JP2014230245A - Colpitts oscillation circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a Colpitts oscillation circuit that prevents abnormal oscillation at frequencies other than a desired one and outputs a frequency signal with a satisfactory phase noise characteristic.SOLUTION: The Colpitts oscillation circuit includes: a resonance circuit including a SAW resonator 1 and an extension inductor L; and an amplification circuit including a transistor 2 for amplifying an output from the resonance circuit. The amplification circuit is so configured that a phase adjusting inductor Lis provided between a base of the transistor 2 and the extension inductor L, and that one end of capacitors C1, C2 connected in series is connected to a point between the extension inductor Land the phase adjusting inductor Lwhile the other end thereof is grounded. This adjusts the phase of a signal input into the base of the transistor 2 to suppress abnormal oscillation at a self-resonant frequency of the SAW resonator 1 and the extension inductor L, and also dispenses with a damping resistor and prevents a drop in Q value to output a desired frequency signal with a satisfactory phase noise characteristic.

Description

  The present invention relates to a Colpitts oscillation circuit used for a voltage-controlled oscillator using a surface acoustic wave resonator, and in particular, outputs a frequency signal having excellent phase noise characteristics while preventing abnormal oscillation other than a desired frequency. The present invention relates to a Colpitts oscillation circuit.

[Description of Prior Art: FIG. 7]
As shown in FIG. 7, a surface acoustic wave (SAW) resonator has an interdigital transducer (IDT) 11 and reflectors 12 formed on both sides of an IDT electrode 11 on a piezoelectric substrate. A voltage is applied to the IDT electrode 11 to generate a surface wave.
FIG. 7 is a schematic explanatory diagram showing the configuration of the electrodes of the SAW resonator.

[Conventional Colpitts oscillation circuit (1): FIG. 8]
Further, in a voltage controlled oscillator (VCSO) using a SAW resonator, for example, a Colpitts oscillation circuit is used.
A conventional Colpitts oscillation circuit will be described with reference to FIG. FIG. 8 is a schematic circuit diagram of a conventional Colpitts oscillation circuit (1).
As shown in FIG. 8, the conventional Colpitts oscillation circuit (1) includes a resonance circuit and an amplification circuit.
Resonant circuit includes a SAW resonator 1, and a stretch inductor L _e connected in series to the SAW resonator 1. The other end of the SAW resonator 1 is grounded.
The amplifier circuit includes a transistor 2, a capacitor C1 and a capacitor C2 that are feedback capacitors, and a resistor R1.

One end of the decompression inductor L _e is connected to the SAW resonator 1 and the other end connected to the base of the transistor 2.
Further, one end of the series connection of the capacitor C1 and the capacitor C2 is connected to a point between the extension inductor _Le and the base of the transistor 2, and the other end is grounded.

The resistor R1 has one end connected to the emitter of the transistor 2 and a point between the capacitors C1 and C2, and the other end grounded.
Further, the output is taken out from the collector side of the transistor 2.
In FIG. 8 and other schematic circuit diagrams, the power supply voltage and the bias voltage are omitted.

By the way, in the SAW resonator, when a material having a large capacity ratio such as quartz is used as the piezoelectric substrate material, there is no problem if a sufficiently large Q value can be secured, but when a sufficient Q value cannot be secured, the characteristics are capacitance. Oscillation becomes impossible due to the nature.
In order to prevent the oscillation from becoming impossible, in the conventional Colpitts oscillation circuit (1), an extension inductor _Le is added to the outside of the SAW resonator in the resonance circuit.

[Reflection (S ₁₁ ) characteristics of SAW resonator: FIG. 9]
The reflection characteristics of the SAW resonator will be described with reference to FIG. FIG. 9 is a characteristic diagram showing an example of the reflection characteristics of the SAW resonator. FIG. 9A shows an example of the characteristic when the extension inductor is not added, and FIG. 9B shows the example of the characteristic when the extension inductor is added.
FIG. 9 shows a reflection (S ₁₁ ) characteristic example of a 2 GHz band 1-port SAW resonator on a quartz substrate. The capacity ratio is about 1000, and the Q value is about 4000.

