Voltage controled oscillator

Abstract: Disclosed is a voltage controlled oscillator which is prevented from generating an abnormal oscillation signal and realizes reduction in the number of parts used including a large part, a smaller structure as a whole, and a lower manufacturing cost. This voltage controlled oscillator comprises: an oscillation transistor (1 1 ) which operates by common-base connection and outputs an oscillation signal from the emitter; an amplification transistor (1 2 ) which operates by common-emitter connection and whose base receives the oscillation signal; and a resonance circuit (2) connected to the collector of the oscillation transistor. In the resonance circuit, an inductor (2 1 ) for resonance is connected in a d.c. circuit provided between the collector of the oscillation transistor and the emitter of the amplification transistor, and a capacitor for resonance is connected between the collector of the oscillation transistor and an earth point.

Description: The present invention relates to a voltage controlled oscillator and, more particularly, to a voltage controlled oscillator which is prevented from generating abnormal oscillation and realizes the smaller number of parts used, in which an oscillation transistor and an amplification transistor are connected in series so as to pass direct currents and a resonance circuit is connected to the collector of the oscillator transistor. Conventionally known voltage controlled oscillators include: a voltage controlled oscillator (hereinbelow, this will be called a known first voltage controlled oscillator) in which an oscillation transistor and an amplification transistor are connected in series so as to pass direct currents and a resonance circuit is connected to the collector of the oscillation transistor; and a band switching type voltage controlled oscillator (hereinbelow, this will be called a known second voltage controlled oscillator) in which an oscillation transistor and an amplification transistor are connected in series so as to pass direct currents and a resonance circuit is connected to the base of the oscillation transistor. In each of the known first and second voltage controlled oscillators, an oscillation signal outputted from the emitter of the oscillation transistor is supplied to the base of the amplification transistor and an amplified oscillation signal is outputted from the collector of the amplification transistor. Another example of a known oscillator can be found in US 5,440,276. This oscillator includes a cascade connection of an oscillating transistor and an amplifying transistor, an RF choke coil connected in series with a resistor between the bases of the two transistors, and a resonant circuit connected to the base of the oscillating transistor. In the known first voltage controlled oscillator, an RF (high-frequency) choke inductor is necessary for the collector of the oscillation transistor to which the resonance circuit is connected. Consequently, there is a case that an abnormal oscillation signal of a frequency much lower than the frequency of the oscillation signal is generated. Since an inherent oscillation signal is interfered by the abnormal oscillation signal generated, an abnormal oscillation suppressing circuit for suppressing the generation of the abnormal oscillation signal, specifically, an abnormal oscillation suppressing circuit in which an inductor and a resistor are connected in series is connected to a d.c. circuit provided between the collector of the oscillation transistor and the emitter of the amplification transistor. In contrast, in the known second voltage controlled oscillator, since the resonance circuit is connected to the base of the oscillation transistor, in a manner similar to the known first voltage controlled oscillator, generation of an abnormal oscillation signal in the collector of the oscillation transistor can be avoided. Fig. 3 is a circuit diagram showing an example of the construction of the known first voltage controlled oscillator. As shown in Fig. 3, the known first voltage controlled oscillator comprises an active circuit section 31 for oscillation, a resonance circuit section 32, an oscillation signal output terminal 33, a power supply terminal 34, a frequency setting voltage supply terminal 35 and an abnormal oscillation suppressing circuit 36. In this case, the active circuit section 31 for oscillation comprises a first transistor (oscillation transistor) 31 1 , a second transistor (amplification transistor) 31 2 , a first feedback capacitor 31 3 , a second feedback capacitor 31 4 , an emitter resistor 31 5 , a first coupling capacitor 31 6 , a first base bias setting resistor 31 7 , a second base bias setting resistor 31 8 , a third base bias setting resistor 31 9 , a first bypass capacitor 31 10 , a second bypass capacitor 31 11 , a second coupling capacitor 31 12 , a load inductor 31 13 , and a smoothing capacitor 31 14 . Other than the above, the active circuit section 31 for oscillation has an abnormal oscillation suppressing circuit 36 comprising an inductor 36 1 and a resistor 36 2 . The resonance circuit section 32 comprises an inductor 32 1 for resonance, a varactor diode 32 2 , a d.c. blocking capacitor 32 3 , a buffer inductor 32 4 , a smoothing capacitor 