224 may be arranged in the same manner as shown in FIG. 19. In the embodiments shown in FIGS. 21 and 20, the auxiliary electrodes 229, 230, and 216 may be cut off at desired positions near open terminals thereof to vary distributed capacitances for variably establishing tuning frequencies and also to adjust balanced conditions. FIG. 22 is illustrative of a tuning amplifier device according to still another embodiment of the present invention. An unbalanced signal applied to an input terminal 231 is converted into balanced signals by a balanced tuning circuit 232, and the balanced signals are applied to and amplified by amplifiers 233, 234 in an opposite-phase mode. Amplified outputs are supplied to balanced terminals 236, 237 of a balanced tuning circuit 235. The balanced tuning circuits 235, 232 have main and auxiliary electrodes 238, 238 and 245, 246 in the same arrangement as that shown in FIG. 19. To the balanced terminals 236, 237 and 243, 244, there are connected voltage-variable capacitance diodes 240, 241 and 247, 148. Designated at 242, 249 are common control voltage terminals for the voltage-variable capacitance diodes 240, 241 and 247, 248, the control voltage terminal 242 serving as an unblanced secondary output tap terminal. Although the unbalanced tuning circuits 232, 235 are shown as being of the same construction as that shown in FIG. 19, they may be constructed as shown in FIG. 20 or 21. With the embodiment of FIG. 22, the balanced variable tuning circuit can variably control a tuning frequency and can be connected to an unbalanced signal circuit system. The voltage-variable capacitance diodes 240, 242 and 247, 248 may be replaced with variable air capacitors. Where the balanced tuning circuit 235 is replaced with one illustrated in FIG. 20 or 21, the auxiliary electrode 216 or the auxiliary electrodes 229, 230 may be cut off at desired positions to select a tuning frequency band as desired and adjust a balanced condition. The balanced amplifier in each of the above embodiments may comprise a push-pull amplifier or a differential amplifier. The unbalanced-to-balanced mode converter may comprise a wide-range balun or a combination of a tuning circuit and a secondary balanced output coil. As described above, the balanced tuning circuit comprises balanced electrodes disposed in confronting relation with a thin dielectric interposed therebetween or in juxtaposed relation on a surface of a dielectric, the balanced tuning circuit being connected between balanced output terminals of a balanced amplifier. Therefore, the balanced tuning circuit has the following advantages: (1) A balanced tuning circuit is composed of a simple unitary construction of an inductor component and a capacitor component, so that a balanced tuning amplifier device is of a highly simple arrangement. (2) Since the balanced tuning circuit is of a low profile and a small size, the balanced tuning amplifier device can also be of a low profile and a small size, and the amount of unwanted radiation from the balanced tuning circuit can be greatly reduced. Therefore, a stable amplifier system can be achieved. (3) Since the inducator and capacitor of the balanced tuning circuit are connected without a lead, the balanced tuning circuit is free from the influence of any lead inductance and stray capacitance, and hence the operation of the tuning circuit is highly stabilized, the tuning accuracy is improved, and a completely balanced condition can be maintained. (4) As the balanced tuning circuit can be fabricated as a module, the constants of the inducator and capacitor are not varied when subjected to mechanical vibrations, and the tuning frequency and the balanced condition are quite stable. (5) By constructing the dielectric substrate of a material which is less temperature-dependent, the balanced tuning circuit has a tuning frequency and a balanced condition that are highly stable against variations in the ambient temperature. (6) Because of the advantages (3), (4), and (5), the balanced condition of the balanced tuning amplifier device is maintained not only during an initial period but also over an extended period of time. Therefore, even-numbered harmonics of the oscillated signal can ideally be cancelled out to stably reduce the distortion rate of the amplified signal. (7) It is possible to reduce the number of parts of the balanced tuning circuit, so that the manufacturing process for the balanced tuning amplifier device can be improved and the cost of manufacture can be lowered. (8) By using the dielectric in the balanced tuning circuit as a circuit board or substrate for the feedback amplifier, the mounting arrangement of the balanced tuning amplifier device is improved. FIG. 23 shows a balanced mixer device according to an embodiment of the present invention. An unbalanced signal applied to an input terminal 250 is converted into balanced signals by an unbalanced-to-balanced mode converter 251, and the balanced signals are applied to mixers 252, 253. An unblanced signal applied to an input terminal 254 in supplied directly to the mixers 252, 253. The signals are mixed in an opposite-phase mode by the mixers 252, 253, and an intermediate-frequency signal output produced in a balanced mode by mixing the applied signals is supplied to balanced terminals 256, 257 of a balanced tuning circuit 255. The balanced tuning circuit 255 has a main electrode 258 having a neutral ground terminal 259 and providing a distributed inductance, the main electrode 258 having balanced terminals 256, 257. An auxiliary electrode 260 is disposed in confronting relation to the main electrode 258 with a dielectric (not shown) interposed therebetween, and has opposite ends serving as ground terminals 261, 262. A hypothetical neutral point may be produced without providing the ground terminal 259 