As shown in FIG. 9A, when the extension inductor is not added, the characteristic of the SAW resonator is biased toward the capacitive characteristic on the Smith chart, and this makes oscillation impossible.
On the other hand, as shown in FIG. 9B, the characteristics when the extension inductor is added are inductive and can oscillate.

[Admittance characteristics of SAW resonator: Fig. 10]
Next, the admittance characteristics of the SAW resonator will be described with reference to FIG. FIGS. 10A and 10B are characteristic diagrams showing the admittance characteristics of the SAW resonator. FIG. 10A shows an example of the characteristics when the extension inductor is not added, and FIG. 10B shows the characteristic example when the extension inductor is added. In FIG. 10, the same 1-port SAW resonator as in FIG. 9 is used.
As shown in FIG. 10 (a), when the extension inductor is not added, there is a peak only in the resonance frequency of the SAW near 2 GHz, but when the extension inductor is added as shown in FIG. 10 (b). In addition to the SAW resonance frequency, there is another peak.
This is due to self-resonance between the extension inductor and the capacitive component of the SAW resonator.

If this self-resonance frequency is in the vicinity of the SAW resonance frequency and the oscillation condition is satisfied, oscillation may occur at the self-resonance frequency instead of the SAW resonance frequency.
Depending on the combination of the SAW resonator and the extension inductor, self-resonance hinders desired oscillation, and a desired frequency signal cannot be obtained.

[Conventional Colpitts oscillation circuit (2): FIG. 11]
There is another Colpitts oscillation circuit as a configuration for suppressing abnormal oscillation due to self-resonance.
The configuration of another conventional Colpitts oscillation circuit (conventional Colpitts oscillation circuit (2)) will be described with reference to FIG. FIG. 11 is a schematic circuit diagram of a conventional Colpitts oscillation circuit (2).
As shown in FIG. 11, the conventional Colpitts oscillation circuit (2), in addition to the configuration shown in FIG. 8, in the resonant circuit, a structure in which a damping resistor Rd in parallel with the stretching inductor L _e.

In the conventional Colpitts oscillation circuit (2), by providing a damping resistor Rd, the Q value of the resonance circuit is intentionally lowered to suppress oscillation due to self-resonance.
However, for that reason, the Q value of resonance by SAW is also deteriorated at the same time.
Since the phase noise characteristic at the oscillation frequency is strongly influenced by the Q value of the resonance circuit, if the Q value deteriorates, the phase noise characteristic also deteriorates, and in the worst case, the oscillation itself may be impossible. .

[Related technologies]
As a technique related to a voltage controlled oscillator using a SAW resonator, there is “Voltage Controlled Oscillator” (Toshiba Corporation, Patent Document 1).
Patent Document 1 describes that a SAW resonator (SAWR) is connected to the cathode of a variable capacitance diode via an extension coil, and the control sensitivity is improved without causing a decrease in the capacity change rate. ing.

Japanese Utility Model Publication No. 63-117107

  As described above, in the conventional Colpitts oscillation circuit, when an extension inductor is added to reliably start oscillation and a damping resistor is provided to suppress oscillation due to self-resonance, the Q value of the resonance circuit decreases. There is a problem that the phase noise characteristic of the output frequency is deteriorated.

  Patent Document 1 does not describe providing a phase adjusting inductor for adjusting the phase immediately before the base of the transistor in addition to the extension inductor.

  The present invention has been made in view of the above circumstances, and it is intended to realize a Colpitts oscillation circuit capable of obtaining an output frequency with good phase noise characteristics while suppressing abnormal oscillation due to self-resonance between the SAW and the extension inductor. Objective.

  The present invention for solving the problems of the conventional example is a Colpitts oscillation circuit including a resonance circuit having a resonator and an amplification circuit having a transistor that amplifies and outputs a signal from the resonance circuit, The resonance circuit includes an extension inductor connected to the output side of the resonator, and the amplifier circuit adjusts the phase of the signal input to the base between the output terminal of the resonance circuit and the base of the transistor. It is characterized by having.