32 5 , and a coupling capacitor 32 6 . In this case, the resonance frequency of the resonance circuit section 32 is set mainly by the resonance inductor 32 1 and the varactor diode 32 2 . The elements 31 1 to 31 14 and 36 1 and 36 2 constructing the active circuit section 31 for oscillation and the elements 32 1 to 32 5 constructing the resonance circuit section 32 are connected as shown in Fig. 3. The known voltage controlled oscillator having the above construction operates as follows. When a frequency setting voltage is supplied to the frequency setting voltage supply terminal 35, the frequency setting voltage is applied to the cathode of the varactor diode 32 2 via the buffer inductor 32 4 to set the capacitance value of the varactor diode 32 2 in correspondence with the magnitude of the frequency setting voltage. At this time, the resonance circuit section 32 resonates to a frequency determined by the inductance value of the resonance inductor 32 1 and the capacitance value of the varactor diode 32 2 which is set by the frequency setting voltage. When the active circuit section 31 for oscillation is seen from the resonance circuit section 32 in a state where the resonance circuit section 32 resonates, a negative resistance occurs in the active circuit section 31 for oscillation, so that a frequency signal equal to the resonance frequency is generated in the resonance circuit section 32. In the case of changing the frequency setting voltage to be supplied to the frequency setting voltage supply terminal 35, the capacitance value of the varactor diode 32 2 changes in correspondence with the change and the frequency of the frequency signal generated in the resonance circuit section 32 changes. When the frequency signal equal to the resonance frequency is generated in the resonance circuit section 32, in the active circuit section 31 for oscillation, the first transistor 31 1 outputs the frequency signal as an oscillation signal from the emitter, which is supplied to the base of the second transistor 31 2 via the first coupling capacitor 31 6 . The second transistor 31 2 amplifies the supplied oscillation signal by common-emitter connection and supplies the amplified oscillation signal from the collector to the oscillation signal output terminal 33 via the second coupling capacitor 31 12 . In this case, in the active circuit section 31 for oscillation, the abnormal oscillation suppressing circuit 36 in which the inductor 36 1 and the resistor 36 2 are connected in series is inserted in the d.c. circuit provided between the collector of the first transistor 31 1 and the emitter of the second transistor 31 2 . The abnormal oscillation suppressing circuit 36 blocks an abnormal oscillation signal which is generated in a frequency band much lower than the oscillation frequency signal supplied from the collector of the first transistor 31 1 to the emitter of the second transistor 31 2 . By connecting the abnormal oscillation suppressing circuit 36, the abnormal oscillation signal is prevented from being supplied from the collector of the second transistor 31 2 to the oscillation signal output terminal 33 via the second coupling capacitor 31 12 . In this case, preferably, the resistance value of the resistor 36 2 used for the abnormal oscillation suppressing circuit 36 is as low as possible since it is connected to the d.c. circuit. Usually, a resistor having a resistance value of 10 to 50 Ω is used. Fig. 4 is a circuit diagram showing an example of the construction of the known second voltage controlled oscillator. As shown in Fig. 4, the known second voltage controlled oscillator comprises an active circuit section 41 for oscillation, a resonance circuit section 42, an oscillation signal output terminal 43, a power supply terminal 44, a frequency setting voltage supply terminal 45, and a band switching voltage supply terminal 46. In this case, the active circuit section 41 for oscillation comprises: a first transistor (oscillation transistor) 41 1 , a second transistor (amplification transistor) 41 2 , an emitter resistor 41 3 , a first emitter bypass capacitor 41 4 , a first coupling capacitor 41 5 , a second emitter bypass capacitor 416, a feedback capacitor 41 7 , a first base bias setting resistor 41 8 , a second base bias setting resistor 41 9 , a third base bias setting resistor 41 10 , a first coupling capacitor 41 11 , a second coupling capacitor 41 12 , a load inductor 41 13 , a smoothing capacitor 41 14 , and a buffer resistor 41 15 . The resonance circuit section 42 comprises a varactor diode 42 1 , a first d.c. blocking capacitor 42 2 , a buffer inductor 42 3 , a first bypass capacitor 42 4 , a first voltage dividing resistor 42 5 , a second voltage dividing resistor 42 6 , a second bypass capacitor 42 7 , a switching diode 42 8 , a coupling capacitor 42 9 , and a second d.c. blocking capacitor 42 10 . Other than the above, the resonance circuit section 42 has an inductor 47 for resonance comprising a first inductor 47 1 and a second inductor 47 2 which are connected in series. In this case, the resonance frequency of the resonance circuit section 42 is set mainly by the resonance inductor 47 and the varactor diode 42 1 . The elements 41 1 to 41 15 constructing the active circuit section 41 for oscillation