Dual band oscillator circuit

Abstract: A dual band oscillator circuit according to the present invention comprises an oscillator circuit portion (1a) that oscillates at a first frequency, an oscillator circuit portion (1b) that oscillates at a second frequency, a buffer amplifier circuit portion (2c) to which an output of the first oscillator circuit portion (1a) is input through a first stage-to-stage coupling element (16a) and an output of the second oscillator circuit portion (1b) is input through a second stage-to-stage coupling element (16b). Operation is switched between the first and second oscillator circuits (1a, 1b) by an externally applied control voltage signal (at 24a,24b).

Description: The present invention relates to a small voltage-controlled oscillator (VCO) used in mobile communication systems such as mobile telephones and portable telephones and in satellite communication equipment. Fig. 5 shows a prior art dual band oscillator circuit for generating signals of two different frequency bands. In Fig. 5, 1a denotes a first oscillator circuit portion, 1b denotes a second oscillator circuit portion, 2a denotes a first buffer amplifier portion, 2b denotes a second buffer amplifier portion, 3 denotes a single-pole double-throw switch, 16a and 16b denote stage-to-stage coupling capacitors, 22 denotes a high-frequency output terminal. The operation of the above conventional dual band oscillator circuit will be described below. In Fig. 5, the first oscillator circuit portion 1a utilizes a first frequency band, and the second oscillator circuit utilizes a second frequency band of the two frequency bands. An oscillation signal from the first oscillator circuit portion 1a is amplified in the first buffer amplifier portion 2a after passing the stage-to-stage coupling capacitor 16a. Similarly, an oscillation signal from the second oscillator circuit portion 1b is amplified in the second buffer amplifier portion 2b after passing the stage-to-stage coupling capacitor 16b. The signals amplified in the buffer amplifier portions 2a and 2b are each input to the single-pole double-throw switch 3. By operating the single-pole double-throw switch 3 according to a control signal provided to a control terminal for switching output frequencies 23, only a desired frequency signal to be output is produced from the high-frequency output terminal 22. Thus, a high-frequency oscillator circuit in two frequency bands is implemented. However, in the above configuration, two buffer amplifier circuits, one for each frequency band, are required. Also, the single-pole double-throw switch is required for switching the outputs of two buffer amplifier portions. For these reasons, the size of the circuit becomes larger, increasing the size of the entire device. A second prior art approach is disclosed in "Switched Resonators Boost the Bandwidth of Microwave VCOs" by Chen Yu. Chi and Chen Y. Ho, Microwaves & RF, May 1990, pp131-pp140. In this document, a resonance circuit portion includes a combination of two inductors and a PIN diode switch. In this circuit, by controlling the on/off condition of the PIN diode switch according to a frequency band to be oscillated, inductors operating as resonators are switched for switching resonance frequencies. Thus, a high-frequency oscillator circuit operating in a broad frequency band is implemented in this document. In the above configuration, oscillation can be achieved for two frequency bands within a broad frequency range. However, there is no configuration for optimizing the C/N ratio (carrier-to-noise ratio) characteristics of the oscillator circuit in desired two frequency bands disclosed. DE 2 607 530 A discloses a multiple band oscillator circuit with oscillators having their output coupled without any switching to a common buffer circuit. The drawback of this known oscillator circuit is that the oscillator transistors and the buffer are not configured to save power. The preferred embodiment seeks to provide a dual band oscillator circuit which overcomes the drawbacks of the prior art oscillator circuits. According to the present invention there is provided a dual band oscillator according to claim 1. According to such a configuration, a single-pole double-throw switch for switching buffer amplifier outputs as used in a conventional circuit is unnecessary, and only one buffer amplifier circuit is necessary, rather than two. A resonance circuit portion constituting the oscillator circuit, such as a resonance circuit portion that changes a resonance frequency according to an externally applied voltage, may be provided in respective first and second oscillator circuits. However, by providing a common dual band resonance circuit, two resonance circuits are reduced to one. In other words, the dual band oscillator circuit may comprise a dual band resonance circuit having both resonance frequencies of the first and second frequency bands, and the dual band resonance circuit is connected to the oscillation transistor of the first oscillator circuit via a first resonator-transistor coupling