  In the Colpitts oscillation circuit, the present invention is characterized in that the phase of the signal is adjusted by adjusting the values of the extension inductor and the phase adjusting inductor.

  According to the present invention, in the Colpitts oscillation circuit, the resonator is a surface acoustic wave resonator.

  According to the present invention, in the Colpitts oscillation circuit, the amplifier circuit includes first and second capacitors connected in series, and one end of the series connection is connected to a point between the phase adjusting inductor and the extension inductor. And the other end is grounded.

  According to the present invention, there is provided a Colpitts oscillation circuit including a resonance circuit having a resonator and an amplifier circuit having a transistor that amplifies and outputs a signal from the resonance circuit, and the resonance circuit is connected to the output side of the resonator. Since the amplifier circuit is a Colpitts oscillation circuit having a phase adjustment inductor for adjusting the phase of the signal input to the base between the output terminal of the resonance circuit and the base of the transistor. There is an effect that an output frequency signal with good phase noise characteristics can be obtained while suppressing abnormal oscillation due to the self-resonant frequency between the resonator and the extension inductor.

1 is a schematic circuit diagram of a Colpitts oscillation circuit according to an embodiment of the present invention. It is a characteristic view which shows the oscillation characteristic of this circuit. It is a characteristic view which shows the oscillation characteristic of the conventional Colpitts oscillation circuit (1). It is a characteristic view which shows the oscillation characteristic of the conventional Colpitts oscillation circuit (2). It is a characteristic figure which shows the oscillation frequency spectrum of this oscillation circuit, the conventional oscillation circuit (1), and the conventional oscillation circuit (2). It is a characteristic view which shows the phase noise characteristic of this oscillation circuit and the conventional oscillation circuit (2). It is a schematic explanatory drawing which shows the structure of the electrode of a SAW resonator. It is a schematic circuit diagram of the conventional Colpitts oscillation circuit (1). It is a characteristic view which shows an example of the reflective characteristic of a SAW resonator, (a) shows the example of a characteristic when not adding an extension inductor, (b) shows the case where an extension inductor is added. It is a characteristic view which shows the admittance characteristic of a SAW resonator, (a) shows the example of a characteristic when not adding an extension inductor, (b) shows the case where an extension inductor is added. It is a schematic circuit diagram of the conventional Colpitts oscillation circuit (2).

Embodiments of the present invention will be described with reference to the drawings.
[Outline of the embodiment]
A Colpitts oscillation circuit according to an embodiment of the present invention includes a resonance circuit including a SAW resonator and an extension inductor, and an amplification circuit including a transistor that amplifies a signal from the resonance circuit and outputs the amplified signal to the outside. in the amplifier circuit, between the base and extension inductor transistor, a structure in which an inductor L _B for phase adjustment to adjust the phase, suppresses the abnormal oscillation due to self-resonance other than the desired frequency, the phase noise characteristics Can obtain a good output.

[Colpitts oscillation circuit according to the embodiment: FIG. 1]
The configuration of the Colpitts oscillation circuit according to the embodiment of the present invention will be described with reference to FIG. FIG. 1 is a schematic circuit diagram of a Colpitts oscillation circuit according to an embodiment of the present invention.
As shown in FIG. 1, the Colpitts oscillation circuit (present oscillation circuit) according to the embodiment of the present invention is an oscillation circuit for VCSO, and the basic configuration is the conventional Colpitts oscillation circuit (shown in FIG. 1) and it is substantially the same, consisting of a resonant circuit having a SAW resonator 1 and decompression inductor L _e, an amplifier circuit having a transistor 2 for outputting to the outside amplifies the signal from the resonant circuit.
By stretching the inductor L _e, is capable to ensure the startup of the oscillation.

