Abstract:
An oscillator which oscillates at a predetermined frequency includes a first inverting amplifier and a second inverting amplifier connected in antiparallel with each other, a surface acoustic wave resonator connected in parallel with one of the first inverting amplifier and the second inverting amplifier, and a filter connected between the first inverting amplifier and the second inverting amplifier for blocking a direct-current signal while allowing a signal of the predetermined frequency to pass.

Description:
BACKGROUND OF INVENTION 
     1. Field of the Invention 
     The present invention relates to oscillators and voltage controlled oscillators, and, more particularly, relates to an oscillator and a voltage controlled oscillator which have a surface acoustic wave resonator. 
     2. Description of the Related Art 
     FIG. 12 schematically illustrates a circuit diagram showing the configuration of a conventional Colpitts voltage controlled oscillator (VCO). 
     In FIG. 12, the Colpitts VCO includes an oscillation circuit  60 , a buffer circuit  66 , and a coupling capacitor  67  for coupling the oscillation circuit  60  and the buffer circuit  66 . The oscillation circuit  60  includes a surface acoustic wave (SAW) resonator  51 , an NPN transistor  52 , capacitors  53  to  56 , a varicap  57 , and resistance elements  58  and  59 . The buffer circuit  66  includes an NPN transistor  61  and resistance elements  62  to  65 . 
     The oscillating frequency of the oscillation circuit  60  is determined by the resonance frequency of the SAW resonator  51  and the capacitance of the oscillation circuit  60 . The oscillating frequency of the oscillation circuit  60  varies because the capacitance of the varicap  57  changes in accordance with change of the input voltage VI to the oscillation circuit  60 . Therefore, an output signal VO having a desired frequency can be obtained by adjusting the input voltage VI. This oscillation circuit is widely used as a reference-signal generator for a television tuner, a portable communication device, and the like. 
     FIG. 13 schematically illustrates a circuit diagram of a conventional differential-LC VCO. In FIG. 13, the differential-LC VCO includes a parallel LC circuit  71 , resistance elements  74  and  75 , NPN transistors  76  and  77 , and a voltage controlled current source  78 . The parallel LC circuit  71 , having an inductor  72  and a capacitor  73  connected in parallel, is connected between the collectors (nodes N 71  and N 72 , respectively) of the NPN transistors  76  and  77 . The collectors of the NPN transistors  76  and  77  are connected to a power line of a source electrical potential Vcc via resistance elements  74  and  75 , respectively. The bases of the NPN transistors  76  and  77  are connected to the collectors of the NPN transistors  77  and  76 , respectively. The emitters of the NPN transistors  76  and  77  are each grounded, via the voltage controlled current source  78 , to a ground line of a ground potential GND. 
     The oscillating frequency of this differential-LC VCO is determined by the parallel resonant frequency of the parallel LC circuit  71  and the internal capacitance of each of the NPN transistors  76  and  77  which are connected in parallel with the parallel LC circuit  71 . Because the capacitance of each of the NPN transistors  76  and  77  is varied by the values of the currents passing through the NPN transistors  76  and  77 , the oscillating frequency of the VCO can be adjusted by causing an external control voltage to control the current of the voltage controlled current source  78 . This circuit is utilized by an integrated circuit for television, and the like. 
     However, the Colpitts VCO shown in FIG. 12 has the following problems: because of the single ended configuration thereof, noise generated by the oscillation may produce adverse effects on other circuits via the power source; and conversely, a signal of the VCO may be modulated by high frequency noise coming through the power source. Furthermore, although it is intended to miniaturize the circuit by integrating the components of the circuit other than the SAW resonator  51 , because there are many capacitance elements  53  to  56  and  67  which occupy more space. than the resistance elements or the transistors, the integration of those elements is difficult. Also, when each element is constructed as a discrete component, since many elements are required for the circuit, miniaturization of the circuit is difficult. 
     In the differential-LC VCO in FIG. 13, although a wire wound variable inductor having a high quality-factor (Q-factor) must be employed as the inductor  72  to obtain an accurate and stable oscillation, miniaturization of the VCO is difficult due to the large size thereof. Moreover, in order to obtain a desired frequency, the inductance of the variable inductor must be adjusted. Additionally, because the VCO requires a variable inductor, the inductance of which must be adjusted by a mechanical operation as a variable component, the reliability is low. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide an oscillator and a voltage controlled oscillator which are highly resistant to noise, have accurate oscillation without adjustment, have high reliability, require less components, and are inexpensive and compact. 