and the elements 42 1 to 42 10 and the elements 47 1 and 47 2 constructing the resonance circuit section 42 are connected as shown in Fig. 4. The known second voltage controlled oscillator having the above construction operates as follows. When a band switching voltage as a ground voltage is supplied to the band switching voltage supply terminal 46, the band switching voltage is applied to the anode of the switching diode 42 8 via the buffer resistor 41 15 . At this time, since a required positive voltage has been supplied via the first and second voltage dividing resistors 42 5 and 42 6 to the cathode of the switching diode 42 8 , the switching diode 42 8 is reverse-biased below cutoff. The resonance inductor 47 in the resonance circuit section 42 therefore enters a state in which the first inductor 47 1 and the second inductor 47 2 are connected in series and the resonance circuit section 42 resonates to a low frequency band. On the other hand, when a band switching positive voltage is supplied to the band switching voltage supply terminal 46, the band switching voltage is supplied to the anode of the switching diode 42 8 . At this time, since the anode voltage of the switching diode 42 8 becomes larger than a voltage obtained by adding a junction voltage to the cathode voltage of the switching diode 42 8 , the switching diode 42 8 is forward biased and is made conductive. Consequently, the resonance inductor 47 in the resonance circuit section 42 is short-circuited by the switching diode 42 8 which is made conductive by the second inductor 47 2 by passing alternating currents, so that a state where only the first inductor 47 1 is connected is obtained and the resonance circuit section 42 resonates to a high frequency band. When the frequency setting voltage is supplied to the frequency setting voltage supply terminal 45, the frequency setting voltage is applied to the cathode of the varactor diode 42 1 via the buffer inductor 42 3 and the capacitance value of the varactor diode 42 1 is set in correspondence with the frequency setting voltage. At this time, the resonance circuit section 42 resonates to a frequency determined by: either the total inductance value of the first and second inductors 47 1 and 47 2 of the resonance inductor 47 (when the resonance circuit section 42 resonates to a low frequency band) or the inductance value of the first inductor 47 1 (when the resonance circuit section 42 resonates to a high frequency band) and the capacitance value of the varactor diode 42 1 which is set by the frequency setting voltage. When the frequency setting voltage to be supplied to the frequency setting voltage supply terminal 45 is changed, the capacitance value of the varactor diode 42 1 changes in correspondence with the change and the frequency of the frequency signal generated in the resonance circuit section 42 changes. When a resonance frequency signal is generated in the resonance circuit section 42, in the active circuit section 41 for oscillation, the frequency signal is received by the base of the first transistor 41 1 , outputted as an oscillation signal from the emitter, and supplied to the base of the second transistor 41 2 via the first coupling capacitor 41 5 . The second transistor 41 2 amplifies the supplied oscillation signal by common-emitter connection and supplies the amplified oscillation signal from the collector to the oscillation signal output terminal 43 via the second coupling capacitor 41 12 . The known first voltage controlled oscillator has an advantage such that generation of the abnormal oscillation signal can be prevented and an oscillation signal in a predetermined frequency band can be outputted. On the other hand, since the abnormal oscillation suppressing circuit 36 in which the inductor 36 1 and the resistor 36 2 are connected in series is inserted in the active circuit section 31 for oscillation to suppress the abnormal oscillation signal, the number of parts used is larger as compared with other voltage controlled oscillators of this kind each in which the abnormal oscillation suppressing circuit 36 is not inserted, and the manufacturing cost is accordingly higher. Since the large inductor 36 1 having a large volume is included in the abnormal oscillation suppressing circuit 36 which is newly inserted in the known first voltage controlled oscillator, it is difficult to reduce the size of the whole structure of the voltage controlled oscillator. The known second voltage controlled oscillator has an advantage such that the generation of the abnormal oscillation signal can be prevented without connecting the abnormal oscillation suppressing circuit 36. On the other hand, since it is necessary to connect the first and second voltage dividing resistors 42 5 and 42 6 , the bypass capacitor 42 7 and the like to the cathode side of the switching diode 42 8 in the resonance circuit section 42, the construction of the band switching type resonance circuit section 42 is complicated as a whole, the number of parts used increases, and the manufacturing cost accordingly rises. The invention has been achieved in consideration of such a technical background and its object is to provide a voltage controlled oscillator which is prevented from generating an abnormal oscillation signal and