In the amplifier circuit, one end of capacitors C1 and C2 connected in series is connected to the base of the transistor 2, and the other end is grounded. One end of a resistor R1 is connected to the emitter of the transistor 2, and the other end is grounded.
Although an output terminal is provided on the collector side of the transistor 2, illustration thereof is omitted here.

Then, as a feature of the present oscillator, the amplifier circuit, between the base of the transistor 2 and the feedback capacitors C1, C2, the phase adjusting inductor L _B is provided for adjusting the phase of the signal inputted to the base .
The phase adjusting inductor L _B prevents abnormal oscillation due to self-resonance by adjusting the phase of the signal input to the base of the transistor 2, thereby eliminating the need for the insertion of a damping resistor and the Q value. Can be prevented.

Inductance value of the phase adjustment inductor L _B, depending on the characteristics of each part constituting the circuit, a suitable value so that the optimum output is obtained is selected.
As a result, in this oscillation circuit, an output signal having good phase noise characteristics can be obtained without lowering the Q value.
In addition to the inductor L _B for phase adjustment, the inductance value of decompression inductor L _e also be appropriately adjusted (selected) combined, at a desired frequency, as it can be the output of the better properties is there.

[Oscillation circuit oscillation enable conditions]
Here, conditions for enabling oscillation in the oscillation circuit will be described.
The impedance of the resonance circuit composed of the SAW and an extended inductor L _e to Z _R, the impedance of the amplifier circuit and Z _A.
The impedance is represented by a complex number, the real part (Re) represents a resistance component, and the imaginary part (Im) represents a reactance component.

The circuit oscillation enable condition is
Re (Z _R ) <− Re (Z _A ) (Formula 1)
The oscillation frequency can be
Im (Z _R ) = − Im (Z _A ) _A frequency satisfying (Expression 2).

[Oscillation characteristics of this oscillator: Fig. 2]
The oscillation characteristics of this oscillation circuit will be described with reference to FIG. FIG. 2 is a characteristic diagram showing the oscillation characteristics of this circuit, where (a) shows the relationship between Re (Z _R ) and -Re (Z _A ), and (b) shows Im (Z _R ) and -Im ( Z _A ) is shown.
FIG. 2 shows a simulation result when about 1.98GHz the resonance frequency of the SAW resonator 1, 5 nH decompression inductor L _e, the phase adjusting inductor L _B and 2.0 nH.

First, as shown in FIG. 2B, in this oscillation circuit, in addition to the SAW resonance frequency near 2.0 GHz, the condition (Im (Z _R ) = −Im (Z _A )).

However, as shown in FIG. 2A, in this oscillation circuit, the condition of Equation 1 (Re (Z _R ) <− Re (Z _A )) is satisfied in the vicinity of the resonance frequency of the SAW. In the vicinity of 7 GHz, Re (Z _R )> − Re (Z _A ), and the condition of (Equation 1) is not satisfied.
Therefore, no oscillation occurs at 2.7 GHz.
Further, as can be seen from FIG. 2A, in this oscillation circuit, it can be seen that the change amount of the real part (Re (Z _R )) of the impedance accompanying the frequency change is small and the phase noise characteristic is good.

The value of the stretch inductor L _e and the phase adjusting inductor L _B is not limited to the above.
However, if the value of the stretch inductor L _e is too large too close and the self-resonant frequency and the SAW resonance frequency, becomes prone to self-oscillation, the oscillation itself does not occur in a stretched insufficient if too small.
If oscillation is possible condition is satisfied, the phase noise characteristic is better value smaller stretch inductor L _e good.
Further, as the phase adjusting inductor L _B, it is provided that following a small 1 nH, no effect of preventing a significant self-excited oscillation is obtained.

For example, it is desirable that the value of the phase adjustment inductor is 1.5 nH to 2.5 nH, and the sum of the extension inductor value and the phase adjustment inductor value is about 7 nH.
However, the optimum value varies depending on the oscillation frequency, the C0 (parallel capacitance) of the resonator, the gain characteristic of the oscillation circuit, and the like, and it is desirable that the optimum value is appropriately selected according to the characteristics of the applied oscillation circuit.