     To this end, according to a first aspect of the present invention, there is provided an oscillator oscillating at a predetermined frequency. The oscillator includes a first inverting amplifier and a second inverting amplifier connected in antiparallel with each other, a surface acoustic wave resonator connected in parallel with one of said first inverting amplifier and said second inverting amplifier and a filter connected between said first inverting amplifier and said second inverting amplifier for blocking a direct-current signal while allowing a signal of said predetermined frequency to pass. 
     Since the direct current signal is blocked by the filter, the oscillating frequency of this oscillator is determined primarily by the parallel resonant frequency of the surface acoustic wave resonator. 
     This oscillator is not susceptible to the influence of external noise flowing via the power source and, conversely, is unlikely to convey noise to other circuits, compared to the Colpitts, due to a differential circuit employed therein. Also, the number of components of the oscillator is small, which makes miniaturization easy and is effective for reducing costs. Furthermore, because a capacitor having large capacitance is not used, it is possible to integrate a circuit. By integrating the entire circuit including the resonator, or the circuit other than the resonator, further miniaturization as well as further reduction in costs is possible. Effects of reduction in the number of components includes improving reliability of the oscillator. 
     Since, in the differential LC oscillator, when the Q-factor of the inductor is low, the Q-factor of the oscillator inevitably becomes low, which makes the stability of the oscillating frequency worse. On the other hand, since this oscillator uses a SAW resonator, accurate and highly stable oscillation can be obtained without adjustment. Moreover, since a variable component which needs mechanical adjustment is not used, reliability becomes high and deterioration with age becomes less. 
     According to a second aspect of the present invention, there is provided a voltage controlled oscillator which oscillates at a frequency in accordance with a control voltage. The voltage controlled oscillator includes a pair of differential transistors, in which a first electrode of one transistor is connected to a first electrode of the other transistor and an input electrode of one transistor is connected to a second electrode of the other transistor with regard to each of the transistors, a surface acoustic wave resonator connected between the second electrodes of the pair of differential transistors, a first capacitor coupled between the input electrode of at least one transistor of the pair of differential transistors and the second electrode of the other transistor, and a voltage controlled current source connected to the first electrodes of the pair of differential transistors for flowing currents having a value in accordance with the control voltage. 
     Because the first capacitor blocks the direct current, the oscillating frequency of this voltage controlled oscillator is determined primarily by the parallel resonant frequency of the surface acoustic wave resonator and the value of current flowing via the pair of differential transistors. This voltage controlled oscillator also enables the same effect as obtained by the invention according to a first aspect of the invention to be obtained. 
     The voltage controlled oscillator may further have a resistance element connected in parallel to the surface acoustic wave resonator. In this case, the variable frequency range is expanded. 
     The voltage controlled oscillator may further have an inductor connected in series to the surface acoustic wave resonator between the second electrodes of the pair of differential transistors. In this case, the variable frequency range is expanded. 
     The voltage controlled oscillator may further have an inductor connected in parallel to the surface acoustic wave resonator. In this case, the variable frequency range is expanded. 
     The voltage controlled oscillator may further have a second capacitor connected in series to the surface acoustic wave resonator between the second electrodes of the pair of differential transistors. In this case, the variable frequency range becomes narrowed, whereby oscillation becomes more stable. 
     The voltage controlled oscillator may further have a second capacitor connected in parallel to the surface acoustic wave resonator. In this case, the variable frequency range becomes narrowed, whereby oscillation becomes more stable. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram showing the configuration of a differential-resonator VCO according to one embodiment of the present invention; 
     FIGS. 2A,  2 B, and  2 C are diagrams illustrating theories of operation of VCOs; 
     FIG. 3 is a circuit diagram showing an equivalent circuit of an SAW resonator shown in FIG. 1; 
     FIG. 4 is a graph showing frequency characteristics of the SAW resonator shown in FIG. 1; 
     FIG. 5 is a graph illustrating effects of the differential-resonator VCO shown in FIG. 1; 
     FIG. 6 is another graph illustrating effects of the differential-resonator VCO shown in FIG. 1; 
     FIG. 7 is a diagram showing a further embodiment of the present invention; 
     FIG. 8 is a diagram showing yet another embodiment of the present invention; 
     FIG. 9 is a diagram showing yet another embodiment of the present invention; 
     FIG. 10 is a diagram showing another embodiment of the present invention; 
     FIG. 11 is a diagram showing another embodiment of the present invention; 
     FIG. 12 is a circuit diagram showing the configuration of a conventional Colpitts VCO; and 
     FIG. 13 is a circuit diagram showing the configuration of a conventional differential-LC VCO. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows a circuit diagram showing the configuration of a differential-resonator VCO according to an embodiment of the present invention. In FIG. 1, the differential-resonator VCO includes an SAW resonator  1 , resistance elements  2 ,  3 ,  8 , and  9 , capacitors  4  and  5 , NPN transistors  6  and  7 , and a voltage controlled current source  10 . 