of the main electrode 258, so that an equivalent ground terminal can be achieved. The balanced terminals 256, 257 may be connected to any positions on the main electrode 258 which present desired impedances, rather than at the ends of the main electrode 258. The ground terminals 261, 262 of the auxiliary electrode 260 may also be connected to desired positions on the auxiliary electrode 260. According to the embodiment of FIG. 23, the balanced tuning circuit can be constructed simply of single main and auxiliary electrodes. FIG. 24 illustrates a balanced mixer device according to another embodiment of the present invention. An unbalanced signal applied to an input terminal 263 is converted into balanced signals by an unbalanced-to-balanced mode converter 264, and the balanced signals are applied to mixers 265, 266. An unblanced signal applied to an input terminal 267 is supplied directly to the mixers 265, 266. The signals are mixed in an opposite-phase mode by the mixers 265, 266, and an intermediate-frequency signal output produced in a balanced mode by mixing the applied signals is supplied to balanced terminals 269, 270 of a balanced tuning circuit 268. The balanced tuning circuit 268 has main electrodes 271, 272 having neutral ground terminals 273, 274 and balanced terminals 269, 270. An auxiliary electrode 257 is disposed in confronting relation to the main electrodes 271, 272 with a dielectric (not shown) interposed therebetween, and has a ground terminal 276 positioned in substantially confronting relation to the balanced terminals 269, 270. The balanced terminals 269, 270 may be connected to any positions on the main electrodes 271, 272 which present desired impedances, rather than at the ends of the main electrodes 271, 272. With the embodiment of FIG. 24, the balanced terminals 269, 270 may be positioned closely to each other, so that signal input and output leads may be short. FIG. 25 illustrates a balanced mixer device according to still another embodiment of the present invention. An unbalanced signal applied to an input terminal 277 is converted into balanced signals by an unbalanced-to-balanced mode converter 278, and the balanced signals are applied to mixers 279, 280. An unblanced signal applied to an input terminal 281 is supplied directly to the mixers 279, 280. The signals are mixed in an opposite-phase mode by the mixers 279, 280, and an intermediate-frequency signal output produced in a balanced mode by mixing the applied signals is supplied to balanced terminals 298, 299 of a balanced tuning circuit 282. The balanced tuning circuit 282 has a main electrode 286 having a neutral ground terminal 285 and balanced terminals 283, 284, the main electrode 286 being of the same construction as that of the main electrode shown in FIG. 23. There are auxiliary electrodes 289, 290 having ground terminals 287, 288. The ground terminals 287, 288, 285, and the balanced terminals 283, 284 may be arranged in the same manner as shown in FIG. 23. In the embodiments shown in FIGS. 25 and 24, the auxiliary electrodes 289, 290, and 275 may be cut off at desired positions near open terminals thereof to vary distributed capacitances for variably establishing tuning frequencies and also to adjust balanced conditions. FIG. 26 is illustrative of a double-balanced mixer device according to a still further embodiment of the present invention. An unbalanced signal applied to an input terminal 291 is converted into balanced signals by an unbalanced-to-balanced mode converter 292, and the balanced signals are applied to mixers 293, 294. An unblanced signal applied to an input terminal 295 is converted by an unbalanced-to-balanced mode converter 296 into balanced signals which are supplied to the mixers 293, 294. The signals are mixed in an opposite-phase mode by the mixers 293, 294, and an intermediate-frequency signal output produced in a balanced mode by mixing the applied signals is supplied to balanced terminals 298, 299 of a balanced tuning circuit 297. The balanced tuning circuit 297 has a main electrode 300, an auxiliary electrode 801, ground terminals 302, 303, 304, and balanced terminals 298, 299, which are arranged in the same manner as shown in FIG. 23. FIG. 27 shows a double-balanced mixer device according to still another embodiment of the present invention. An unbalanced signal applied to an input terminal 305 is converted into balanced signals by an unbalanced-to-balanced mode converter 306, and the balanced signals are applied through balanced terminals 307, 308 to mixers 309, 310. An unbalanced signal applied to an input terminal 311 is converted into balanced signals by an unbalanced-to-balanced mode converter 312, and the balanced signals are applied through balanced terminals 313, 314 to the mixers 309, 310. The signals are mixed in an opposite-phase mode by the mixers 309, 310, and an intermediate-frequency signal output produced in a balanced mode by mixing the applied signals is supplied to balanced terminals 306, 317 of a balanced tuning circuit 315. The balanced tuning circuits 306, 312, 315 have main electrodes 318, 319, 320 and auxiliary electrodes 321, 322, 323 in the same arrangement as that shown in FIG. 23. To the balanced terminals 307, 308; 313, 314; and 316, 317, there are connected voltage-variable capacitance diodes 324, 325; 326, 327; and 328, 329. Designated at 330, 331, 332 are control voltage terminals, and 333 an unblanced secondary output tap terminal. With the embodiment of FIG. 27, the balanced variable tuning circuit can variably control a tuning frequency and can be connected to an unbalanced signal circuit system. The voltage-variable capacitance diodes 324 through 329 may be replaced with variable air capacitors. The balanced tuning circuits 297, 306, 312, 315 in the mixer devices shown in FIGS. 26 and 27 may be