JP4102333B2 - Oscillation circuit and voltage controlled oscillator - Google Patents

Description

  The present invention relates to an oscillation circuit using a piezoelectric resonator and a voltage controlled oscillator.

  In recent years, the market for information devices using wireless communication such as mobile phones and wireless LANs has expanded, and services have become highly functional. Along with this, the frequency band to be used tends to gradually shift to a high frequency of gigahertz or higher. In the future, with the arrival of a ubiquitous society, the demand for ultra-compact wireless systems is expected to increase. High-frequency circuits used in such wireless systems require passive components such as resonators and filters in addition to semiconductor integrated circuits.

  Conventionally, as resonators and filters of this type of high frequency band, surface acoustic wave elements (hereinafter referred to as “SAW” (Surface Acoustic Wave)) and resonators and filters using ceramic dielectrics have been used. .

  However, in SAW resonators and SAW filters, higher frequency is considered to be close to the limit because of the limit of fine processing of the comb-shaped electrode or the reliability. On the other hand, ceramic dielectric resonators and ceramic dielectric filters are generally unsuitable for miniaturization and unsuitable for mobile communication devices that are desired to be small and light.

  In order to solve these problems, a thin film bulk elastic resonator (hereinafter referred to as “FBAR” (Film Bulk Acoustic Resonator)) that utilizes the longitudinal vibration of the piezoelectric thin film has been recently proposed (see Patent Document 1). Some have already been commercialized. FBAR does not require the fine processing of electrodes required for SAW devices, and has the advantage that it can be significantly reduced in size compared to a dielectric resonator using a bulk dielectric.

  By arranging a plurality of FBARs in series or in parallel to form a ladder type filter, it can be used as an RF filter for a mobile communication device. Further, by combining a variable capacitance element connected in parallel with the FBAR and a variable capacitance element connected in series, and combining them in a ladder shape, a tunable filter having a variable pass band can be configured.

On the other hand, this FBAR can be used as a voltage controlled oscillator (VCO) for a local oscillator used for mobile communication by combining with a variable capacitance element, a capacitor, a CMOS inverter, a resistor, and the like.
JP 2000-69594 A

  Conventionally, when a piezoelectric resonator such as crystal or piezoelectric ceramic is used as a part of a tank circuit to form an oscillation circuit, a circuit configuration using a CMOS inverter or a circuit configuration using a bipolar transistor is generally used. These circuit configurations are all called Colpitts type oscillation circuits and have the following characteristics.

  (1) Oscillates in series resonance of the resonance circuit.

  (2) Single phase output.

  On the other hand, in recent years, with the advance of CMOS analog application technology and Bi-CMOS technology, an IC built-in type oscillation circuit is provided inside a high-frequency integrated circuit used in a wireless communication device. However, this type of IC built-in oscillation circuit has the following characteristics.

  (1) Oscillates in parallel resonance of a resonance circuit (tank circuit).

  (2) Differential output.

  In addition to using bipolar transistors and CMOS, the circuit configuration of the built-in IC oscillation circuit may be composed of only NMOS, and various variations are possible. However, the basic idea is that a parallel resonance that maximizes the impedance of the tank circuit is achieved by cross-connecting the input and output of the two inverter pairs and connecting a tank circuit consisting of LCRs between the input and output terminals. It oscillates near the frequency.

  In such a circuit configuration, due to the symmetry of the circuit, the output waveforms at the two output ends can obtain a differential output whose phase is inverted by 180 °. In high-frequency integrated circuits for communication, single-balance and double-balance types are used as mixer circuit configurations for up-converters and down-converters, but both input differential voltage waveforms at the local oscillator input terminal. It is assumed that Therefore, these differential output oscillators have better circuit matching than a single-phase output Colpitts oscillator.

  If FBAR is used instead of the LCR tank circuit of these differential output type oscillators, it is expected that a differential output oscillator with low phase noise can be constructed by using a high Q of FBAR. However, if FBAR is connected between the input and output terminals of the inverter instead of the tank circuit consisting of LCR, the two terminal potentials of this circuit will be fixed at a constant potential of either “H” or “L”. Therefore, the oscillator operation cannot be obtained.