     The SAW resonator  1  is connected between the collector (a node N 1 ) of the NPN transistor  6  and the collector (a node N 2 ) of the NPN transistor  7 . The collectors of the NPN transistors  6  and  7  are connected via the resistance elements  2  and  3 , respectively, to a power line of a source potential Vcc. The bases of the NPN transistors  6  and  7  are connected via the capacitors  4  and  5  to the collectors of the NPN transistors  7  and  6 , respectively. The emitters of the NPN transistors  6  and  7  are connected via the resistance elements  8  and  9 , respectively, to a node N 10 . The NPN transistors  6  and  7  constitute a pair of differential transistors. The voltage controlled current source  10  is connected between the node N 10  and a ground line of a ground potential GND, and provides current in accordance with an external control voltage. A voltage between the nodes N 1  and N 2  corresponds to an output signal. 
     The operation of the differential-resonator VCO will now be described with comparison to that of conventional VCOs. The Colpitts type VCO is basically constructed by looping an amplifier  11  and a resonator  12 , as shown in FIG.  2 A. When the resonator  12  oscillates at the resonant frequency, the impedance of the resonator  12  has a minimum value and the loop gain of the circuit has a maximum value. Therefore, the Colpitts type VCO oscillates at the resonant frequency of the resonator  12 . 
     The differential-LC type VCO is basically constructed by, as shown in FIG. 2B, connecting a parallel LC circuit  15  in parallel with loop-connected inverting amplifiers  13  and  14 . The impedance of the parallel LC circuit  15  has a maximum at the parallel resonant frequency thereof. When the loop gain thereof is 1 or more, the phase of the loop is 2 np (n is an integer). Therefore, the differential-LC type VCO oscillates at the parallel resonant frequency of the parallel LC circuit  15 . 
     The impedance of the parallel LC circuit  15  has a maximum at the parallel resonant frequency. The more the frequency of the parallel LC circuit  15  deviates from the parallel resonant frequency, the more the impedance thereof monotonically decreases. However, a model circuit of the SAW resonator  1  includes, as shown in FIG. 3, a resistance element  21 , a capacitor  22 , and a inductor  23 , which are connected in series between the nodes N 1  and N 2 , and a capacitor  24 , which is connected in parallel with the above components. As shown in FIG. 4, the impedance of the resonator has a peak value at a parallel resonant frequency f 0 , and further goes to infinity at a frequency of 0 Hz, i.e., that of a direct current. In the oscillation circuit, the loop gain of the circuit obtained at direct current exceeds the loop gain obtained at the current of the parallel resonant frequency f 0 . Accordingly, the VCO in FIG. 2B cannot be oscillated at the parallel resonant frequency f 0  only by replacing the parallel LC circuit  15  with the SAW resonator  1 . 
     A differential-resonator type VCO, as shown in FIG. 2C, includes inverting amplifiers  16  and  17  connected in series, a filter  18  connected between the output node of the inverting amplifier  17  and the input node of the inverting amplifier  16  for blocking the direct-current signal while allowing the signal having the parallel resonant frequency f 0  to pass, and a SAW resonator  19  connected in parallel with the inverting amplifier  17 . Because the filter  18  prevents the direct current from being input to the inverting amplifiers  16  and  17 , the loop gain of the oscillation circuit decreases at direct current, and has a maximum value at the current of the parallel resonant frequency f 0  of the SAW resonator  19 . Therefore the differential-resonator type VCO oscillates at the parallel resonant frequency f 0  of the SAW resonator  19 . 
     The circuit diagram in FIG. 1 is an embodiment of the circuit block diagram in FIG.  2 C. The capacitors  4  and  5  constitute a direct-current blocking filter. The resistance elements  2  and  3  are provided for bias of the NPN transistors  6  and  7 , and the resistance elements  8  and  9  are provided for limiting currents. 
