Patent Application
20100052799
1. Field of the Invention
The present invention relates to a voltage controlled oscillator, an MMIC, and a high frequency wireless device. In particular, the present invention relates to a voltage controlled oscillator, an MMIC, and a high frequency wireless device that work in a microwave or millimeter wave range.
2. Description of the Related Art
In conjunction with the widespread proliferation of high frequency wireless devices including a car-mounted radar device and a mobile phone, there has been a growing requirement for high performance of oscillators having output frequency of 1 GHz or higher. The oscillator is a circuit for generating oscillation of a high frequency electric signal inside the circuit so as to deliver the high frequency electric signal to the outside. In particular, an oscillator having a control voltage terminal for changing the output frequency is called a voltage controlled oscillator (VCO). The oscillator includes an active device such as a transistor for amplifying the high frequency electric signal and a resonator for oscillating a high frequency electric signal having a specific frequency. In order to realize a variable output function, the VCO includes a variable resonator having mainly a varactor (variable capacitor). A control voltage is applied to the varactor for changing capacitance of the varactor so that the output frequency can be changed.
As important characteristics of the VCO, there are phase noise and output frequency. The phase noise is an indicator of stability of the output frequency. When the high frequency wireless device is used for a radar device or a communication device, the phase noise affects its accuracy in measuring distances or its communication error rate. Therefore, it is desirable that the phase noise should be a lower value.
One of methods for controlling (or suppressing) the phase noise of the VCO is to improve a Q value of the resonator (an indicator of an energy amount that the resonator can store for an electric signal of a specific frequency). As an example of the method, there is a reported method in which a plurality of stubs are used for the resonator so as to make the resonator having a high Q value (see, for example, “A Low Phase Noise 19 GHz-band VCO using Two Different Frequency Resonators”, IEEE MTT-S Int. Microwave Symp. Digest, pp. 2189-2191, 2003).
Further, as another method for controlling (or suppressing) the phase noise of the VCO, there is a method of controlling a phenomenon that a voltage fluctuates at the terminal of a transistor on the resonator side in the VCO by using harmonic signals such as the second harmonic signal, the third harmonic signal, and so on (see, for example, “A Ka-Band Second Harmonic Oscillator with Optimized Harmonic Load”, IEICE technical report, Vol. 107, No. 355, pp. 29-32, November, 2007).
Although many methods for controlling the phase noise are proposed as described above, it is difficult to make a resonator having a high Q value in the VCO having an output frequency above 30 GHz. Therefore, it is impossible to obtain sufficiently low phase noise characteristics.
In addition, it is desirable that the VCO directly deliver a signal of the frequency to be handled by the high frequency wireless device. It is possible to use the VCO that delivers a signal of a frequency lower than the frequency handled by the wireless device together with a frequency multiplier, but it is not advantageous for cost reduction because the structure of the wireless device becomes complicated. In today's circumstances where high frequencies to be handled by wireless devices have become higher and higher, it is desired to improve the output frequency of the VCO.
As the output frequency is increased, the phase noise of the VCO is increased, in other words, deteriorated in principle. If the output frequency is increased up to a frequency in the millimeter wave band or higher (above 30 GHz), it is difficult to make a resonator having a high Q value. Therefore, it is impossible to make a VCO having sufficiently low phase noise characteristics.
In the method of using a plurality of resonators as described above in “A Low Phase Noise 19 GHz-band VCO using Two Different Frequency Resonators”, IEEE MTT-S Int. Microwave Symp. Digest, pp. 2189-2191, 2003, the Q value is improved only for a fundamental wave frequency, i.e., an oscillation frequency, and hence a circuit load for a harmonic frequency cannot be optimized. Further, in the method of controlling voltage fluctuations by using harmonic signals described above in “A Ka-Band Second Harmonic Oscillator with Optimized Harmonic Load”, IEICE technical report, Vol. 107, No. 355, pp. 29-32, November, 2007, only the circuit loads for the harmonic frequencies are taken into account, but the Q value for the fundamental wave frequency cannot be improved. Therefore, these methods have a problem in that sufficiently low phase noise characteristics cannot be obtained in the VCO having an output frequency above approximately 30 GHz in particular.