  To control the bandwidth of the tunable filter with variable passband, the capacitance value of the variable capacitance element connected in parallel to the FBAR and the capacitance value of the variable capacitance element connected in series are accurately controlled independently of each other. There is a need. For this purpose, an oscillator circuit is configured by using a FBAR, a parallel variable capacitor, and a series-connected variable capacitor connected as a tank circuit, and by monitoring this oscillation frequency, the series resonance frequency of this tank circuit, Alternatively, a method of knowing the parallel resonance frequency and controlling the pass band of the tunable filter has been proposed.

  In addition, an oscillator circuit is configured by connecting FBAR and a parallel variable capacitance element as a tank circuit, and by monitoring this oscillation frequency, the parallel resonance frequency of this tank circuit is known and the passband of the tunable filter is controlled. This method has been proposed.

  In order to realize such a control method, an oscillator that oscillates in the vicinity of a parallel resonance frequency of a tank circuit constituted by a piezoelectric resonator is required. However, in the case of a Colpitts type oscillator using a conventional piezoelectric resonator, it oscillates in the vicinity of the series resonance frequency of the tank circuit, that is, the frequency at which the impedance is minimized, and at parallel resonance, that is, at the frequency at which the impedance is maximized. An oscillation circuit that oscillates could not be constructed.

  An object of the present invention is to provide an oscillation circuit that can oscillate stably near the parallel resonance frequency of a piezoelectric resonance circuit.

According to one aspect of the present invention, the first and second non-inverting amplifiers connected in a loop have one end connected to the input terminal of the first non-inverting amplifier, and the third and the third non-inverting amplifiers connected in a loop. A piezoelectric resonator having the other end connected to an input terminal of the third non-inverting amplifier of a fourth non-inverting amplifier;
A differential output terminal having one end connected to the output terminal of the first non-inverting amplifier and the other end connected to the output terminal of the third non-inverting amplifier is provided. Is done.

  According to the present invention, it is possible to realize an oscillation circuit that can oscillate stably near the parallel resonance frequency of the piezoelectric resonance circuit.

  Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

(First embodiment)
FIG. 1 is a circuit diagram of an oscillation circuit according to a first embodiment of the present invention. The oscillation circuit of FIG. 1 includes first and second non-inverting amplifiers 1 and 2 and a piezoelectric resonator 3 connected in a loop. One end of the piezoelectric resonator 3 is connected to a connection path between the output terminal of the first non-inverting amplifier 1 and the input terminal of the second non-inverting amplifier 2, and the other end is grounded.

  The oscillation circuit of FIG. 1 oscillates near the parallel resonance frequency where the impedance of the piezoelectric resonator 3 is maximum.

  The first non-inverting amplifier 1 is constituted by a gate ground circuit as shown in FIG. The gate ground circuit of FIG. 2A includes an NMOS transistor Q1, an NMOS transistor Q2, and a current source 4 connected in series between a power supply line and a ground line. The drain and gate of the NMOS transistor Q1 are both connected to the power supply line and function as a resistance element. The bias voltage Vb is applied to the gate of the NMOS transistor Q2, and the output voltage VX of the second non-inverting amplifier 2 is supplied to the connection path between the source of the NMOS transistor Q2 and the current source 4. An output voltage VY is output from a connection path between the source of the NMOS transistor Q1 and the drain of the NMOS transistor Q2, and is supplied to the second non-inverting amplifier 2.

  The second non-inverting amplifier 2 is composed of a source follower circuit as shown in FIG. The source follower circuit of FIG. 2B includes an NMOS transistor Q3 and a current source 5 connected in series between a power supply line and a ground line. The output voltage VX of the first non-inverting amplifier 1 is supplied to the gate of the NMOS transistor Q3, and the output voltage VX is output from the connection path between the source of the NMOS transistor Q3 and the power supply line, and is supplied to the first non-inverting amplifier 1. Supplied.