composed of one shown in FIG. 24 or 25, in addition to one shown in FIG. 23. Where the balanced tuning circuits 297, 306, 312, 315 are replaced with one illustrated in FIG. 24 or 25, the auxiliary electrode 275 or the auxiliary electrodes 289, 290 may be cut off at desired positions to select a tuning frequency band as desired and adjust a balanced condition. The mixer device can perform the desired function without the voltage-variable capacitance diodes 328, 329 disposed at an intermediate-frequency output stage in the embodiment of FIG. 27. The balanced mixer or the double-balanced mixer in each of the above embodiments may be composed of diodes, transistors, or FETs. The unbalanced-to-balanced mode converter may comprise a wide-range balun or a combination of a general tuning circuit and a secondary balanced output coil. As described above, the balanced tuning circuit comprises balanced electrodes disposed in confronting relation with a thin dielectric interposed therebetween or in juxtaposed relation on a surface of a dielectric, the balanced tuning circuit being connected between balanced intermediate-frequency output terminals of a balanced or double-balanced mixer. Therefore, the balanced tuning circuit has the following advantages: (1) A balanced tuning circuit is composed of a simple unitary construction of an inductor component and a capacitor component, so that a balanced mixer device is of a highly simple arrangement. (2) Since the balanced tuning circuit is of a low profile and a small size, the balanced mixer device can also be of a low profile and a small size, and the amount of unwanted radiation from the balanced tuning circuit can be greatly reduced. Therefore, a stable mixer system can be achieved. (3) Since the inducator and capacitor of the balanced tuning circuit are connected without a lead, the balanced tuning circuit is free from the influence of any lead inductance and stray capacitance, and hence the operation of the tuning circuit is highly stabilized, the tuning accuracy is improved, and a completely balanced condition can be maintained. (4) As the balanced tuning circuit can be fabricated as a module, the constants of the inducator and capacitor are not varied when subjected to mechanical vibrations, and the tuning frequency and the balanced condition are quite stable. (5) By constructing the dielectric substrate of a material which is less temperature-dependent, the balanced tuning circuit has a tuning frequency and a balanced condition that are highly stable against variations in the ambient temperature. (6) Because of the advantages (3), (4), and (5), the balanced condition of the balanced mixer device is maintained not only during an initial period but also over an extended period of time. Therefore, even-numbered harmonics of the oscillated signal can ideally be cancelled out to stably reduce the distortion rate of the amplified signal. The balanced mixer device can thus well prevent spurious interference and intermodulation interference which would be caused when a signal of a high voltage level is applied, with the result that the distortion rate of the intermediate-frequency output signal can stably be reduced and the S/N ratio can be improved. (7) It is possible to reduce the number of parts of the balanced tuning circuit, so that the manufacturing process for the balanced mixer device can be improved and the cost of manufacture can be lowered. (8) By using the dielectric in the balanced tuning circuit as a circuit board or substrate for the mixer, the mounting arrangement of the balanced balanced mixer device is improved. FIG. 28 is shows in block diagram of a superheterodyne tuning system according to an embodiment of the present invention. A signal applied to an antenna terminal 334 is amplified by a tuning amplifier devcie 335 into an amplified signal which is supplied to a mixer device 337. An oscillated output signal from a tuning oscillator device 336 is supplied to the mixer device 337. The antanna input signal and the oscillated output signal are mixed by the mixer device 337 to issue an intermedaite-frequency signal to an intermediate-frequency output terminal 338. The tuning amplifier device 335 may comprise one shown in FIGS. 7 through 9 or FIGS. 19 through 22, and the tuning oscillator device may comprise one shown in FIG. 5, 6, or 12 through 18. The mixer device 337 may comprise one illustrated in FIG. 10, 11, or 23 through 27. The tuning circuits 23, 33, 43, 48, 54, 58, 72, 79, 80, 81, 97, 112, and 129 in the tuning systems in the embodiments shown in FIGS. 5 through 14 will be described hereinbelow. FIGS. 29(a), 29(b) and 29(c) through 32(a), 32(b) and 32(c) illustrate the constructions of tuning circuits according to embodiments of the present invention. In FIGS. 29(a), 29(b) and 29(c), a dielectric 339 in the form of a plate is made of ceramic and supports an electrode 340 forming an inductor on a surface of the dielectric 339 and another electrode 341 disposed on an opposite surface of the dielectric 339 in confronting relation to the electrode 340. The electrodes 340, 341 jointly form a distributed-constant circuit forming a capacitor. The electrode 340 includes a ground terminal 340a and an open terminal 340b, and the electrode 341 includes a ground terminal 341a and an open terminal 341b which are opposite to the terminals 340b, 340a, respectively. Each of the electrodes 340, 341 has one bent portion having a desired bent angle and direction. In FIGS. 30(a) through 30(c), electrodes 343, 344 are mounted on opposite surfaces of a dielectric 342 in the from of a plate, and in FIGS. 31(a) through 32(c), electrodes 346, 347 are mounted on opposite surfaces of a dielectric 345 in the from of a plate. As illustrated in FIGs. 30(a) through 30(c) and 31(a) through 31(c), the electrodes are mounted on the