capacitor and is connected to the oscillation transistor of the second oscillator circuit via a second resonator-transistor coupling capacitor. It is preferable that the above dual band resonance circuit comprises a strip line resonator having a ground end and an open end and a varactor diode whose anode terminal is grounded, the open end of the strip line resonator and a cathode terminal of the varactor diode being connected by a capacitor, and a predetermined position between the ground end and the open end of the strip line resonator and the cathode terminal of the varactor diode being connected by another capacitor. Furthermore, it is preferable that the other end of the first resonator-transistor coupling capacitor connected to the oscillation transistor of the first oscillator circuit is connected to a position between the ground end and the open end of the strip line resonator of the dual band resonance circuit, the position being a node of a voltage standing wave caused by resonance of the second frequency band. Thus, oscillating in the second frequency band, the effect of capacitive coupling to the first oscillator circuit and the resonator is reduced. For a similar purpose, it is preferable that a switch element is interposed between the first resonator-transistor coupling capacitor and the oscillation transistor of the first oscillator circuit. This switch element is interposed to be in a conductive state during the operation of the first oscillator circuit and in a disconnected state during the operation of the second oscillator circuit. Also, it is preferable that an impedance of the first stage-to-stage coupling element is low so as to be in a conductive state in the first frequency band and is high so as to be in an open state in the second frequency band. Thus, when oscillating in the second frequency band, the effect of coupling the first oscillator circuit and the common buffer amplifier can be reduced. Fig. 1 shows a circuit diagram of a first embodiment of a dual band oscillator circuit according to the present invention; Fig. 2 shows a circuit diagram of a second embodiment of a dual band oscillator circuit according to the present invention; Fig. 3 shows a circuit diagram of a third embodiment of a dual band oscillator circuit according to the present invention; Fig. 4 shows a circuit diagram of a fourth embodiment of a dual band oscillator circuit according to the present invention; and Fig. 5 shows a circuit diagram of a conventional dual band oscillator circuit. (First Embodiment) In Fig. 1, 1a denotes a first oscillator circuit portion, 1b denotes a second oscillator circuit portion, 2c denotes a buffer amplifier portion, 4a denotes a first resonance circuit portion, 4b denotes a second resonance circuit portion, 5a and 5b denote resonators, 6a and 6b denote resonator-varactor diode coupling capacitors, 7a and 7b denote varactor diodes, 8a, 8b, and 8c denote high-frequency chokes, 9a, 9b, and 9c denote high-frequency ground capacitors, 10a and 10b denote oscillation transistors, 11a and 11b denote collector-emitter capacitors, 12a and 12b denote collector-base capacitors, 13a and 13b denote emitter-ground capacitors, 14a and 14b denote base-ground capacitors, 15a and 15b denote resonator-transistor coupling capacitors, 16a and 16b denote stage-to-stage coupling capacitors, 17c denotes an output coupling capacitor, 18c denotes a buffer amplifier transistor, 19a, 19b, and 19c denote bias resistors, 20a and 20b denote tuning voltage supply terminals, 21c denotes a power supply terminal, 22 denotes a high-frequency output terminal, 24a denotes a first base bias control terminal, 24b denotes a second base bias control terminal, and 30a and 30b denote base bias resistors. The collector-emitter capacitor 11a, the collector-base capacitor 12a, the emitter-ground capacitor 13a, the base-ground capacitor 14a, and the resonator-transistor coupling capacitor 15a, which constitute the first oscillator circuit portion 1a, have respective capacitance values selected to be in an optimal oscillation state in a first frequency band. Also, the collector-emitter capacitor 11b, the collector-base capacitor 12b, the emitter-ground capacitor 13b, the base-ground capacitor 14b, and the resonator-transistor coupling capacitor 15b, which constitute the second oscillator circuit portion 1b, have respective capacitance values selected to be in an optimal oscillation state in a second frequency band. The first oscillator circuit portion 1a oscillates at a first frequency according to a voltage externally applied to the tuning voltage supply terminal 20a, and its oscillation output is provided to the buffer amplifier portion 2c through the stage-to-stage coupling capacitor 16a. Also, the second oscillator circuit portion 1b oscillates at a second frequency according to a voltage externally applied to the tuning voltage supply terminal 20b, and its oscillation output is provided to the buffer amplifier portion 2c through the stage-to-stage coupling capacitor 16b. The first base bias control terminal 24a is provided to