The present invention has been created as a solution to the above-mentioned problem, and it is an object thereof to obtain a voltage controlled oscillator (VCO), a monolithic microwave integrated circuit (MMIC), and a high frequency wireless device that can realize low phase noise characteristics even at an output frequency in the microwave band (1 GHz or more) or the millimeter wave band (30 GHz or more).
The present invention provides a voltage controlled oscillator including: a variable resonator; and at least one open-end stub connected in parallel to the variable resonator, the at least one open-end stub having a length smaller than or equal to an odd multiple of one quarter of a wavelength of a harmonic signal plus one sixteenth of the wavelength of the harmonic signal and larger than or equal to an odd multiple of one quarter of the wavelength of the harmonic signal minus one sixteenth of the wavelength of the harmonic signal.
The present invention also provides a voltage controlled oscillator including: a variable resonator; and at least one short-end stub connected in parallel to the variable resonator, the at least one short-end stub having a length smaller than or equal to an integral multiple of a wavelength of a harmonic signal plus one sixteenth of the wavelength of the harmonic signal and larger than or equal to an integral multiple of the wavelength of the harmonic signal minus one sixteenth of the wavelength of the harmonic signal.
The present invention provides a voltage controlled oscillator including: a variable resonator; and at least one open-end stub connected in parallel to the variable resonator, the at least one open-end stub having a length smaller than or equal to an odd multiple of one quarter of a wavelength of a harmonic signal plus one sixteenth of the wavelength of the harmonic signal and larger than or equal to an odd multiple of one quarter of the wavelength of the harmonic signal minus one sixteenth of the wavelength of the harmonic signal. In addition, the present invention provides a voltage controlled oscillator including: a variable resonator; and at least one short-end stub connected in parallel to the variable resonator, the at least one short-end stub having a length smaller than or equal to an integral multiple of a wavelength of a harmonic signal plus one sixteenth of the wavelength of the harmonic signal and larger than or equal to an integral multiple of the wavelength of the harmonic signal minus one sixteenth of the wavelength of the harmonic signal. Thus, it is possible to realize low phase noise characteristics even at an output frequency in the microwave band (1 GHz or more) or the millimeter wave band (30 GHz or more).
In the accompanying drawings:
This structure of the VCO circuit is an MMIC, for instance, and it may also be realized by using a microwave integrated circuit (MIC) or discrete elements. Its substrate may be made of a material such as gallium arsenide (GaAs), gallium nitride (GaN), indium phosphide (InP), Si or the like.
The material of the transistor 1 is not limited, and silicon, gallium arsenide, gallium nitride or the like can be used. The structure of the transistor 1 is also not limited, and a bipolar transistor, a field effect transistor, a high electron mobility transistor or the like can be used. It can be a vacuum tube.
Next, operations are explained. A noise signal such as thermal noise inside the circuit is supplied to the transistor 1, which amplifies the noise signal. Then, the noise signal is fed back to the base of the transistor 1 from the emitter line 14 of the transistor 1 or reflected back from the fundamental wave reflection stub 15 via the line 13 and the transistor 1 and is supplied to the transistor 1 again to be amplified. Thus, oscillation of the fundamental wave frequency occurs inside the VCO, but the transistor 1 also generates harmonic signals having frequencies of twice, three times, and so on of the fundamental wave frequency (second harmonic signal, third harmonic signal, and so on). Since the fundamental wave reflection stub 15 is open for the second harmonic signal, the second harmonic signal is directed to the output terminal 4 and is delivered to the outside of the oscillator. Since the fundamental wave signal does not propagate to the output side farther from the fundamental wave reflection stub 15, it is not delivered to the outside of the oscillator.
If these harmonic signals propagate to the control voltage terminal 4 and make the control voltage Vt fluctuate, the output frequency fluctuates without intention. In other words, stability of the output frequency is lost and phase noise is increased. In order to suppress this fluctuation of the control voltage Vt, the open-end stub 5 is added between the transistor 1 and the line 12 so that the fundamental wave signal can pass therethrough while the harmonic signal is absorbed. This open-end stub 5 disables the harmonic signal to propagate to the control voltage terminal 3. In contrast, the fundamental wave signal can propagate to the varactor 2, and hence the oscillation frequency can be changed by changing the control voltage Vt externally so as to change capacitance of the varactor 2.