realizes a small structure as a whole by reducing the number of parts used including a large part at a low manufacturing cost. In order to achieve the object, according to the invention, there is provided a voltage controlled oscillator comprising: an oscillation transistor which operates by common-base connection and outputs an oscillation signal from the emitter; an amplification transistor which operates by common-emitter connection and whose base receives the oscillation signal; and a resonance circuit connected to the collector of the oscillation transistor, wherein in the resonance circuit, an inductor for resonance is connected in a d.c. circuit provided between the collector of the oscillation transistor and the emitter of the amplification transistor, and a capacitor for resonance is connected between the collector of the oscillation transistor and an earth point. According to the construction, since the inductor for resonance constructing the resonance circuit which also serves as a power supply line is inserted in place of the abnormal oscillation suppressing circuit in the d.c. circuit provided between the collector of the oscillation transistor and the emitter of the amplification transistor, the resonance circuit can be constructed by the inductor for resonance and the capacitor for resonance and an abnormal oscillation signal can be prevented from being generated. By using at least the inductor for resonance while reducing the number of parts used in the voltage controlled oscillator, the number of parts of the abnormal oscillation suppressing circuit is reduced. Moreover, since one of the reduced parts is an inductor which is a large part, the whole structure of the voltage controlled oscillator can be made smaller than that of a known voltage controlled oscillator of this kind. According to the invention, in the case of forming a band switching voltage controlled oscillator, preferably, a resonance frequency band of the resonance circuit is selectively switched to either a high band or a low band according to either one polarity or the other polarity of a band switching voltage and, when the resonance frequency band is switched to the high band, a part of the inductor for resonance or one of two inductors for resonance is short-circuited by passing alternating currents by the band switching voltage of the polarity. With the construction, according to the polarity of the band switching voltage to be supplied, the whole inductor for resonance or only a part of the inductor is selectively used as a resonance circuit. Thus, the resonance frequency band of the resonance circuit can be selectively switched to either the high band or the low band relatively easily. Further, according to the invention, in the case of constructing a band switching voltage controlled oscillator, it is preferable that a part of the inductor for resonance or one of the two inductors for resonance is short-circuited by passing alternating currents by a switching diode whose cathode is directly connected to one end of the inductor for resonance and whose anode is connected to a middle point of the inductor for resonance or a connection point of the two inductors for resonance via a capacitor. With such a construction, it is unnecessary to connect a plurality of voltage dividing resistors and a bypass capacitors on the cathode side of the switching diode. Consequently, the construction of the band switching type voltage controlled oscillator is simplified, the reduction in the number of parts used and the miniaturization of the oscillator are achieved, and the manufacturing cost can be also reduced. According to the invention, the inductor for resonance may take the form of a microstrip line. With such a construction, the inductor for resonance can be miniaturized. Moreover, the resonance circuit can be made adapted to high frequency oscillation and the whole structure of the voltage controlled oscillator can be miniaturized. Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a circuit construction diagram showing an embodiment of a voltage controlled oscillator according to the invention. Fig. 2 is a circuit construction diagram showing another embodiment of a voltage controlled oscillator according to the invention. Fig. 3 is a circuit diagram showing an example of the construction of a known first voltage controlled oscillator. Fig. 4 is a circuit diagram showing an example of the construction of a known second voltage controlled oscillator. As shown in Fig. 1, the voltage controlled oscillator according to the embodiment comprises an active circuit section 1 for oscillation, a resonance circuit section 2, an oscillation signal output terminal 3, a power supply terminal 4, and a frequency setting voltage supply terminal 5. In this case, the active circuit section 1 for oscillation comprises a first transistor (oscillation transistor) 1 1 , a second transistor (amplification transistor) 12, a first feedback capacitor 1 3 , a second feedback capacitor 1 4 , an emitter resistor 1 5 , a first coupling capacitor 1 6 , a first base bias setting resistor 1 7 , a second base bias setting resistor 1 8 , a third base bias setting resistor 1 9 , a first bypass capacitor 1 10 , a second bypass capacitor 1 11 , a second coupling capacitor 