apply a control voltage externally to the base of the first oscillation transistor 10a through the base bias resistor 30a. Similarly, the second base bias control terminal 24b is provided to apply a control voltage externally to the base of the second oscillation transistor 10b constituting the second oscillator circuit portion 1b through the base bias resistor 30b. According to the above configuration, when producing the oscillation output at the first frequency, a predetermined voltage is applied to the first base bias control terminal 24a, and no voltage is applied to the second base bias control terminal 24b. As a result, only the first oscillator circuit portion 1a becomes operational, and its oscillation output is amplified in the buffer amplifier portion 2c, after passing the stage-to-stage coupling capacitor 16a, to be output from the high-frequency output terminal 22. When producing the oscillation output at the second frequency, a predetermined voltage is applied to the second base bias control terminal 24b, and no voltage is applied to the first base bias control terminal 24a. As a result, only the second oscillator circuit portion 1b becomes operational, and its oscillation output is amplified in the buffer amplifier portion 2c, after passing the stage-to-stage coupling capacitor 16b, to be output from the high-frequency output terminal 22. As indicated above, in this embodiment the outputs of the first and second oscillator circuit portions 1a and 1b are connected to one buffer amplifier portion 2c to selectively operate either the first or second oscillator circuit portion 1a or 1b depending on control voltages applied to the first and second base bias control terminals 24a and 24b. Therefore, no single-pole double-throw switch for switching the outputs of buffer amplifier portions is required, and only one buffer amplifier is required, rather than two. As a result, the size of the circuit may be reduced, thus permitting a reduction in the size of the entire device. (Second Embodiment) In Fig. 2, 25 denotes a dual band resonance circuit portion, 26 denotes a strip line resonator, 6a and 6b denote resonator-varactor diode coupling capacitors, 7c denotes a varactor diode, 8c denotes a high-frequency choke, 9c denotes a high-frequency ground capacitor, 20c denotes a tuning voltage supply terminal, and other circuit elements are identical to those shown in Fig. 1, like elements being given like reference numbers. The strip line resonator 26 has one end grounded and the other end connected to the cathode terminal of the varactor diode 7c via the resonator-varactor diode coupling capacitor 6a. Also, the resonator-varactor diode coupling capacitor 6b is connected between a point on the strip line resonator 26b and the cathode terminal of the varactor diode 7c. The measured values of resonance frequency and Q values of the dual band resonance circuit portion 25 having such a configuration are shown in Table 1. Resonance frequency characteristics and Q value of dual band resonance circuit Resonance frequency (MHz) Tuning sensitivity (MHz/V) Q First frequency band 762 804 42 44 Second frequency band 1650 1693 43 44 The dual band resonance circuit 25 that resonates at two different frequencies is connected to the first oscillation transistor 10a via the resonator-transistor coupling capacitor 15a, and to the second oscillation transistor 10b via the resonator-transistor coupling capacitor 15b. In the above configuration, when a predetermined voltage is applied to the first base bias control terminal 24a, and no voltage is applied to the second base bias control terminal 24b, the first oscillation transistor 10a is turned on, and the second oscillation transistor 10b is turned off. Since the capacitance values of each capacitor connected to the first oscillation transistor 10a are selected so that an optimal oscillation state can be obtained at the first frequency, an oscillator circuit is formed which utilizes the inductive reactance of the dual band resonance circuit portion 25 at a frequency band somewhat lower than the resonance frequency of the first frequency. When a predetermined voltage is applied to the second base bias control terminal 24b, and no voltage is applied to the first base bias control terminal 24a, the second oscillation transistor 10b is turned on, and the first oscillation transistor 10a is turned off. Since the capacitance values of each capacitor connected to the second oscillation transistor 10b are selected so that an optimal oscillation state can be obtained at the second frequency, an oscillator circuit is formed which utilizes the inductive reactance of the dual band resonance circuit portion 25 at a frequency band somewhat lower than the resonance frequency of the second frequency. As mentioned above, according to the dual band oscillator circuit in this embodiment, by using the dual band resonance circuit portion 25 that resonates at two frequencies, two conventional resonance circuit portions can be integrated into one. As a result, the size of the circuit can be reduced, resulting