In this embodiment, since the open-end stub 5 has the length corresponding to one quarter of the wavelength of the second harmonic signal, the open-end stub 5 has a load that is neither a short circuit nor an open circuit for the fundamental wave frequency. Therefore, the fundamental wave signal fed back from the emitter line 14 of the transistor 1 or reflected back from the fundamental wave reflection stub 15 propagates to both the open-end stub 5 and the varactor 2. Thus, a resonator made up of a plurality of stubs for the fundamental wave is structured so that a high Q value can be realized for the fundamental wave. In this case, since the fundamental wave and the harmonic have a relationship of 6 dB/oct in the oscillator, phase noise can be reduced in both the fundamental wave and the harmonic. In contrast, the open-end stub 5 has a short circuit load for the second harmonic frequency, and hence the second harmonic signal propagates to the open-end stub 5 entirely and thus does not propagate to the varactor 2. Therefore, fluctuation of the control voltage Vt due to the second harmonic signal can be controlled, and the phase noise generated in the variable resonance circuit having the varactor 2 is reduced. In addition, electric field fluctuation due to the second harmonic signal is not generated at a connection node of the open-end stub 5. Therefore, fluctuation of a base voltage of the transistor 1 due to the second harmonic signal can be suppressed, and further the phase noise is reduced. Thus, a VCO with low phase noise can be realized.
Although the line length of the open-end stub 5 is one quarter of the wavelength of the second harmonic signal as an example in
In addition, it is not necessary to set the length of the open-end stub 5 strictly to be the length defined by Expression (1), but the length may have an error of approximately ±λ/16. It is because this range of error can be expected sufficiently to obtain the effect of suppressing the phase noise within the range of about 0.8 to 1.4 dB drop, as compared with the phase noise in the case where the length is set strictly according to Expression (1), with reference to a result of calculation of the level of the phase noise for the load impedance of the second harmonic on the resonance circuit side.
In this embodiment, the harmonic signal is the second harmonic signal. However, if a third harmonic signal, a fourth harmonic signal or the like is a dominant factor of deteriorating the phase noise, it is possible to use the open-end stub having the line length satisfying Expression (1) for the wavelength λ of the third harmonic signal, the fourth harmonic signal or the like so that the short circuit load is formed for the third harmonic frequency, the forth harmonic frequency or the like. In this case too, the effect of suppressing the phase noise can be expected even if the ±λ/16 error is included.
Although the example illustrated in
As described above, in this embodiment, at least one open-end stub is connected in parallel to the variable resonator, and a length of the open-end stub is smaller than or equal to an odd multiple of one quarter of the wavelength of a harmonic signal plus one sixteenth of the wavelength of the harmonic signal and is larger than or equal to an odd multiple of one quarter of the wavelength of the harmonic signal minus one sixteenth of the wavelength of the harmonic signal. Since the open-end stub 5 has a load that is neither a short circuit nor an open circuit for the fundamental wave frequency, and it has a short circuit load for the harmonic frequency, the fundamental wave signal can propagate to both the open-end stub 5 and the varactor 2 at the fundamental wave frequency. In other words, the resonator using a plurality of stubs is structured so that a high Q value can be realized. In contrast, the open-end stub 5 has a short circuit load for the harmonic frequency so that the harmonic signal propagates to the open-end stub 5 entirely. Therefore, the harmonic signal does not propagate to the varactor 2 so that the fluctuation of the control voltage Vt due to the harmonic signal can be suppressed. In addition, since the electric field fluctuation due to the harmonic signal is not generated at the connection node of the open-end stub 5, the fluctuation of the base voltage of the transistor 1 due to the harmonic signal can be suppressed. Thus, in this embodiment, the Q value for the fundamental wave frequency can be improved, and the fluctuation of the voltage to be applied to the varactor and the transistor due to the harmonic signal can be suppressed. As a result, a VCO having low phase noise can be realized.