  When the oscillation circuit of FIG. 1 is configured using the gate grounding circuit of FIG. 2A and the source follower circuit of FIG. 2B, a circuit as shown in FIG. 2C is obtained. In the oscillation circuit of FIG. 2C, the gate ground circuit and the source follower circuit share one current source 4.

  FIG. 3 is a diagram showing the relationship between the input and output voltages of the first and second non-inverting amplifiers 1 and 2, and the horizontal axis represents the input of the first non-inverting amplifier 1 (the output of the second non-inverting amplifier 2). VX represents the input of the second non-inverting amplifier 2 (output of the second non-inverting amplifier 2) VY.

  FIG. 3 shows a waveform when the bias voltage Vb is changed from 0 V to 3 V in increments of 0.5 V and an input / output voltage waveform of the source follower circuit alone of FIG. It can be seen that there is an intersection with the input / output voltage waveform of the source follower circuit alone for each bias voltage Vb. These intersections are operating points.

  These intersections are loops in which the output voltage of the first non-inverting amplifier 1 is input to the second non-inverting amplifier 2 and the output voltage of the second non-inverting amplifier 2 is input to the first non-inverting amplifier 1. When the configuration is adopted, it is shown that a stable state exists in terms of direct current for both. Even if the piezoelectric resonator 3 is connected to this loop circuit, since the piezoelectric resonator 3 is an insulator in terms of direct current, there is no influence that changes the direct current stable point. Therefore, the circuit shown in FIG. 2 (c) diverges in a DC manner by appropriately designing, unlike an oscillator composed of an inverting amplifier (inverter) and a piezoelectric resonator 3 as shown in FIG. There is no possibility.

  FIG. 5 is a circuit diagram showing an example in which the circuit of FIG. 2C is further embodied. The oscillation circuit of FIG. 5 uses a current mirror circuit using NMOS transistors Q4 and Q5 and a current source 6 as a current source 4, and has a variable capacitance element 7 connected in parallel to the piezoelectric resonator 3.

  FIG. 6 is a diagram showing the frequency characteristics of the admittance Ytank when the left side is viewed from the line XX ′ in FIG. From FIG. 6, when the capacitance value of the variable capacitance element 7 connected in parallel to the piezoelectric resonator 3 changes, the series resonance frequency of Ytank (frequency at which the admittance becomes maximum) remains constant, and the parallel resonance frequency (the admittance becomes minimum). It can be seen that only the frequency) changes.

  FIG. 7 is a diagram showing the relationship between the capacitance value of the variable capacitor and the oscillation frequency of the oscillation circuit of FIG. FIG. 7 shows that the oscillation frequency decreases when the value of the variable capacitor connected in parallel is increased.

  From the results of FIGS. 6 and 7, the circuit of FIG. 5 indicates that the oscillation frequency is determined by the parallel resonance frequency of the piezoelectric resonator 3.

  FIG. 8 is a circuit diagram of an oscillation circuit using an LCR tank circuit 8 having a circuit configuration similar to that of the present embodiment. In this circuit, a portion corresponding to a resistance load of the grounded-gate amplifier is constituted by an LCR tank circuit 8. It is known that the circuit of FIG. 8 oscillates near the parallel resonance frequency of the LCR tank circuit 8.

  However, oscillation cannot be obtained with the circuit configuration of FIG. 9 in which the LCR tank circuit 8 of FIG. That is, the circuit in FIG. 9 should be considered to have a completely different circuit configuration even though it is similar to the present embodiment.

  The present embodiment is different from the circuit of FIG. 9 in that the piezoelectric resonator 3 and the first and second non-inverting amplifiers 1 and 2 are independent from each other. In addition, a loop is formed in advance so that the operating points of the DC input / output voltages of the first and second non-inverting amplifiers 1 and 2 are stabilized, and then the piezoelectric resonator 3 is connected to the loop including these non-inverting amplifiers. By connecting these, oscillation in the vicinity of the parallel resonance frequency of the piezoelectric resonator 3 can be obtained.