in a reduction in the size of the entire device. One end of the resonator-transistor coupling capacitor 15a is connected to the oscillation transistor 10a that oscillates at the first frequency, and the other end is connected to a point that is a node of the voltage standing wave resonating at the second frequency, in the strip line resonator 26. Thus, the adverse effects of connecting the oscillation transistor 10a via the resonator-transistor coupling capacitor 15a, for example, a decrease in tuning voltage sensitivity with respect to the oscillation frequency of the second frequency band, are reduced. The measured values of changes in oscillation frequency and changes in tuning voltage sensitivity in the second frequency band when changing the connection point of the resonator-transistor coupling capacitor 15a and the strip line resonator 26 are shown in Table 2. Effect of changing connection point of resonator-transistor coupling capacitor 15a and strip line resonator 26 Position of connection point Oscillation frequency of second frequency Tuning voltage sensitivity of second frequency 3.5 mm 1700.9 MHz 15.4 MHz/V 8.5 mm 1676.8 MHz 17.2 MHz/V 16.0 mm 1662.0 MHz 21.4 MHz/V (Third Embodiment) In Fig. 3, 25 denotes a dual band resonance circuit portion, 15a and 15b denote resonator-transistor coupling capacitors, 27 denotes a switching diode, 28 denotes a switching diode bias choke coil, 29 denotes a switching diode bias resistor, 24a denotes a first base bias control terminal, 30a denotes a base bias resistor, 10a denotes a first oscillation transistor, 13a denotes an emitter-ground capacitor, 31a denotes a base-emitter capacitor, 9a denotes a high-frequency ground capacitor, 19a denotes a bias resistor, 16a denotes a stage-to-stage coupling capacitor, and other components are identical to those shown in Fig. 2. The emitter-ground capacitor 13a, the base-emitter capacitor 31a, the high-frequency ground capacitor 9a, and the resonator-transistor coupling capacitor 15a have respective capacitance values selected so that optimal oscillation occurs at a first frequency. 10b denotes the second oscillation transistor. The collector-emitter capacitor 11b, the collector-base capacitor 12b, the emitter-ground capacitor 13b, the base-ground capacitor 14b, the resonator-transistor coupling capacitor 15b have respective capacitance values selected so that optimal oscillation occurs at a second frequency. The dual band resonance circuit portion 25 that resonates in two different frequencies is connected to the first oscillation transistor 10a that oscillates at the first frequency via the resonator-transistor coupling capacitor 15a and the switching diode 27, and is also connected to the second oscillation transistor 10b that oscillates at the second frequency via the resonator-transistor coupling capacitor 15b. When a predetermined voltage is applied to the first base bias control terminal 24a, and no voltage is applied to the second base bias control terminal 24b, the first oscillation transistor 10a and the switching diode 27 are turned on, and the second oscillation transistor 10b is turned off. Since the capacitance of each capacitor connected to the first oscillation transistor 10a is selected for optimal oscillation at the first frequency, an oscillator circuit is formed which utilizes the inductive reactance of the dual band resonance circuit portion 25 at a frequency somewhat lower than the resonance frequency of the first frequency. When a predetermined voltage is applied to the second base bias control terminal 24b, and no voltage is applied to the first base bias control terminal 24a, the second oscillation transistor 10b is turned on, and the first oscillation transistor 10a and the switching diode 27 are turned off. Since the capacitance of each capacitor connected to the second oscillation transistor 10b is selected for optimal oscillation at the second frequency, an oscillator circuit is formed which utilizes the inductive reactance of the dual band resonance circuit portion 25 at a frequency somewhat lower than the resonance frequency of the second frequency. The impedance of the circuit including the oscillation transistor 10a at the connection point of the resonance circuit portion 25 and the resonator-transistor coupling capacitor 15a can be made very large because the switching diode 27 is off. Also, the capacitance value of the resonator-transistor coupling capacitor 15b can be selected so that the impedance is high at the first frequency, and that the impedance is low at the second frequency. In this example, when producing the second frequency as an output, the impedance of the circuit including the first oscillation transistor 10a at the connection point of the resonance circuit portion 25 and the resonator-transistor coupling capacitor 15a is very large because the switching diode 27 is off. Therefore, the adverse effects of connecting the first oscillation transistor 10a of the oscillator circuit on the non-operation side, for example, a decrease in tuning voltage sensitivity with respect