1 12 , a load inductor 1 13 , and a smoothing capacitor 1 14 . The resonance circuit section 2 comprises an inductor 2 1 for resonance, a varactor diode 2 2 , a buffer inductor 2 3 , a smoothing capacitor 2 4 , and a coupling capacitor 2 5 . The resonance frequency of the resonance circuit section 2 is mainly set by the resonance inductor 2 1 and the varactor diode 2 2 . In the active circuit section 1 for oscillation, the load inductor 1 13 , the collector-emitter path of the second transistor 1 2 , the resonance inductor 2 1 in the resonance circuit section 2, the collector-emitter path of the first transistor 1 1 , and the emitter resistor 1 5 are sequentially connected in series between the power supply terminal 4 and the earth point. In parallel with the series connection, the third base bias setting resistor 1 9 , the second base bias setting resistor 1 8 , and the first base bias setting resistor 1 7 are sequentially connected in series. The connection point between the third and second base bias setting resistors 1 9 and 1 8 is connected to the base of the second transistor 1 2 . The connection point between the second and first base bias setting resistors 1 8 and 1 7 is connected to the base of the first transistor 1 1 . The base of the first transistor 1 1 is connected to the earth point via the first bypass capacitor 1 10 . The emitter of the first transistor 1 1 is connected to the base of the second transistor 1 2 via the second coupling capacitor 1 6 . The collector of the first transistor 1 1 is connected to the coupling capacitor 2 5 in the resonance circuit section 2. Besides, in the first transistor 1 1 , the first feedback capacitor 1 3 is connected between the collector and the emitter, and the second feedback capacitor 1 4 is connected between the emitter and the earth point. In the second transistor 1 2 , the emitter is connected to the earth point via the second bypass capacitor 1 11 and the collector is connected to the oscillation signal output terminal 3 via the second coupling capacitor 1 12 . In the resonance circuit section 2, the cathode of the varactor diode 2 2 is connected to the active circuit section 1 for oscillation via the coupling capacitor 2 5 and to the frequency setting voltage supply terminal 5 via the buffer inductor 2 3 . The anode of the varactor diode 2 2 is connected to the earth point. The smoothing capacitor 2 4 is connected between the frequency setting voltage supply terminal 5 and the earth point. The voltage controlled oscillator having the above construction according to the embodiment operates as follows. When a frequency setting positive voltage is supplied to the frequency setting voltage supply terminal 5, the frequency setting voltage is applied to the cathode of the varactor diode 2 2 via the buffer inductor 2 3 . Since the anode of the varactor diode 2 2 has been connected to the earth point and an earth voltage is applied to the anode, the varactor diode 2 2 is reverse-biased and is set to a capacitance value corresponding to the frequency setting positive voltage supplied. At this time, the resonance circuit constructed by the resonance inductor 2 1 and the varactor diode 2 2 resonates to a frequency determined by the inductance value of the resonance inductor 2 1 and the set capacitance value of the varactor diode 2 2 . When the active circuit section 1 for oscillation is seen from the resonance circuit section 2 in a state where the resonance circuit section 2 resonates, a negative resistance occurs in the active circuit section 1 for oscillation. An oscillation signal of a frequency equal to the resonance frequency of the resonance circuit section 2 is generated in the active circuit section 1 for oscillation. In the case where the frequency setting positive voltage to be supplied to the frequency setting voltage supply terminal 5 is changed, the capacitance value of the varactor diode 2 2 changes in correspondence with the changed frequency setting voltage, and the frequency of the resonance signal generated in the resonance circuit section 2 changes. An oscillation signal generated in the active circuit section 1 for oscillation is outputted from the emitter of the first transistor 1 1 and is supplied to the base of the second transistor 1 2 via the first coupling capacitor 1 6 . The second transistor 1 2 amplifies the supplied oscillation signal by common-emitter connection and supplies the amplified oscillation signal from the collector to the oscillation signal output terminal 3 via the second coupling capacitor 1 12 . In the voltage controlled oscillator in the embodiment, the resonance inductor 2 1 is connected in the d.c. circuit provided between the collector of the first transistor 1 1 and the emitter of the second transistor 1 2 in the active circuit section 1 for oscillation. In the case of using the resonance inductor 2 1 in the d.c. circuit as described above, an abnormal oscillation signal in a frequency band much lower than the oscillation frequency signal can be prevented from being generated. Moreover, it is unnecessary to use the resistor 36 2 of a small resistance value. When the voltage controlled oscillator in the embodiment is compared with the known first voltage controlled oscillator shown in Fig. 3, by using the resonance