dielectrics and have a terminal arrangement in the same manner as described with reference to the embodiment of FIGS. 29(a) through 29(c). However, each of the electrodes has more than one bent portion having a desired bent angle and direction. In FIGS. 32(a) through 32(c), electrodes 349, 350 are mounted on opposite surfaces of a dielectric 348 in the from of a plate. The electrodes are mounted on the dielectrics and have a terminal arrangement in the same manner as described with reference to the embodiment of FIGS. 29(a) through 29(c). However, each of the electrodes has a spiral configuration. Although in the embodiments shown in FIGS. 29(a), 29(b) and 29(c) through 32(a), 32(b) and 32(c) each electrode has a bent portion formed as an angular pattern having a desired bent angle, the electrode may instead have a bent portion formed as an arcuate pattern having a desired curvature. FIGS. 33(a) and 33(b) show the construction of a tuning circuit according to another embodiment of the present invention. A tubular dielectric 351 supports an electrode 352 on an inner peripheral surface thereof and an electrode 353 on an outer peripheral surface thereof, the electrodes 352, 353 extending in confronting relation to each other. The electrodes 352, 353 have ground terminals located at opposite ends thereof. The dielectric 351 may be in the form of a hollow structure having a rectangular cross section. The terminals that are designated as ground terminals in the foregoing embodiments may not be used as ground terminals but may serve as general common terminals for connection to other circuits (not shown). The tuning circuits shown in FIGS. 29(a), 29(b) and 29(c) through 32(a), 32(b) and 32(c) can form relatively large distributed inductors and capacitors though they take up small areas. Therefore, small tuning circuits having relatively low tuning frequencies can be achieved with a resulting increased space factor. The tuning circuit shown in FIGS. 33(a) and 33(b) is smaller in size than the tuning circuits shown in FIGS. 29(a), 29(b) and 29(c) through 32(a), 32(b) and 32(c), but can form a sufficient larger inductor and capacitor. Therefore, a much smaller tuning circuit having a sufficiently tuning frequency can be accomplished. The tuning circuit of FIGS. 33(a) and 33(b) can be mass-produced with ease by continuously forming electrodes 352, 353 on an elongate continuous tubular dielectric 351 and then cutting off the tubular dielectric 351 to desired lengths. The electrodes, or transmission-line electrodes, in the foregoing embodiments may be constructed of a metal conductor, a printed metal-foil conductor, a printed thick-film conductor, or a thin-film conductor, or may comprise a combination of these conductors. The dielectric may be made of alumina ceramic, barium titanate, plastics, Teflon (trademark), glass, mica, or in the form of a printed-circuit board of resin base. The balanced tuning circuits 149, 160, 172, 184, 197, 209, 222, 232, 235, 255, 268, 282, 297, 306, 312, and 315 in the balanced tuning systems in the embodiments shown in FIGS. 15 through 27 will be described hereinbelow. FIGS. 34(a), 34(b) and 34(c) and 35(a), 35(b) and 35(c) are illustrative of the balanced tuning circuit for use in the balanced tuning systems shown in FIGS. 15, 18, 19, 22, 23, 26, 27, and also of the construction of main and auxiliary electrodes and a dielectric for the balanced tuning circuits shown in FIGS. 31 and 32.. In FIGS. 34(a) through 34(c), the tuning circuit has a dielectric base plate or substrate 354 and electrodes 355, 356 forming a distributed-constant circuit providing a distributed inductor and a distributed capacitor. The electrodes 355, 356 have ground terminals positioned remotely from each other between the confronting electrodes as shown in FIGS. 34(a) through 34(c). (The ground terminals in FIGS. 35(a) through 35(c) are similarly arranged.) Sides A, B shown in FIG. 34(a) correspond respectively to sides A, B in FIG. 34(c) (This also holds true in FIGS. 35(a) through 35(c).) In FIGS. 34(a) through 34(c), the electrodes 355, 356 of a meandering configuration are disposed in confronting relation to each other with the dielectric substrate 354 interposed therebetween. In FIGS. 35(a) through 35(c), electrodes 360, 361 of a spiral configuration are disposed in confronting relation to each other with a dielectric substrate 337 interposed therebetween. While in the embodiments of FIGS. 34(a) through 34(c) and 35(a) through 35(c) the electrodes disposed in confronting or juxtaposed relation are of the same shape and fully confront each other over their entire surfaces, one of the electrodes may have a different equivalent length than that of the other electrode, or the companion electrodes may confront each other only partially. Designated in FIGS. 34(a) through 34(c) and 35(a) through 35(c) at 357, 358, 362, 363 are balanced terminals. Operation of the tuning circuit for the tuning system in each of the foregoing embodiments will now be described. FIGS. 36(a) through 36(e) are circuit diagrams of equivalent circuits of the tuning circuit of the present invention. In FIG. 36(a), transmission lines are composed of transmission-line electrodes 364, 365 having an electrical length ℓ and including ground terminals disposed at opposite ends. A signal source 366 for generating a voltage e is connected to the transmission-line electrode 364 for supplying a signal thereto. When the signal is applied, a traveling-wave voltage e A is excited at the open terminal on the distal end of the transmission-line electrode 364. Since the transmission-line electrode 365 is disposed in confronting or juxtaposed relation to the transmission-line electrode 364 adjacent thereto, a voltage is induced in the transmission-line electrode 365 by mutual induction. A traveling-wave