     The oscillating frequency of this differential-resonator VCO is determined by the resonant frequency f 0  of the SAW resonator  1  and the internal capacitance of each of the NPN transistors  6  and  7  in parallel with the SAW resonator  1 . Because the capacitance of each of the NPN transistors  6  and  7  is varied by the values of currents passing through the NPN transistors  6  and  7 , the oscillating frequency of the VCO can be controlled by causing the external control voltage to control the current value of the current source  10 . The oscillating frequency signal is output from across the nodes N 1  and N 2 . 
     FIG. 5 is a diagram showing rates of change of oscillating frequencies versus temperature for the differential-resonator type VCO, the Colpitts type VCO, and the differential-LC type VCO. FIG. 6 is a diagram showing the rates of change of the oscillating frequency versus aging for each type of VCO. In each diagram, the change rate of frequency of the differential-resonator VCO is small, which is preferable, whereas the change rate of frequency of the differential-LC VCO is large. Because the inductor  72  (FIG. 13) has a low Q-factor, the differential-LC VCO inevitably has a low Q-factor, which leads to frequency instability. On the other hand, the differential-resonator VCO can oscillate highly accurately and stably without adjustment because the differential-resonator VCO employs the SAW resonator  1  instead of the inductor  72 . Furthermore, since a variable component which needs mechanical adjustment to change inductance is not used for the VCO of the present invention, change in frequency with age is small. 
     The resistance elements  8  and  9  may optionally be omitted. Although both capacitors  4  and  5  are preferable in order to maintain equilibrium of the circuit, one of them may suffice when there are limitations concerning the layout of the circuit. Also, the circuit-may be constructed in discrete components. However, in this case the NPN transistors  6  and  7  having uniform quality must be selected. Miniaturization of the VCO can be achieved by integrating the components other than the resonator  1 . When the resonator  1  can be prepared by molding, the resonator  1  can be made in a single chip. 
     Hereinbelow, various modifications of this embodiment are described. In a modification illustrated in FIG. 7, instead of the resonator  1  in FIG. 1, a parallel connection body having the resonator  1  and a resistance element  31  connected in parallel is provided in the VCO in FIG.  1 . In this case, because the Q-factor of the resonance system including the resonator  1  and the resistance element  31  decreases, a range of variable frequencies of the VCO can be expanded. 
     In a modification illustrated in FIG. 8, instead of the resonator  1  in FIG. 1, a series connection body having the resonator  1  and an inductor  32  connected in series is provided in the VCO in FIG.  1 . In this case, the series resonant frequency shifts to a lower frequency. Also, because the Q-factor of the inductor  32  is smaller than that of the resonator  1  by one order of magnitude, the Q-factor of the resonance system decreases, which causes a range of variable frequencies of the VCO to be expanded. 
     In a modification illustrated in FIG. 9, instead of the resonator  1  in FIG. 1, a parallel connection body having the resonator  1  and an inductor  33  connected in parallel is provided in the VCO in FIG.  1 . In this case, the parallel resonant frequency shifts to a higher frequency. Also, because the Q-factor of the inductor  33  is smaller than that of the resonator  1  by one order of magnitude, the Q-factor of the resonance system decreases, which causes a range of variable frequencies of the VCO to be expanded. 
     In a modification illustrated in FIG. 10, instead of the resonator  1  in FIG. 1, a series connection body having the resonator  1  and a capacitor  34  connected in series is provided in the VCO in FIG.  1 . In this case, the series resonant frequency shifts to a higher frequency. Also, because the Q-factor of the capacitor  34  is smaller compared with that of the resonator  1 , the Q-factor of the resonance system decreases, which causes a range of variable frequencies of the VCO to be narrowed, whereby the oscillation of the VCO becomes more stable. 
     In a modification in FIG. 11, instead of the resonator  1  in FIG. 1, a parallel connection body having the resonator  1  and a capacitor  35  connected in parallel is provided in the VCO in FIG.  1 . In this case, the parallel resonance frequency shifts to a lower frequency. Also, because the Q-factor of the capacitor  35  is smaller compared with that of the resonator  1 , the Q-factor of the resonance system decreases, which causes a range of variable frequencies of the VCO to be narrowed, whereby the oscillation of the VCO becomes stable. 
     It should be understood that the present disclosure of preferred forms of the present invention are exemplary and not limited in every respect. The scope of the invention is defined by the appended claims rather than by the description preceding them, and all changes that fall within the scope of the claims, or equivalence of such scope of the claims, are intended to be included by the claims.