Electrical Tuning Circuit

The conventional electronic tuning circuit which has hitherto been employed for UHF applications comprises a straigth transmission line segment, a varactor having one of its electrodes connected to one end of the transmission line and a DC blocking capacitor having one of its electrodes connected to the opposite end of the transmission line. The other electrodes of the varactor and the blocking capacitors are both connected to ground so as to form a closed loop resonance circuit. To control the capacitance of the varactor, a DC control signal is applied through an RF choke coil to one electrode of the varactor so the one electrode is biased with respect to the other electrode. Since the connections to ground terminals constitute a part of the resonance circuit, the UHF energy is partially wasted by a high impedance which may be introduced by the ground connections. Furthermore, because of the straight-line configuration, the prior art tuning circuit tends to dissipate its energy through its environment without serving any useful purposes.

The primary object of the invention is to provide an electronic tuning circuit which operates with a minimum of energy loss.

Another object is to provide an electronic tuning circuit which is suitable for adaptation to integrated circuit fabrication.

The primary object of the invention is realized by formation of a transmission line in a generally C-shaped configuration and connecting a varactor and a DC blocking capacitor in series with the transmission line to form a closed loop radio-frequency resonance circuit. The invention contemplates the use of two RF chokes, one of which is connected between one terminal of the varactor and a control voltage source and the other being connected between the other terminal of the varactor and ground. The RF choke coils allow the DC control current to pass through the varactor while preventing the passage of RF current therethrough. The DC blocking capacitor has a low impedance at radio frequencies and prevents the varactor from being short-circuited by the transmission line which also serves as a path for the DC control current.

This and other objects, features and advantages of the invention will be understood by the following description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a preferred embodiment of the electronic tuning circuit of the invention;

FIG. 2a is a plan view of the electronic tuning circuit shown mounted on a dielectric substrate, and FIGS. 2b-2c are cross-sectional views taken along the lines A--A' of FIG. 2a;

FIG. 3 is a graph showing an electrical characteristic according to the present embodiments in comparison with the prior art electronic tuning circuit;

FIGS. 4a-4c are illustrations of modified forms of the present invention;

FIG. 4d is a cross-sectional view taken on the line A--A' of FIG. 4a;

FIG. 5a is a modification of the preferred embodiment of FIG. 4a;

FIG. 5b is a cross-sectional view taken through the lines A--A', FIG. 5a;

FIGS. 6-9 illustrate applications of the preferred embodiments of the present invention; and

FIG. 10 is a graph showing an electrical characteristic of the application of FIG. 9.

Reference is now made to FIG. 1 which illustrates an electronic tuning circuit 10 embodying the present invention. As shown, the tuning circuit 10 comprises a transmission line formed by identical, generally C-shaped conductive strips 26 and 28. The strip 26 constitutes a first section of the transmission line which is only reactively coupled with an input circuit 40 and the strip 28 constitutes a second section of the transmission line which is only reactively coupled with an output circuit 42. A DC blocking capacitor 30 is provided between the ends 26a and 28a of strips 26 and 28, respectively, and a varactor 32 is provided between the ends 26b and 28b. A DC control voltage is supplied to varactor 32 from a terminal 34 through an RF choke coil 36 and strip 28. A second RF control choke coil 38 is connected between end portion 26b and ground to block high-frequency currents and allows the second RF control current to flow therethrough to ground.

In operation, the input microwave energy is coupled through input circuit 40 to the first section 26 of the transmission line and then coupled through the DC blocking capacitor 30 to the second section 28. The DC blocking capacitor 30 offers a low impedance to the radio frequency current so that strips 26 and 28 act as a single transmission line. The microwave energy in the second section 28 is coupled to output circuit 42. Tuning is effected by controlling the voltage applied at terminal 34 to vary the capacitance of the varactor 32 and therefore the resonant frequency of the tuning circuit 10. Therefore, the microwave energy extracted from the output circuit 42 is tuned to the resonant frequency of the circuit 10. Since the microwave current is allowed to pass through the closed loop low loss circuit, and no ground connection exists in the closed loop, the present invention offers a higher Q value than the prior art tuning circuit. Furthermore, the closed-loop configuration of the tuning circuit 10 confines the microwave energy to a limited area, so there are strong reactive couplings with the input and output circuits and microwave energy is transferred from the input to the output with a minimum of wasted energy.

As shown in FIG. 2a, the tuning circuit 10 with the RF choke coils removed is shown mounted on a dielectric substrate 44 which is mounted in a metal housing 46 preferably with a close spacing to the bottom wall of the housing as illustrated in FIG. 2b. Substrate 44 should be positioned as illustrated, and not midway between the top and bottom of casing 46. The illustrated mounting of dielectric support 44 imparts a high circuit Q value to the tuning circuit, which in turn allows the use of an inexpensive material of high dielectric loss, such as glass or epoxy-glass laminates, etc. The dielectric substrate 44 may be mounted on the bottom wall as illustrated in FIG. 2c, in which case the dielectric loss of the substrate 44 tends to adversely affect the Q value of the tuning circuit, and thus the use of a relative lower dielectric material such as ceramics of polytetrafluorethylene, is preferred. FIG. 3 includes two curves wherein a curve a denotes the unloaded Q as a function of resonant frequency according to the circuit of the type shown in FIG. 2b, and a curve b obtained from the prior art straight-line type tuning circuit. For the purpose of exact comparison of the characteristics as shown in FIG. 3, the circuits of the prior art and FIG. 2b of the invention are designed such that each of the substrates used is 1.6 mm thick and 4 mm wide, and the inner height of the housings of both prior art and the invention is 15 mm, and the varactors are of silicon type. It is seen from the graph of FIG. 3 that the circuit Q of the FIG. 2b circuit is especially high in the lower range of the resonant frequency. This is desirable since the silicon type varactors have larger series resistance in the lower range of the resonant frequency than in the higher. Therefore, according to the present embodiments, the high Q in the lower range can improve the noise figure of the circuit.