inductor 2 1 in the former oscillator, not only a part of the components (resistor 36 2 ) of the abnormal oscillation suppressing circuit 36 in the latter oscillator but also a part (d.c. blocking capacitor 32 3 ) of components constructing the resonance circuit section 32 become unnecessary. The number of components used in the former oscillator is therefore smaller than that of the latter oscillator. Fig. 2 is a circuit diagram showing another embodiment of a voltage controlled oscillator, and shows an example in which a band switching type resonance circuit is used as the resonance circuit section. As shown in Fig. 2, a voltage controlled oscillator (hereinbelow, called a band switching type voltage controlled oscillator) in the embodiment comprises an active circuit section 1' for oscillation, a resonance circuit section 2', the oscillation signal output terminal 3, the power supply terminal 4, the frequency setting voltage supply terminal 5, and the band switching voltage supply terminal 6. When the voltage controlled oscillator in the embodiment is compared with the voltage controlled oscillator (called a single-band voltage controlled oscillator) in the embodiment of Fig. 1, the band switching voltage supply terminal 6 is newly provided. Further, as will be described hereinlater, the active circuit section 1 for oscillation and the resonance circuit section 2 are slightly modified, thereby obtaining the active circuit section 1' for oscillation and the resonance circuit section 2'. In this case, in a manner similar to the active circuit section 1 for oscillation, the active circuit section 1' for oscillation comprises: the first transistor (oscillation transistor) 1 1 , second transistor (amplification transistor) 1 2 , first feedback capacitor 1 3 , second feedback capacitor 1 4 , emitter resistor 1 5 , first coupling capacitor 1 6 , first base bias setting resistor 1 7 , second base bias setting resistor 1 8 , third base bias setting resistor 1 9 , first bypass capacitor 1 10 , second bypass capacitor 1 11 , second coupling capacitor 1 12 , load inductor 1 13 , and smoothing capacitor 1 14 . Different from the active circuit section 1 for oscillation, the active circuit section 1' for oscillation further comprises a buffer resistor 1 15 . In a manner similar to the resonance circuit section 2, the resonance circuit section 2' comprises the varactor diode 2 2 , buffer inductor 2 3 , smoothing capacitor 2 4 , and coupling capacitor 2 5 . The resonance circuit section 2' is different from the resonance circuit section 2 with respect to a point that the resonance inductor 2 1 which also serves as an abnormal oscillation suppressing circuit is replaced by a series circuit of a first resonance inductor 2 6 which also serves as an abnormal oscillation suppressing circuit and a second resonance inductor 2 7 and with respect to a point that the resonance circuit section 2' further comprises a first d.c. blocking capacitor 2 8 , switching diode 2 9 , and second d.c. blocking capacitor 2 10 . In the case of a high band, the resonance frequency of the resonance circuit section 2' is set mainly by the first resonance inductor 2 6 and the varactor diode 2 2 . In the case of a low band, the resonance frequency is set mainly by the series circuit of the first and second resonance inductors 2 6 and 2 7 and the varactor diode 2 2 . In the active circuit section 1' for oscillation, the buffer resistor 1 15 is connected between the band switching voltage supply terminal and the connection point T of the first d.c. blocking capacitor 2 8 , the switching diode 2 9 , and the second d.c. blocking capacitor 2 10 in the resonance circuit section 2'. In the resonance circuit section 2', the series circuit of the first and second resonance inductors 2 6 and 2 7 is connected between the collector of the first transistor 11 and the emitter of the second transistor 1 2 . The d.c. blocking capacitor 2 8 is connected between the connection point of the first and second resonance inductors 2 6 and 2 7 and the connection point T. The cathode of the switching diode 2 9 is connected to the connection point of the second resonance inductor 2 7 and the emitter of the second transistor 1 2 and the anode of the switching diode 2 9 is connected to the connection point T. The coupling capacitor 2 10 is connected between the connection point T and the first resonance inductor 2 6 and the collector of the first transistor 1 1 . The operation of the band switching type voltage controlled oscillator having the construction is as follows. When the band switching type voltage controlled oscillator is used as a voltage controlled oscillator in a low frequency band to output an oscillation signal in a low frequency band, a band switching voltage which is an earth voltage is supplied to the band switching voltage supply terminal 6. When the band switching voltage which is an earth voltage is supplied to the anode of the switching diode 2 9 via the buffer resistor 1 15 , the cathode of the switching diode 2 9 is connected to the emitter of the second transistor 1 2 . Since a required positive voltage has been supplied to the cathode, a reverse bias is applied to the switching diode 2 9 and a cutoff state is obtained. Consequently, in the resonance circuit section 