Although
In this embodiment, even if the frequency of the fundamental wave signal or the harmonic signal is lower than 1 GHz, the same effect as described above can be obtained as long as the line length of the open-end stub 5 is set to be the length defined in Expression (1). Although the variable resonator is made up of the varactor 1 and the line 12 in the structure illustrated in
nλ(n=1, 2, . . . ) (2)
The short-end stub 6 having the line length defined by Expression (2) becomes a short circuit load also for the fundamental wave frequency at a low frequency lower than approximately 1 GHz. Therefore, the fundamental wave signal cannot propagate to the variable resonator including the varactor 2 so that the oscillation frequency cannot be varied. In contrast, as the frequency becomes higher, the line length defined by Expression (2) deviates from an integral multiple of the half wavelength of the fundamental wave signal due to a parasitic capacitance (C) component and a parasitic inductance (L) component included in the line of the short-end stub 6. Therefore, the short-end stub 6 having the line length defined by Expression (2) becomes to have a load that is neither a short circuit nor an open circuit for the fundamental wave frequency. Therefore, as for the VCO oscillating at a fundamental wave frequency of approximately 1 GHz or higher, it is possible to use the short-end stub 6 having the line length defined by Expression (2) instead of the open-end stub 5 in Embodiment 1.
An operation principle of the VCO according to this embodiment is fundamentally the same as that of the VCO according to Embodiment 1. In order to verify the operation of the VCO according to this embodiment,
It is understood that the fundamental wave signal of 38 GHz propagates to both the short-end stub 6 and the varactor 2 from the electric field distribution as illustrated in
Table 1 illustrates an example of calculation results of the phase noise of the VCO according to Embodiment 2. It is understood from Table 1 that there is no large difference between the output frequency in the case where the short-end stub 6 is disposed and the output frequency in the case where it is not disposed, and that the phase noise can be reduced by adding the shorting stub 6. In addition, it is possible to change the frequency by approximately 1 GHz by the voltage applied to the control voltage terminal 3 in both cases.
Note that in this embodiment too, similarly to Embodiment 1, it is not necessary to set the short-end stub 6 to have the exact length defined by Expression (2). The effect of suppressing the phase noise can be expected even if the ±λ/16 error is included.
In addition, it is possible to use the short-end stub having the line length that is adapted to satisfy Expression (2) for the wavelength of the third harmonic signal, the fourth harmonic signal or the like so that it becomes a short circuit load for the third harmonic frequency, the fourth harmonic frequency or the like. In this case too, the effect of reducing the phase noise can be expected even if the ±λ/16 error is included.
Although only one short-end stub 6 is connected in parallel to the variable resonator in the example illustrated in
In this embodiment, one or more short-end stubs are connected in parallel to the variable resonator, in which the length of the short-end stub is smaller than or equal to an integral multiple of the wavelength of a harmonic signal plus one sixteenth of the wavelength of the harmonic signal and is larger than or equal to an integral multiple of the wavelength of the harmonic signal minus one sixteenth of the wavelength of the harmonic signal. The short-end stub 6 has a load that is neither a short circuit nor an open circuit for the fundamental wave frequency while it has a short circuit load for the harmonic frequency. Therefore, the fundamental wave frequency propagates to both the short-end stub 6 and the varactor 2. In other words, a resonator using a plurality of stubs is constituted so that a high Q value can be realized. In contrast, the open-end stub 5 has a short circuit load for the harmonic frequency so that the harmonic signal propagates to the short-end stub 6 entirely. Therefore, the harmonic signal does not propagate to the varactor 2 so that the fluctuation of the control voltage Vt due to the harmonic signal can be suppressed. In addition, the electric field fluctuation due to the harmonic signal is not generated at the connection node of the short-end stub 6 so that the fluctuation of the base voltage of the transistor 1 due to the harmonic signal can be suppressed. Thus, in this embodiment too, similarly to Embodiment 1, a VCO having low phase noise can be realized.