  FIG. 10 is a circuit diagram showing a modification of the present embodiment. The oscillation circuit of FIG. 10 includes a piezoelectric resonator 3 connected between the output terminal of the first non-inverting amplifier 1 and a power supply voltage that is an AC ground potential. Also in this modification, it goes without saying that oscillation is obtained near the parallel resonance frequency of the piezoelectric resonator 3.

  As described above, in the first embodiment, since the piezoelectric resonator 3 is connected to the first and second non-inverting amplifiers 1 and 2 connected in a loop, the piezoelectric resonator 3 is stable in the vicinity of the parallel resonance frequency. Oscillation can be performed.

(Second Embodiment)
The second embodiment is characterized in that it has a differential oscillation output.

  FIG. 11 is a circuit diagram of an oscillation circuit according to the second embodiment of the present invention. The oscillation circuit of FIG. 11 includes first and second non-inverting amplifiers 1 and 2 connected in a loop, third and fourth non-inverting amplifiers 11 and 12 connected in a loop, and piezoelectric resonance. 3 is provided. One end of the piezoelectric resonator 3 is connected to the output terminal of the first non-inverting amplifier 1 (the input terminal of the second non-inverting amplifier 2), and the other end is the output terminal (fourth of the third non-inverting amplifier 11). Of the non-inverting amplifier 12).

  The oscillation circuit of FIG. 11 also oscillates in the vicinity of the parallel resonance frequency where the impedance of the piezoelectric resonator 3 is maximum.

  FIG. 12 is a circuit diagram of an oscillation circuit embodying FIG. 11 and shows an example in which the circuit is constituted by CMOS transistors. A similar circuit configuration can be realized with a bipolar transistor. In the oscillation circuit of FIG. 12, the first and third non-inverting amplifiers 1 and 11 are constituted by grounded-gate amplifiers, and the second and fourth non-inverting amplifiers 2 and 12 are constituted by source follower circuits. The first and second non-inverting amplifiers 1 and 2 are composed of NMOS transistors Q1 to Q4, and the third and fourth non-inverting amplifiers 11 and 12 are composed of NMOS transistors Q11 to Q14.

  13 is an oscillation voltage waveform diagram at the differential output terminal of the oscillation circuit of FIG. The two waveforms in FIG. 13 indicate that a differential output voltage whose phase is inverted is obtained reflecting the symmetry of the circuit. This is because, since the piezoelectric resonator 3 is basically a capacitive element, when one electrode is charged, the other electrode is discharged and the voltage phase across the piezoelectric resonator 3 is reversed.

  As described above, in the second embodiment, two sets of two non-inverting amplifiers connected in a loop are provided, and the piezoelectric resonator 3 is connected between each set. Therefore, in the vicinity of the parallel resonance frequency of the piezoelectric resonator 3. Thus, a differential output voltage capable of stable oscillation can be obtained.

(Third embodiment)
The third embodiment constitutes a differential output type voltage controlled oscillator.

  FIG. 14 is a circuit diagram of an oscillation circuit according to the third embodiment of the present invention. The basic configuration of the circuit of FIG. 14 is the same as that of FIG. 12, but instead of the piezoelectric resonator 3, two variable piezoelectric resonators 3 a and 3 b using a ferroelectric thin film, and these variable piezoelectric resonators. And a control voltage terminal Vcontrol connected to the three connection paths via a resistance element R1.

  As the ferroelectric thin film, a thin film such as barium titanate (BaTiO3) or lead zirconate titanate (Pb (Zr, Ti) O3) can be used. When the ferroelectric thin film is used as the variable frequency variable piezoelectric resonator 3, the parallel resonance frequency can be changed more greatly than the series resonance frequency by controlling the DC bias voltage applied to the control voltage terminal Vcontrol. it can. Therefore, since the oscillation frequency of the oscillation circuit according to the present embodiment also changes greatly, a differential output type voltage controlled oscillator (VCO) can be obtained.

  Thus, in the third embodiment, since the two variable piezoelectric resonators 3 using the ferroelectric thin film and the control voltage terminal Vcontrol are provided, by controlling the voltage applied to the control voltage terminal Vcontrol, The parallel resonance frequency of the variable piezoelectric resonator 3 can be greatly changed, and a highly accurate voltage-controlled oscillator with a differential output can be realized.