[Oscillation characteristics of conventional Colpitts oscillation circuit (1): FIG. 3]
The oscillation characteristics of the conventional Colpitts oscillation circuit (1) will be described with reference to FIG. FIG. 3 is a characteristic diagram showing the oscillation characteristics of the conventional Colpitts oscillation circuit (1), where (a) is the relationship between Re (Z _R ) and -Re (Z _A ), and (b) is Im (Z _R ) and -Im (Z _A ).
Conventional Colpitts oscillation circuit (1) (conventional oscillation circuit (1)) is a circuit shown in FIG. 8, a configuration in which except for the phase adjustment inductor L _B from the oscillation circuit of FIG.
As shown in FIG. 3B, in the conventional oscillation circuit (1), in addition to the SAW resonance frequency (1.98 GHz) near 2.0 GHz, the condition of (Equation 2) is satisfied near 2.3 GHz. is there.

Furthermore, as shown in FIG. 3A, in the conventional oscillation circuit (1), the frequencies near the SAW resonance frequency and the frequencies near 2.3 GHz are both Re (Z _R ) <− Re (Z _A ). Thus, the oscillation enabling condition of (Equation 1) is satisfied.
Therefore, the conventional oscillation circuit (1) may oscillate at either the SAW resonance frequency or the frequency near 2.3 GHz.

[Oscillation characteristics of conventional Colpitts oscillation circuit (2): Fig. 4]
The oscillation characteristics of the conventional Colpitts oscillation circuit (2) will be described with reference to FIG. FIG. 4 is a characteristic diagram showing the oscillation characteristics of the conventional Colpitts oscillation circuit (2), where (a) is the relationship between Re (Z _R ) and -Re (Z _A ), and (b) is Im (Z _R ) and -Im (Z _A ).
A conventional Colpitts oscillation circuit (2) (conventional oscillation circuit (2)) is the circuit shown in FIG. 11, and includes an extension inductor _Le and a damping resistor Rd.
Stretching inductor L _e is 5 nH, damping resistor Rd is set to 370Omu (ohms).

FIG. 4B shows that the conventional oscillation circuit (2) satisfies the condition of (Equation 2) near 2.5 GHz in addition to the SAW resonance frequency (1.98 GHz) near 2.0 GHz.
However, as shown in FIG. 4A, it is understood that only the SAW resonance frequency satisfies the condition of the expression (1), and oscillation at 2.5 GHz does not occur.

However, as shown in FIG. 4A, in the conventional oscillation circuit (2), the real part of the impedance (Re (Z _R )) increases with a large slope with frequency. This indicates that the Q value is lowered by the damping resistor Rd and the phase noise characteristic is deteriorated.

[Comparison of oscillation frequency spectra: Fig. 5]
Next, the simulation results of the oscillation frequency spectra of the present oscillation circuit, the conventional oscillation circuit (1), and the conventional oscillation circuit (2) are compared using FIG. FIG. 5 is a characteristic diagram showing an oscillation frequency spectrum of each oscillation circuit, where (a) is the present oscillation circuit, (b) is the conventional oscillation circuit (1), and (c) is the conventional oscillation circuit (2). Show properties.
As shown in FIG. 5A, in this oscillation circuit, the main oscillation frequency matches the SAW resonance frequency (near 2.0 GHz), and a desired frequency signal can be obtained.
Further, as shown in FIG. 5A, in this oscillation circuit, it is understood that the oscillation output around 4.0 GHz which is the second harmonic of the main oscillation frequency is suppressed, and the phase noise characteristic is good. .

In the conventional oscillation circuit (1), as described above with reference to FIG. 3, there is a point that can oscillate in both the vicinity of 2.0 GHz and the vicinity of 2.3 GHz. Thus, the main oscillation of the oscillation frequency spectrum is around 2.3 GHz.
This is consistent with the self-resonant frequency due to the capacitive component of the SAW resonator and an extended inductor L _e, in the conventional oscillator circuit (1), the frequency signal near 2.0GHz intended can not be obtained.