Reference is now made to FIGS. 4a-4d, which illustrate another preferred embodiment of the present invention, in which the DC blocking capacitor 30 is formed by overlapping portions of the strips 26 and 28 with the dielectric substrate between them as clearly shown in FIG. 4d. The capacitance may be increased as desired by increasing the overlapped area relative to the other areas as illustrated in FIG. 4c. This is also possible by the use of a thin dielectric substrate of a material of low dielectric loss. A tuning circuit of a frequency range from 470 MHz to 920 MHz was obtained from the following manufacturing parameters:

(1) Substrate material: Polytetrafluorethylene glass laminate

(2) Substrate thickness: 0.4 mm

(3) Capacitor area: 15 mm.sup.2 (approx. 11 pF)

(4) capacitance ratio of varactor: 1:7.6

(5) Substrate spacing from bottom wall: 15 mm

FIGS. 5a and 5b are views showing the actual physical construction of a closed loop tuned circuit similar to that illustrated in FIG. 4a. In FIGS. 5a and 5b, diode 32 has a pair of oppositely extending leads that are connected to metallic coatings 26 and 28 on substrate 44. A lumped parameter capacitor subsists between a portion of metallic coating 28 on the upper portion of substrate 44 and the circular, closed loop metallic partial ring 26. A fixed capacitance is thereby provided between metallic coatings 26 and 28 on the upper face of substrate 44, while a variable capacitance is provided between the leads of varactor diode 32.

FIG. 6 illustrates an application of the circuit of FIG. 1 to a UHF tuner without an r-f amplifier. A UHF signal is applied to the tuner through an input terminal 45 and then fed through a conducting line 46 to a double-tuned bandpass filter circuit including circuits 10a and 10b each. The signal from the double-tuned circuit is applied to a diode 48, which serves as a mixer, and to which a signal is also applied from a local oscillator including a circuit 10c and a transistor 50. The mixer, as is well known in the art, generates an intermediate frequency signal by mixing the two received signals. The IF signal is fed through a terminal 52 to the next stage (not shown). A variable d.c. voltage is applied to varactors 32a-32c through a terminal 54 for the purpose of changing resonant frequencies of the circuits 10a-10c, respectively. Choke coils 38a-38c are provided between the circuits 10a-10c and a conductive strip 56, respectively, in order to make direct current paths. As shown, conductive strips 56 and 58 are grounded.

FIG. 7 is a modification of FIG. 6 in which each tuning circuit is replaced with the circuit of FIG. 4c. This form of tuner is more suitable for integrated circuit fabrication.

FIG. 8 is an illustration of a bandpass filter utilizing N of the tuning circuits of FIG. 1, where N is a positive integer greater than one. A UHF signal is applied to an input terminal 80 and then transmitted to an output terminal 82 through a plurality of successively arranged tuning circuits 10d-10g The resonant frequency of each of the circuits 10d-10g is determined by a variable d.c. voltage applied to a terminal 83.

The tuning circuits are arranged such that each linear portion of each tuning circuit of the transmission line is adjacent to and parallel with a linear portion of the adjacent tuning circuit so that microwave energy is transferred with a minimum of energy loss from the input terminal 80 to the output terminal 82. High frequency-selectivity can be obtained by providing as many such tuning circuits as desired.

FIG. 9 is an illustration of a directional coupler utilizing the embodiment of FIG. 1. The directional coupler comprises first and second transmission lines 85 and 91. Line 85 includes an input port 84, to which microwave energy is applied, and an output port 86, while line 91 includes second and third output ports 88 and 90. Between transmission lines 85 and 91 is tuning circuit 10 having first and second half-sections 26 and 28 respectively extending parallel with the first and second transmission lines 85 and 91.

The operation of the FIG. 9 embodiment is best understood with reference to FIG. 10. The input energy at port 84 is coupled with low attenuation to output port 86 when the frequency of the energy is outside the resonant frequency f.sub.r of the tuning circuit 10. When the input frequency approaches the resonant frequency f.sub.r, the input signal is transmitted through the tuning circuit 10 to the third output port 90 and the attenuation between ports 84 and 90 is at a minimum at the resonant frequency. On the other hand, the attenuation between the input port 84 and the second output port 88 is remarkably high so that no signal coupling occurs between them. It is thus understood that the arrangement of FIG. 9 operates as a directional coupler by varying the DC potential, +V, on terminal 34. For example, an input signal at frequency f.sub.r can be switched so energy at input port 84, initially coupled to output port 86, is transferred to port 90 by increasing the resonant frequency of circuit 10 to a level above f.sub.r.