(Fourth embodiment)
In the fourth embodiment, a direct conversion type receiver is configured using a differential output type voltage controlled oscillator having a piezoelectric resonator according to the present invention.

  FIG. 15 is a block diagram showing a configuration of a receiver according to the fourth embodiment of the present invention. This receiver includes an antenna 21, a band pass filter 22, a low noise amplifier 23, a down conversion mixer 24, a differential output type voltage controlled oscillator 25 according to the present embodiment, and a π / 2 phase shifter 26. A crystal oscillator 27, a PLL 28, a low-pass filter 29, a variable gain amplifier 30, an A / D converter 31 and the like.

  Only a desired band is extracted from the high-frequency signal received by the antenna 21 by the band-pass filter 22, and a weak radio wave is amplified using the low-noise amplifier 23. By multiplying the high-frequency received signal thus obtained by the waveform oscillated by the differential output type voltage-controlled oscillator 25 according to the present embodiment using the down-conversion mixer 24, the frequency is converted to a low frequency. To do.

  In order to increase resistance to noise, a balanced circuit configuration is used for the low-noise amplifier 23, and a double-balanced type is used for the down-conversion mixer 24. In addition, in order to demodulate a signal modulated by the four-phase phase modulation (QPSK) system, two down-conversion mixers 24-1 and 24-2 are provided, and one down-conversion mixer includes a π / 2 phase shifter 26. The signal whose phase is shifted by is input.

  On the other hand, in order to keep the frequency of the voltage controlled oscillator 25 according to the present embodiment constant, feedback control is performed using the PLL 28 based on the reference signal generated by the crystal oscillator 27, and the phase of the oscillation frequency of the voltage controlled oscillator 25 is determined. Lock.

  The signal converted to a low frequency removes an extra frequency component remaining in the band by the low-pass filter 29, is amplified to an appropriate amplitude by the variable gain amplifier 30, and then is converted to a digital signal by the A / D converter 31. Is done. The signal converted into the digital signal is digitally processed by a baseband circuit (not shown).

  In the fourth embodiment, the differential output of the oscillator 25 is directly supplied to the double balance type (differential type) down-conversion mixer 24 by using the differential output type voltage controlled oscillator 25 of the present embodiment. Can do. Since the differential circuit configuration is excellent in resistance to common-mode noise, it is advantageous when the receiving circuit is configured by an integrated circuit. The differential output type voltage controlled oscillator has less influence of noise on the external circuit than the single phase output type oscillator. Therefore, the differential output type voltage controlled oscillator using the piezoelectric resonator of this embodiment is advantageous in integrating with other balanced type analog circuits.

  Conventionally, a voltage-controlled oscillator built in an integrated circuit generally has a spiral inductor formed on the same substrate and is used as a resonator. The spiral inductor generates an alternating magnetic field having the same frequency component as the oscillation frequency around the oscillation circuit. On the other hand, in the low noise amplifier 23, inductive source degeneration is often used in order to reduce noise generation. In this method, an inductor is connected to the source of the MOS transistor used in the amplifying unit, thereby attempting to match the input impedance without using a resistance element that becomes a noise source. As a result, when the low noise amplifier 23 and the voltage controlled oscillator are integrated on the same substrate, the oscillation waveform of the voltage controlled oscillator leaks to the low noise amplifier 23 through the coupling between the spiral inductors existing on the same substrate. This phenomenon is called LO leakage, and causes a large DC offset voltage to the output of the down conversion mixer 24. This is said to be one of the disadvantages of the direct conversion method.

  On the other hand, in the fourth embodiment, a piezoelectric resonator is used as a resonator instead of a spiral inductor. Since the piezoelectric resonator basically has a capacitor structure, it is not electromagnetically coupled to an inductor of another circuit. Therefore, by using the differential output type voltage controlled oscillator 25 according to the present embodiment for the direct conversion type receiver, the influence of the DC offset due to the LO leakage can be greatly suppressed, and the performance of the receiver is greatly improved. Can be improved.