In the conventional oscillation circuit (2), as described above, only the SAW resonance frequency satisfies the oscillation condition. As shown in FIG. 5C, the main oscillation frequency is the SAW resonance near 2.0 GHz. It matches the frequency.
However, in the conventional oscillation circuit (2), the output in the vicinity of 4.0 GHz which is the second harmonic is larger than the characteristic of the present oscillation circuit shown in (a), and the phase noise characteristic is deteriorated. Yes.

[Comparison of phase noise characteristics: Fig. 6]
Next, the phase noise characteristics of this oscillation circuit and the conventional oscillation circuit (2) will be compared using FIG. 6A and 6B are characteristic diagrams showing the phase noise characteristics, where FIG. 6A shows the characteristics of the present oscillation circuit, and FIG. 6B shows the characteristics of the conventional oscillation circuit (2).
As shown in FIG. 6A, in this oscillation circuit, the phase noise exceeds −111 dBc / Hz with respect to the offset of 1 kHz, and good characteristics are obtained.

  On the other hand, as shown in FIG. 6B, in the conventional oscillation circuit (2), it remains at −108 dBc / Hz with respect to the offset of 1 kHz, and the phase noise characteristic is higher than that of the present oscillation circuit shown in FIG. not good. This indicates that the Q value itself of the resonance circuit is lowered by the damping resistor Rd.

[Effect of the embodiment]
According to Colpitts oscillation circuit according to the embodiment of the present invention, consisting of a resonant circuit provided with a SAW resonator 1 and the stretching inductor L _e, an amplifier circuit having a transistor 2 for amplifying the output from the resonant circuit a Colpitts oscillator circuit, in the amplifier circuit, for a phase adjusting inductor L _B, decompression inductor L _e and the phase adjustment between the base of the extension inductor L _e and transistor 2 provided to the output end of the resonant circuit the point between the inductor L _B, is connected to one end of the capacitor C1, C2 connected in series and the other end has a configuration which is grounded, by adjusting the phase of the signal input to the base of the transistor 2 , it is possible to suppress the oscillation due to self-resonance frequency of the SAW resonator 1 and an extended inductor L _e, also prevents a decrease in the Q value of the damping resistor Rd as required There is an effect that a desired frequency signal having good phase noise characteristics can be output.

Further, according to the present Colpitts oscillation circuit, by appropriately adjusting the value of the phase adjustment inductor L _B and decompression inductor L _e, there is an effect that it is possible to obtain an output signal having good characteristics with a simple configuration.

  The present invention is suitable for a Colpitts oscillation circuit that can prevent abnormal oscillation at a frequency other than a desired frequency and output a frequency signal with good phase noise characteristics.

1 ... SAW resonator, 2 ... transistor, 11 ... interdigital electrodes (IDT), 12 ... reflector, L _e ... stretching inductor, L _B ... phase adjustment inductor, C1, C2 ... capacitor, R1 ... resistor, Rd ... damping resistor

Claims (4)

A Colpitts oscillation circuit comprising a resonance circuit having a resonator and an amplifier circuit having a transistor that amplifies and outputs a signal from the resonance circuit,
The resonant circuit comprises an extension inductor connected to the output side of the resonator;
The Colpitts oscillation circuit, wherein the amplifier circuit includes a phase adjusting inductor for adjusting a phase of a signal input to the base between an output terminal of the resonance circuit and a base of the transistor.   2. The Colpitts oscillation circuit according to claim 1, wherein the phase of the signal is adjusted by adjusting values of the extension inductor and the phase adjusting inductor.   3. The Colpitts oscillation circuit according to claim 1, wherein the resonator is a surface acoustic wave resonator. The amplifier circuit includes first and second capacitors connected in series,
4. The Colpitts oscillation circuit according to claim 1, wherein one end of the series connection is connected to a point between the phase adjusting inductor and the extension inductor and the other end is grounded.

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