voltage e B is then induced at the open terminal on the distal end of the transmission-line electrode 365. With the ground terminals of the transmission-line electrodes 364, 365 being disposed in opposite relation or remotely from each other, the induced traveling-wave voltage e B is opposite in phase to the excited traveling-wave voltage e A . As the distal ends of the transmission lines are open, the traveling-wave voltages e A , e B form voltage standing waves in the transmission lines which are composed of the transmission-line electrodes 364, 365, respectively. Assuming that a voltage distribution coefficient indicative of the distribution of the voltage standing wave in the transmission-line electrode 364 is expressed by K, a voltage distribution coefficient of the transmission-line electrode 365 can be expressed by (1 - K). A potential difference V between any desired confronting portions of the transmission-line electrodes 364, 365 is expressed by: V = Ke A - (1 - K)ε B (1) On the condition that the transmission-line electrodes 364, 365 are of the same electrical length ℓ. e B = - e A (2) Thus, the potential difference V given by the above equation (1) can now be expressed by: Therefore, the potential difference V can be generated between all confronting portions of the transmission-line electrodes 364, 365. It is assumed here that the transmission-line electrodes 364, 365 have a width W (with their thickness being small) and are spaced a distance d from each other by a dielectric having a dielectric constant ε s . A capacitance C O formed per unit length of the transmission lines is given as follows: Therefore, C O = ε o ε s W d (6) Accordingly, the transmission lines shown in FIG. 36(a) are equivalent to transmission lines as shown in FIG. 36(b) having distributed capacitors 367 of C O determined by the equation (6) per unit length. As illustrated in FIG. 36(c), the transmission lines can be equivalently expressed as a distributed-constant circuit composed of total distributed inductors 368, 369 due to distributed inductor components of the transmission lines and lumped inductor components produced by the bent configuration of the transmission lines and a distributed capacitor 367. Now, the relationship between the formation of the distributed capacitor 367 and the electrical length ℓ of the transmission lines will be described. A characteristic impedance Z O per unit length of the transmission lines shown in FIG. 37(a) can be expressed by an equivalent circuit illustrated in FIG. 37(b). The characteristic impedance Z O is generally given by: If the transmission lines cause no loss, This assumption can be applied to many of the resonator filters according to the embodiments of the invention. For the sake of brevity, the characteristic impedance Z O defined by the equation (8) will hereinafter be used. The capacitance C O in the equation (8) is the same as the capacitance C O per unit length of the transmission lines as determined by the equation (6). More specifically, the characteristic impedance Z O per unit length of the transmission lines is a function of the capacitance C O , and also a fuction of the dielectric constant ε s of the dielectric, the width W of the transmission-line electrodes, and the distance d between the transmission-line electrodes. An equivalent reactance X at a terminal of a transmission line having a characteristic impedance Z O per unit length thereof, an electrical length ℓ, and an open distal end can be expressed by: X = - Z O cotϑ (9) where ϑ = 2π ℓ λ (10) When ϑ = 0∿ π 2 = π∿ 3 4 π (11) the equivalent reactance X is given by: X ≦ 0 (12) Therefore, the equivalent reactance at the terminal of the transmission line can be a capacitive reactance. Where ε falls in the ranges of the equation (11) because of the electrical length ℓ of the transmission line, or by selecting the electrical length ℓ to be λ/4 or less, a capacitor can be formed. The capacitance C of the capacitor thus formed is: Therefore, any desired capacitance C can be achieved by varying ϑ or selecting the electrical length ℓ of the tranmission line. FIG. 38 is illustrative of the mode of operation of the transmission lines described with reference to the above equations (9) through (13). In FIG. 38, the equivalent reactance X generated at the terminals is shown as being varied as the electrical length of the transmission lines with open distal ends varies. As is apparent from FIG. 38, it is possible to form a negative terminal reactance if the electrical length ℓ is λ/4 or below or in the range of from λ/2 to 4λ/3, that is, a capacitor can equivalently be formed. Furthermore, under the condition to form a negative terminal reactance, the capacitance C can be of a desired value by selecting any desired electrical length ℓ of the transmission lines. The capacitor C can be equivalently replaced with a lumped-constant capacitor 370 shown in FIG. 36(d). The inductor formed by the total of distributed inductor components present in the transmission lines and lumped inductor components generated by the bending of the transmission lines can be equivalently replaced with a lumped-constant inductor 371. By expressing the ground terminals in FIG. 36(d) as a common ground terminal, the arrangement can finally be equivalent to a parallel resonant circuit as shown in FIG. 36(e) which is composed of a lumped-constant capacitor 370 and a lumped-constant inductor 371, thus realizing a tuning circuit. The tuning circuit of the invention is realized on the basis of the arrangement and operation described above The construction and principles of operation of the tuning circuit according to the present invention are entirely different from those of the conventional tuning circuits. To indicate that the tuning circuit of the invention differs entirely from a conventional