to the oscillation frequency of the second frequency band, is reduced. In this embodiment, the first oscillation transistor 10a that oscillates at the first frequency and the dual band resonance circuit portion 25 that resonates at two different frequencies are connected via the resonator-transistor coupling capacitor 15a and the switching diode 27. Thus, when producing an oscillation output at the second frequency, with the oscillation transistor 10b in an operation state, the adverse effects of the oscillation transistor 10a side are reduced. When oscillating at the first frequency, the resonator-transistor coupling capacitor 15b has a high impedance at the first frequency. Therefore, the adverse effect of the second oscillation transistor 10b on the oscillator circuit is similarly reduced. (Fourth Embodiment) Fig. 4 is an embodiment of the present invention that implements the dual band resonance circuit portion 25 in Fig. 2 in another form. In Fig. 4, one end of a strip line 36b is grounded, and the other end is connected to the cathode terminal of a switching diode 27 as a switch means. The anode terminal of the switching diode 27 is connected to one end of a strip line 36a, the other end of which is connected to a first transistor circuit portion 1c constituting a first oscillator circuit via a resonator-transistor coupling capacitor 15a. Furthermore, a point on the strip line 36b is connected to a second transistor circuit portion 1d constituting a second oscillator circuit via a resonator-transistor coupling capacitor 15b. The anode terminal of the switching diode 27 is connected to a band switching terminal 35 via a switching diode bias choke coil 28, and the band switching terminal 35 is grounded by a bypass capacitor 34. The sum L1 of the lengths of the strip lines 36a and 36b is set to a 1/4 of the wavelength of a required first resonance frequency. Also, the length L2 of the strip line 36b is a 1/4 of the wavelength of a required second resonance frequency. The first resonance frequency is lower than the second resonance frequency. The resonance frequencies are switched depending on whether a positive voltage is applied to the band switching terminal 35. When a positive voltage is applied, the switching diode 27 is forward biased to be conductive. Therefore, the strip lines 36a and 36b operate co-operatively as an end grounded resonator. Then, the resonance frequency of the resonance circuit is determined by the sum of the lengths of the two lines. When no positive voltage is applied to the band switching terminal 35, the switching diode 27 is in an off state. Therefore, the strip line 36b operates as an end grounded resonator independently. In this configuration, switching the oscillation frequencies is achieved by turning the switching diode 27 on and off, depending on whether a positive voltage is applied to the band switching terminal 35 or not, and by switching the control voltages applied to the first and second base bias control terminals 24a and 24b When producing an oscillation output at the first frequency, a predetermined voltage is applied to the first base bias control terminal 24a, no predetermined voltage is applied to the second base bias control terminal 24b, and a positive voltage is applied to the band switching terminal 35. Thus, the first transistor circuit portion 1c is turned on, the second transistor circuit portion 1d is turned off, and the switching diode 27 is turned on. Then, the resonance circuit serves as a 1/4-wavelength strip line resonator having one end grounded, corresponding to the sum L1 of the lengths of the strip lines 36a and 36b. At the first oscillation frequency corresponding to its resonance frequency, only the first transistor circuit portion 1c operates. After passing a stage-to-stage coupling capacitor 16a, its oscillation output is amplified in a buffer amplifier portion 2c to be output from a high-frequency output terminal 22. When producing an oscillation output at the second frequency, a predetermined voltage is applied to the second base bias control terminal 24b, no voltage is applied to the first base bias control terminal 24a, and no positive voltage is applied to the base switching terminal 35. Thus, the first transistor circuit portion 1c is turned off, the second transistor circuit portion 1d. is turned on, and the switching diode 27 is turned off. Thus, the resonance circuit is operated only by the strip line 36b and serves as a 1/4-wavelength strip line resonator having a grounded end. At the second oscillation frequency, corresponding to its resonance frequency, only the second transistor circuit portion 1d is operational. After passing a stage-to-stage coupling capacitor 16b, its oscillation output is amplified in the buffer amplifier portion 2c to be output from the high-frequency output terminal 22. According to the above configuration, particularly, when the switching diode 27 is off, the first transistor circuit 1c is disconnected. Therefore, advantageously, any adverse effect of connecting the first transistor circuit 1c is reduced.