Although an example of the harmonic extraction oscillator including the fundamental wave reflection stub 15 is illustrated in
Although the variable resonator is made up of the varactor 1 and the line 12 in the structure illustrated in
The same effect as that of Embodiment 2 in which the short-end stub is added can be obtained by letting the bias circuit make a short circuit via the capacitor 11 at the portion separated from the connection node by a distance satisfying Expression (2), without newly adding the short-end stub 6 as described above in Embodiment 2.
Although the line length of the bias circuit 7 is adapted to be a length corresponding to the wavelength of the second harmonic signal according to the above-mentioned description, this structure is not a limitation. It is sufficient that the line length of the bias circuit 7 is a length corresponding to an integral multiple of the wavelength of the second harmonic signal. In addition, it may be a length corresponding to an integral multiple of the wavelength of the harmonic that is not limited to the second harmonic signal but can be a third or higher harmonic signal.
In addition, even if the line length of the bias circuit 7 includes the ±λ/16 error, the effect of reducing the phase noise can be expected.
Thus, according to this embodiment, the bias circuit is connected in parallel to the variable resonator, in which the line length from the connection node of the bias circuit to the ground connection portion via the capacitor is smaller than or equal to an integral multiple of the wavelength of a harmonic signal plus one sixteenth of the wavelength of the harmonic signal and is larger than or equal to an integral multiple of the wavelength of the harmonic signal minus one sixteenth of the wavelength of the harmonic signal. Therefore, similarly to Embodiment 2, a VCO having low phase noise can be realized in this embodiment, too.
In addition,
Note that each of the LCR circuit 8 and the waveguide circuit 9 has a short circuit load or a load close to the short circuit load, having e.g., an impedance within the range of −30 j ohms to +30 j ohms for the frequency of the harmonic signal. The load in this range corresponds to the range of λ±λ/16 of the short-end stub 6 illustrated in Embodiment 2 in the system of the characteristic impedance of 50 ohms. As understood from
It is sufficient that the circuit that is added in the above-mentioned Embodiment 1 or 2 should has a load that is neither a short circuit nor a open circuit for the fundamental wave frequency and a short circuit load for the harmonic frequency, and is not necessarily the line stub. Therefore, it is possible to use the LCR circuit 8 or the waveguide circuit 9 as described in this embodiment.
Thus, according to this embodiment, at least one LCR circuit 8 or waveguide circuit 9, which is not a short circuit for the fundamental frequency and has a load including a real number component within the range of 0 to 15 ohms and an imaginary number component within the range of −30 j to +30 j ohms for the frequency of the harmonic signal, is connected in parallel to the variable resonator. Therefore, similarly to the above-mentioned Embodiment 2 or 3, a VCO having low phase noise can be realized.
The circuit that is added in the above-mentioned Embodiment 1, 2 or 4 can also be a plurality of circuits as illustrated in
In addition, if a plurality of orders of the harmonic signals become a factor of deteriorating the phase noise, it is possible to make the plurality of added circuits be short circuit loads for the different orders of the harmonic signals as illustrated in
In addition, as illustrated in
Thus, also in this embodiment, a VCO having low phase noise can be realized similarly to the above-mentioned Embodiment 1, 2 or 4.
A voltage controlled oscillator 22 oscillates at a frequency based on a voltage signal from a frequency control device 21, an amplifier 23 amplifies the oscillation signal, and an output antenna 24 transmits a microwave or a millimeter wave. A reception antenna 25 receives the microwave or the millimeter wave. A mixer 28 performs frequency conversion of the oscillation signal delivered by a voltage controlled oscillator 27 based on the voltage signal of a frequency control device 26 and the reception signal from the reception antenna 25 so as to deliver a desired signal.
The transmission antenna 24 and the reception antenna 25 may be one unit. The frequency control devices 21 and 26, as well as the voltage controlled oscillators 22 and 27 may be one unit, respectively. In addition, it is possible to use the voltage controlled oscillator according to any one of Embodiments 1 to 5 to only one of the transmission portion and the reception portion.
When the high frequency wireless device uses the voltage controlled oscillator according to any one of Embodiments 1 to 5, a high quality microwave or millimeter wave with little phase noise can be transmitted. In addition, noise in the reception mode can be reduced.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2008-223274 | Sep 2008 | JP | national |