2', the series circuit of the first and second resonance inductors 2 6 and 2 7 is connected as a resonance inductor, a state that the total inductance of the resonance inductors is large is obtained, and the resonance circuit in the resonance circuit section 2' resonates to a low frequency band. On the other hand, when the band switching type voltage controlled oscillator is used as a voltage controlled oscillator in a high frequency band to output an oscillation signal in a high frequency band, a band switching positive voltage is supplied to the band switching voltage supply terminal 6. When the band switching positive voltage is supplied to the anode of the switching diode 2 9 via the buffer resistor 1 15 , the anode voltage of the switching diode 2 9 becomes larger than a voltage obtained by adding the junction voltage of the diode 2 9 to the cathode voltage, the switching diode 2 9 is forward biased and enters an ON state. Consequently, in the resonance circuit section 2', the second resonance inductor 2 7 is short-circuited by passing alternating currents by the switching diode 2 9 in an ON state, a state in which only the first resonance inductor 2 6 is connected as the resonance inductor is obtained, the inductance of the resonance inductor is reduced, and the resonance circuit in the resonance circuit section 2' resonates to a high frequency band. When a frequency setting positive voltage is supplied to the resonance frequency setting voltage supply terminal 5 in a state where the band switching type voltage controlled oscillator is switched to a low frequency band, the frequency setting positive voltage is supplied to the cathode of the varactor diode 2 2 via the buffer inductor 2 3 , and an earth voltage is supplied to the anode of the varactor diode 2 2 by the connection to the earth point. Consequently, the varactor diode 2 2 is reverse-biased and is set to a capacitance value corresponding to the frequency setting voltage. At this time, the resonance circuit constructed by the series circuit of the first and second resonance inductors 2 6 and 2 7 and the varactor diode 2 2 resonates to frequencies in a low frequency band determined by the total inductance value of the first and second resonance inductors 2 6 and 2 7 and the capacitance value set in the varactor diode 2 2 . On the other hand, when the frequency setting positive voltage is supplied to the resonance frequency setting voltage supply terminal 5 in a state where the band switching type voltage controlled oscillator is switched to the high frequency band, the frequency setting positive voltage is supplied to the cathode of the varactor diode 2 2 via the buffer inductor 2 3 , and an earth voltage is supplied to the anode of the varactor diode 2 2 . Consequently, the varactor diode 2 2 is reverse-biased and is set to a capacitance value corresponding to the frequency setting voltage. At this time, the resonance circuit constructed by the first resonance inductor 2 6 and the varactor diode 2 2 resonates to frequencies in a high frequency band determined by the inductance value of the first resonance inductor 2 6 and the capacitance value set in the varactor diode 2 2 . The subsequent operation is the same as that of the signal-band voltage controlled oscillator irrespective of the case where the band switching type voltage controlled oscillator is switched to either the low frequency band or the high frequency band. That is, when the active circuit section 1' for oscillation is seen from the resonance circuit section 2' in a state where the resonance circuit resonates, a negative resistance occurs in the active circuit section 1' for oscillation. Consequently, an oscillation signal of a frequency equal to the resonance frequency of the resonance circuit is generated in the active circuit section 1' for oscillation. In this case as well, when the frequency setting positive voltage to be supplied to the frequency setting voltage supply terminal 5 is changed, the capacitance value of the varactor diode 2 2 changes according to the changed frequency setting voltage and the frequency of the resonance signal generated in the resonance circuit section 2' changes. The oscillation signal generated in the active circuit section 1' for oscillation is outputted from the emitter of the first transistor 1 1 and is supplied via the first coupling capacitor 1 6 to the base of the second transistor 1 2 . The second transistor 1 2 amplifies the supplied oscillation signal by common emitter connection and supplies the amplified oscillation signal via the second coupling capacitor 1 12 to the oscillation signal output terminal 3. In the band switching type voltage controlled oscillator as well, the resonance inductor which comprises the first and second resonance inductors 2 6 and 2 7 and also serves as the abnormal oscillation suppressing circuit is connected in the d.c. circuit provided between the collector of the first transistor 1 1 and the emitter of the second transistor 1 2 in the active circuit section 1' for oscillation. By using such a resonance inductor which also serves as the abnormal oscillation suppressing circuit, in a manner similar to the known abnormal oscillation suppressing circuit 36 which is made by the series circuit of the inductor 36 1 