tuning circuit or another tuning circuit arrangement using the same transmission lines as those in the tuning circuit of the invention, the construction and operation of the conventional tuning circuit or the other tuning circuit arrangement using the same transmission lines will hereinafter be described. This will serve to clarify the difference between the tuning circuit of the invention and the conventional tuning circuit and also the novelty of th tuning circuit of the invention. FIG. 39 is illustrative of the circuit arrangement o a quarter-wave resonator which has heretofore been used most widely. The illustrated prior resonator is fully different from the tuning circuit of the invention as to the distal ends of the transmission lines, selection of the length thereof, and selection of ground terminals. In FIG. 39, balanced-mode transmission-line electrodes 372, 373 have an electrical length ℓ equal to λ/4 at a resonance frequency and their distal ends short-circuited. The transmission-line electrodes are driven in a balanced mode by a balanced signal source 374 which generates a voltage e. A ground terminal is selected to be located at a neutral point of the balanced signal source 374, and no ground terminal is placed at any terminal of the transmission-line electrodes. An equivalent terminal reactance X produced at the terminals of the transmission lines is given by: X = Z O tan ϑ (14) where Z O is a characteristic impedance of the transmission lines and equal to the one indicated by the equation (8) and ϑ is equal to the one indicated by the equation (10). The electrical length ℓ of the transmission lines of this resonator is: ℓ = λ/4 (15) and hence, ϑ = π/2 (16) Therefore, the terminal reactance X in the equation (14) becomes: X = Z O tan(π/2) = ∞ (17) As a consequence, parallel resonance characteristics can equivalently be obtained. The construction of the quarter-wave resonator above will be compared with that of the tuning circuit of the present invention. With respect to the terminal condition, the terminals of the tuning circuit of the invention are open, whereas those of the conventional quarter-wave resonator are short-circuited, so that the terminal conditions are entirely different. As to the electrical length ℓ of the transmissionlines, the electrical length of the tuning circuit of the invention is selected to be λ/4 or shorter, and about λ/16 in reality. However, the electrical length of the prior quarter-wave resonator is strictly selected to be λ/4 of the resonance frequency. Therefore, it is apparent that the electrical lengths ℓ of the transmission lines are also basically different from each other. Because of the difference betwen the electrical lengths of the transmission lines, the tuning circuit of the invention can be of a smaller size when the tuning circuit of the invention and the prior resonator are designed for use at the same tuning frequency or resonance frequency. The quarter-wave resonator is required to have much longer transmission lines, and has to be larger in size. Some prior quarter-wave resonators are rendered smaller in size by shortening the transmission lines with a dielectric having quite a large dielectric constant. The dielectric with a large dielectric constant is generally subjected to a high dielectric loss tan δ, and the Q of the resonator tends to be greatly reduced. Furthermore, the dielectric constant of the dielectric with a high dielectric constant is generally highly temperature-dependent, and it is difficult to maintain the stability of a tuning frequency. FIGS. 40(a) through 40(c) are illustrative of the construction of a tuning circuit according to still another embodiment of the present invention, as representing the embodiment shown in FIGS. 32(a) through 32(c). Electrodes 376, 377 are mounted on opposite surfaces of a dielectric 375 in the form of a plate. The electrodes are mounted on the dielectrics and have a terminal arrangement in the same manner as described with reference to the embodiment of FIGS. 32(a) through 32(c). Each of the electrodes has a spiral configuration. If it is desired to adjust the capacitance of the distributed capacitor or the inductance of the distributed inductor, the electrode 377 is cut off at a desired portion 378. The severance of the electrode will be described in greater detail. By cutting off the electrode, a distributed inductor 379 (FIG. 41(a)) is cut off at an electrode portion 380. As a result, the values of a distributed capacitance 381 and the distributed inductance 379 can be varied as desired. FIG. 41(b) illustrates the tuning circuit in the form of a lumped-constant equivalent circuit which comprises a parallel-connected circuit of a variable inductor 382 and a variable capacitor 383. The inductance of the inductor of the tuning circuit can be designed as desired by the number of turns of the spiral electrodes or the length of the spiral electrodes. The capacitance of the distributed capacitor can be designed as desired by the area in which the spiral electrodes confront each other, the dielectric constant and thickness of the dielectric. The formation of the distributed capacitance will be described further with reference to FIG. 42. The confronting spiral electrodes has a transmission-line equivalent length ℓ which is designed to be shorter than λ/4 at an operating frequency taking into account a wavelength shortening coefficient 1/√ε determined by the dielectric cosntant ε of the dielectric used. By selecting the ratio of the transmission-line equivalent length ℓ to the λ/4 as desired, the value of a capacitive reactance Xc can be designed as desired. A capacitance C = 1/2 πfo Xc can be determined from the capacitive reactance Xc and the operating frequency f o . If the transmission-line equivalent length ℓ is shortened to a transmission-line