Claims: A dual band oscillator circuit for producing an oscillating output at either one of first and second frequencies, comprising: a first oscillator circuit (1a) operable at a first frequency, including a first oscillation transistor (10a); a second oscillator circuit (1b) operable at a second frequency, including a second oscillation transistor (10b); a buffer amplifier circuit (2c) to which an output of the first oscillator circuit is input through a first stage-to-stage coupling element (16a) and an output of the second oscillator circuit is input through a second stage-to-stage coupling element (16b), the buffer amplifier circuit including a buffer transistor (18c); and first and second input (24a,24b) to apply a first and second voltages for controlling operation of the first and second oscillator circuits respectively so as to alternatively operate one of the first and second oscillator circuits; wherein a direct current component flowing through the buffer transistor (18c) also flows through the first or second oscillation transistors (10a, 10b) that is under operation. The dual band oscillator circuit according to claim 1, comprising a dual band resonance circuit (25) resonant at both the first and second frequencies, wherein the dual band resonance circuit (25) is connected to the first oscillation transistor (10a) via a first resonator-transistor coupling capacitor (15a) and is connected to the second oscillation transistor (10b) via a second resonator-transistor coupling capacitor (15b). The dual band oscillator circuit according to claim 2, wherein the dual band resonance circuit comprises a strip line resonator (26) having a ground end and an open end, a varactor diode (7c) whose anode terminal is grounded, a first capacitor (6a) connected between the open end of the strip line resonator and a cathode terminal of the varactor diode, and a second capacitor (6b) connected between the cathode terminal of the varactor diode and a predetermined position between the ground end and the open end of the strip line resonator. The dual band oscillator circuit according to claim 2 or 3, wherein the first resonator-transistor coupling capacitor (15a) is connected to the first oscillation transistor (10a) at a first side and is connected to a predetermined position between a ground end and an open end of a strip line resonator (26) of the dual band resonance circuit, the position being a node of a voltage standing wave resonating at the second frequency. The dual band oscillator circuit according to claim 2, 3 or 4, wherein a switch element (27) is interposed between the first resonator-transistor coupling capacitor (15a) and the first oscillation transistor (10a), and the switch element conducts when the first oscillator circuit (1a) operates and is in a disconnected state when the second oscillator circuit (1b) operates. The dual band oscillator circuit according to claim 2, wherein the dual band resonance circuit comprises a first strip line (36b) whose first terminal is grounded, a switch means (27) whose first terminal is connected to a second terminal of the first strip line, and a second strip line (36a) connected to a second terminal of the switch means, and wherein one of a resonance frequency of the first strip line and a resonance frequency of the first and second strip lines is selected by switching the switch means. The dual band oscillator circuit according to claim 6, wherein a sum of lengths of the first and second strip lines is a 1/4 of a wavelength of the first resonance frequency, and a length of the first strip line is a 1/4 of a wavelength of the second resonance frequency in the dual band resonance circuit.

Inventors: FUSE NOBUO, HASHIMOTO KOUJI, ISHIDA KAORU, KOSUGI HIROAKI, MIYOSHI HISANAGA, SAKAKURA MAKOTO, FUSE, NOBUO, HASHIMOTO, KOUJI, ISHIDA, KAORU, KOSUGI, HIROAKI, MIYOSHI, HISANAGA, SAKAKURA, MAKOTO
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD
Application year: 1997
Main classification: H03B5
IPC codes: H03B5