and the resistor 36 2 in the known first voltage controlled oscillator, the abnormal oscillation signal in the frequency band much lower than the oscillation frequency signal can be prevented from being generated and, moreover, it is unnecessary to use the resistor 36 2 of a small resistance value. When the band switching type voltage controlled oscillator is compared with the known second voltage controlled oscillator shown in Fig. 4, in the former oscillator, by directly connecting the cathode of the switching diode 2 9 to the emitter of the second transistor 1 2 (an end of the second resonance inductor 2 7 ), the components (the first and second voltage dividing resistors 42 5 and 42 6 and the bypass capacitor 42 7 which are used in the latter oscillator) for applying a bias voltage to the cathode of the switching diode 42 8 become unnecessary. The number of components used in the former oscillator is therefore smaller than that of the latter oscillator. Since each of the single-band voltage controlled oscillator and the band switching type voltage controlled oscillator generates an oscillation signal in a high frequency band in the foregoing embodiments, it is preferable to use an inductor for high frequencies such as a microstrip line as each of the resonance inductor 2 1 , the first resonance inductor 2 6 and the second resonance inductor 2 7 . Although the case where the first and second resonance inductors 2 6 and 2 7 used in the band switching type voltage controlled oscillator are two separate resonance inductors has been described in the other embodiment, the resonance inductor of the invention is not limited to two separate resonance inductors but may be a single inductor having a center tap. As described above, the resonance inductor constructing the resonance circuit is inserted in the d.c. circuit which is provided between the collector of the oscillation transistor and the emitter of the amplification transistor and the resonance inductor is also used as an inductor for the d.c. circuit. Thus, effects such that the resonance circuit can be constructed by the resonance inductor and the resonance capacitor and generation of an abnormal oscillation signal can be suppressed are produced. The number of parts used in the voltage controlled oscillator can be reduced by an amount corresponding to at least the arrangement of using the resonance inductor also as the d.c. circuit. Moreover, since the omitted component is an inductor which is a large component, an effect such that the whole structure of the voltage controlled oscillator can be miniaturized as compared with the whole structure of the known first voltage controlled oscillator is produced. Further, in the case of forming the band switching type voltage controlled oscillator, the following effects are produced. As compared with the whole structure of the known second voltage controlled oscillator, since it is unnecessary to connect a voltage dividing resistor and a bypass capacitor to the cathode of the switching diode, the construction of the band switching type voltage controlled oscillator is simplified, the number of parts used can be reduced, the size can be reduced, and the manufacturing cost can be reduced.

Claims: A voltage controlled oscillator comprising: an oscillation transistor (1 1 ) which operates by common-base connection and outputs an oscillation signal from the emitter; an amplification transistor (1 2 ) which operates by common-emitter connection and whose base receives the oscillation signal; and a resonance circuit (2, 2') connected to the collector of the oscillation transistor, wherein in the resonance circuit, an inductor for resonance (2 1 , 2 6 , 2 7 ) is connected in a d.c. circuit provided between the collector of the oscillation transistor and the emitter of the amplification transistor, and a capacitor for resonance (2 2 ) is connected between the collector of the oscillation transistor and an earth point. A voltage controlled oscillator according to claim 1, wherein a resonance frequency band of the resonance circuit is selectively switched to either a high band or a low band according to either one polarity or the other polarity of a band switching voltage (6) and, when the resonance frequency band is switched to the high band, a part of the inductor for resonance (2 1 ) or one of two inductors for resonance (2 7 ) is short-circuited by passing alternating currents by the band switching voltage of the polarity. A voltage controlled oscillator according to claim 2, wherein a part of the inductor for resonance (2 1 ) or one of the two inductors for resonance (2 7 ) is short-circuited by passing alternating currents by a switching diode (2 9 ) whose cathode is directly connected to one end of the inductor for resonance and whose anode is connected to a middle point of the inductor for resonance or a connection point of the two inductors for resonance via a capacitor (2 8 ). A voltage controlled oscillator according to claim 1, 2 or 3, wherein the inductor for resonance is a microstrip line.

Inventors: IKARASHI YASUHIRO, IKARASHI, YASUHIRO, Ikarashi, Yasuhiro, Alps Electric Co., Ltd.
Applicants: ALPS ELECTRIC CO LTD, ALPS ELECTRIC CO., LTD.
Application year: 2000
Main classification: H03B5
IPC codes: H03B5, H03B1, H03L7