1 is a circuit diagram of an oscillation circuit according to a first embodiment of the present invention. The circuit diagram which shows the specific example of FIG. The figure which shows the relationship of the input-output voltage of the 1st and 2nd non-inverting amplifiers 1 and 2. FIG. FIG. 3 is a circuit diagram showing an example of an oscillator composed of an inverting amplifier (inverter) and a piezoelectric resonator 3. FIG. 3 is a circuit diagram showing an example in which the circuit of FIG. The figure which shows the frequency characteristic of admittance Ytank at the time of seeing the left side from the line XX 'of FIG. The figure which shows the relationship between the capacitance value of a variable capacitor, and the oscillation frequency of the oscillation circuit of FIG. The circuit diagram of the oscillation circuit using the LCR tank circuit 8 which has a circuit structure similar to this embodiment. 4 is a circuit diagram in which the LCR tank circuit 8 is simply replaced with a piezoelectric resonator 3. FIG. The circuit diagram which shows the modification of this embodiment. The circuit diagram of the oscillation circuit by the 2nd Embodiment of this invention. FIG. 12 is a circuit diagram of an oscillation circuit that embodies FIG. 11. FIG. 13 is an oscillation voltage waveform diagram at a differential output terminal of the oscillation circuit of FIG. 12. The circuit diagram of the oscillation circuit by the 3rd Embodiment of this invention. The block diagram which shows the structure of the receiver by the 4th Embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 1st non-inverting amplifier 2 2nd non-inverting amplifier 3 Piezoelectric resonator 4-6 Current source 7 Variable capacity element 8 LCR tank circuit 11 3rd non-inverting amplifier 12 4th non-inverting amplifier 21 Antenna 22 Band pass Filter 23 Low noise amplifier 24 Down conversion mixer 25 Voltage controlled oscillator 26 Phase shifter 27 Crystal oscillator 28 PLL
29 Low-pass filter 30 Variable gain amplifier 31 A / D converter

Claims (6)

One ends of the first and second non-inverting amplifiers connected in a loop are connected to the input terminals of the first non-inverting amplifiers, and the third and fourth non-inverting amplifiers connected in a loop form. A piezoelectric resonator having the other end connected to the input terminal of the non-inverting amplifier 3;
An oscillation circuit comprising: a differential output terminal having one end connected to the output terminal of the first non-inverting amplifier and the other end connected to the output terminal of the third non-inverting amplifier. The first and third non-inverting amplifiers each have a grounded-gate amplifier;
2. The oscillation circuit according to claim 1 , wherein each of the second and fourth non-inverting amplifiers includes a source follower circuit. 3. The oscillation circuit according to claim 2 , wherein the first and second non-inverting amplifiers to the third and fourth non-inverting amplifiers share one current source. The piezoelectric resonator includes first and second variable piezoelectric resonance circuits that are connected in series with each other and can change a parallel resonance frequency by a control voltage;
Oscillator circuit according to any one of claims 1 to 3, further comprising a control voltage terminal for supplying the control voltage to the connection path of the first and second variable pressure electric resonant circuit. 5. The oscillation circuit according to claim 4 , wherein the first and second variable piezoelectric resonance circuits are formed using a ferroelectric thin film as a material. One ends of the first and second non-inverting amplifiers connected in a loop are connected to the input terminals of the first non-inverting amplifiers, and the third and fourth non-inverting amplifiers connected in a loop form. A piezoelectric resonator having the other end connected to the input terminal of the non-inverting amplifier 3;
A differential output terminal having one end connected to the output terminal of the first non-inverting amplifier and the other end connected to the output terminal of the third non-inverting amplifier;
The piezoelectric resonator includes first and second variable piezoelectric resonance circuits that are connected in series with each other, can change a parallel resonance frequency by a control voltage, and are formed using a ferroelectric thin film as a material.
A voltage-controlled oscillator comprising a control voltage terminal for supplying the control voltage to a connection path of the first and second variable piezoelectric resonance circuits.

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