equivalent length ℓ', the capacitive reactance Xc is changed to a capacitive reactance Xc'. A capacitance C' = 1/2 πfo Xc' can be determined from the capacitive reactance Xc' and the operating frequency f o . The capacitance is thus varied since C' < C. The capacitor having the capacitance C is equivalent to the variable capacitor 108 shown in FIG. 41(b). The length of the spiral electrode (the spiral electrode 377 in FIG. 40(c)) forming the capacitor electrode which is grounded has been illustrated as being the same as the length of the spiral electrode (the spiral electrode 376 in FIG. 40(a)) forming the inductor electrode. However, the capacitor electrdoe may be of any desired length shorter than the inductor electrdoe, and may be formed at any desired position confronting the inductor electrode. FIGS. 43, 44, 45 and 46 are illustrative of the manner in which the variable capacitor and the variable inductor of the tuning circuit of FIGS. 40(a) through 40(c), as an example, are variably adjusted. FIGS. 43 and 44 are explanatory of a mode for adjusting the variable capacitor by cutting off the capacitor electrode. FIG. 44 shows the relationship between an electrode length d from the open terminal to the cut-off position, a distributed capacitance C over the electrode length d, a distributed inductance L over the electrode length d, and a self-resonant frequency f o for the electrode length d. As the electrode length d increases, the distributed capacitance C is reduced but the distributed inductance L remains unchanged, and the self-resonant frequency f o goes higher. FIGS. 45 and 46 are explanatory of a mode for simultaneously adjusting the variable inductor and the variable capacitor by cutting off the inductor electrode. FIG. 46 shows the relationship between an electrode length d from the open terminal to th cut-off position, a distributed capacitance C over the electrode length d, a distributed inductance L over the electrode length d, and a self-resonant frequency fo for the electrode length d. As the electrode length d increases, both the distributed inductance L and the distributed capacitance C are reduced, and the self-resonant frequency f o goes higher. The electrode may be cut off by non-contant cutter means such as a laser cutter or a sand blaster which does not affect the tuning frequency during cutting operation. With the above embodiment, the inductor electrode an the capacitor electrode are shared by each other and the inductance of the capacitor electrode which is grounded is cancelled out, so that the variable inductor and the variable capacitor are of an integral construction. FIG. 47 shows a tuning system according to an embodiment of the present invention, representing the tuning circuits for adjusting tuning frequencies as described with reference to FIGS. 32(a) through 32(c), 40(a) through 40(c), and 46. Tuning circuits 385, 386, 387, 388 are scattered on a single-layer or multilayer circuit board or substrate 384 (the electrodes on the reverse side and intermediate electroes are not shown). The tuning circuits 385, 386 are connected to each other by another circuit element 389, and the tuning circuits 387, 388 are directly connected to each other. The tuning circuits 386, 388 are connected to each other by another circuit element 390. The circuit elements 389, 390 may comprise amplifiers, oscillators, mixers, modulators, demodulators, or detectors. The tuning circuits 385, 386, 387, 388 and the other circuit elements 389, 390 may be positioned at any desired locations on the circuit substrate 384, and as many tuning circuits and other circuit elements as desired may be mounted. The tuning frequencies of the tuning circuits 385 through 388 can independently be adjusted. The electrodes in the tuning circuits 385 through 388 may be formed of metal conductors or printed conductors, and mutually confronting electrodes may be formed of different conductors. The electrodes may be disposed within the circuit substrate, rather than on its surfaces, or may be disposed on an intermediate layer of a multilayer circuit substrate. The circuit substrate 384 may be made of alumina ceramic, plastics, Teflon (trademark), glass, or mica, for example. The tuning circuits 385 through 388 which are in the form of metal conductors can be bonded simultaneously to the circuit substrate 384. The tuning circuits 385 through 388 which are in the form of printed conductors can be printed simultaneously on the circuit substrate 384 at the time other circuit patterns are formed thereon. It is possible to connect other variable reactance elements (not shown) to the tuning circuits 385 through 388 so that the tuning frequencies of the tuning circuits 385 through 388 will be varied in ganged relation. With the present invention, as described above, each tuning circuit is composed of a variable inductor electrode doubling as a variable capacitor electrode, and a variable capacitor electrode, the variable inductor and capacitor electrodes being disposed in adjacent and confronting relation with a common dielectric circuit substrate interposed therebetween, and other circuit elements are formed on the dielectric circuit substrate and connected to the tuning circuits. The tuning frequency of each tuning circuit can be selected as desired by cutting off an electrode or adjusting the position of a ground terminal connected to the electrode. The tuning circuit of the present invention has the following advantages: (1) The variable inducator and the variable capacitor may be integrally formed in the circuit substrate. (2) The tuning circuit is of an extremely low profile and an extremely small size, and a tuning system which is of a low profile and a small size that have not been achieved heretofore can be realized by employing other circuit elements which are also of a low profile and a small size.. (3) Since the adjusted tuning circuit has no mechanically moving parts, the tuning circuit is subjected to only extremely small tuning frequency shifts. (4) Since the variable inductor and the variable capacitor are interconnected without leads, the circuit operation of the tuning circuit is highly stable without being affected by any lead inductance and stray capacitance. (5) Because the electrodes of the tuning circuit can be fabricated together with other circuit elements, the number of parts used can be reduced, and the manufacturing process is improved, with the result that the cost of manufacture can be lowered. (6) Where the electrodes are cut off for adjusting the tuning frequency of the tuning circuit, a noncontacting frequency adjusting means may be employed, so that the tuning circuit can be adjusted without affecting the tuning frequency. (7) Where the position of the ground terminal is adjusted for adjusting the tuning frequency, a nondestructive electrode adjusting means may be employed, so that the tuning frequency can repeatedly be increased or lowered. (8) The speed of trimming the tuning frequency of the tuning circuit is increased. The initial setting values of the variable inductance and the variable capacitance in the tuning circuit are dependent on a simple artwork for an electrode pattern, so that the tuning circuit can be designed with increased freedom and the constants can easily be corrected. Th electrode conductors of the tuning circuit may partially be formed in an intermediate layer of a multilayer circuit substrate, with the result that the packaging of the tuning circuit can be designed with more freedom.

Claims: A tuning system comprising: a dielectric; a pair of first and second electrodes (29, 30) confronting each other with said dielectric interposed therebetween, each of said first and second electrodes having at least one bent portion to present primarily a lumped-constant inductance and having a desired equivalent electrical length; said first and second electrodes having ground or common terminals located either adjacent mutually opposite ends of the electrodes or at an intermediate position of one electrode and adjacent both ends of the other electrode so that voltage signals induced by mutual induction between the electrodes will be opposite in phase with respect to each other, thereby producing a parasitic distributed-constant capacitance due to a potential difference between the electrodes and a dielectric constant of the dielectric; one of said first and second electrodes having a first terminal (27) at a given position thereof, the electrodes forming a two-terminal circuit network, said ground or common terminals serving as a second terminal, the two-terminal circuit network forming an equivalent parallel resonant circuit composed of a lumped-constant inductance formed by said one electrode and a parasitic distributed-constant capacitance between the first and second electrodes; and one or more of the following items;- a) a feedback amplifier (26) connected to said parallel resonant circuit for generating an inherent voltage vibration to produce a tuning oscillating signal at the resonance frequency of said parallel resonant circuit; b) an amplifier (40) connected to said parallel resonant circuit for generating a tuning amplifying action at the resonance frequency of said parallel resonant circuit to produce a selective tuning amplified signal; c) a mixer (68) connected to said parallel resonant circuit and responsive to frequency signals to be mixed for generating a frequency converting action to produce an intermediate frequency tuning signal, said resonance frequency of said parallel resonant circuit corresponding to the frequency of the signal present at the mixer terminal to which said parallel resonant circuit is connected. A tuning system as claimed in claim 1 including at least two of said items (a), (b) and (c) wherein said dielectric is shared by said at least two items. The tuning system according to claim 1 or 2, wherein each of said electrodes has a spiral shape (360, 361). The tuning system according to claim 1 or 2, wherein each of said electrodes has a zigzag shape (343, 344). The tuning system according to any of claims 1 to 4, wherein said electrodes are disposed on one surface of said dielectric. The tuning system according to any of claims 1 to 4, wherein at least one of said electrodes is at least partly embedded in said dielectric. The tuning system according to any of claims 1 to 6, wherein said dielectric is in the form of a hollow cylinder (351). The tuning system according to any of claims 1 to 6, wherein said dielectric is in the form of a ring. The tuning system according to any of claims 1 to 6, wherein said dielectric is in the form of a plate. The tuning system according to any of claims 1 to 9, wherein at least one of said electrodes has a portion cut off for adjusting the oscillation tuning frequency. The tuning system according to any of claims 1 to 10, further comprising a reactance element (51) connected between said first terminal and said ground or common terminal. The tuning system according to claim 11, wherein said reactance element comprises a voltage-variable capacitance diode. The tuning system according to claim 11, including a phase-locked loop for controlling the oscillation frequency of the tuning oscillator circuit, the arrangement being such that a control bias voltage from said phase-locked loop (117) for said voltage-variable capacitance diode is employed as required as a tuning control voltage for said selective tuning amplifier circuit.

Inventors: KANE, JOHJI
Applicants: MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1984
Main classification: H03J3
IPC codes: H03B1, H03B5, H03H5, H03F3, H03D9, H03L7, H03J3, H03J1