JP2000165144A - Voltage controlled piezoelectric oscillator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To secure a wide variable frequency region by suppressing blunting of increase in the frequency against controlling voltage in a high frequency band by inserting an element formed by serially connecting a resistance element and capacitance element in parallel as a variable voltage capacitance element to constitute an oscillator. SOLUTION: The capacitance element 106 is provided to prevent dividing voltage of resistance due to a high resistance element 104 and the resistance element 107 between a controlled voltage input terminal 105 and the variable voltage capacitance element 103 from being caused. The combined value and a capacitance value of a phase control element are necessary to be set so that the real part of the combined impedance between the variable voltage capacitance element 103 and a phase control element when the capacitance value of the variable voltage capacitance element 103 becomes minimum within a range of controlled voltage have a value of a certain degree or higher to extend width of variable frequency. The larger the real part of the combined impedance between the variable voltage capacitance element 103 and the phase control element when the capacitance value of the variable voltage capacitance element 103 becomes minimum within the range of the control voltage becomes, the larger external quantity of the width of variable frequency becomes.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a voltage controlled piezoelectric oscillator whose frequency can be controlled by a control voltage applied from the outside.

[0002]

2. Description of the Related Art Among piezoelectric oscillators, those whose frequency can be controlled by a control voltage applied from the outside are called voltage-controlled piezoelectric oscillators (commonly called VCXOs) and are widely used in the field of mobile communication and image display. .

[0003]

(Equation 1)

It is well known that adding a capacitance in series to a piezoelectric vibrator changes the resonance frequency.
The amount of frequency shift at that time is approximated by Equation 1 above. In Equation 1, fs represents the series resonance frequency of the vibrator, and C0 and C0
1 indicates a parallel equivalent capacitance and a series equivalent capacitance of the vibrator, respectively, and CL indicates a load capacitance.

FIG. 2 shows a basic configuration of a voltage controlled piezoelectric oscillator. It comprises a piezoelectric vibrator 201 connected in series, a voltage variable capacitance element 203, and an oscillation circuit 202 for exciting the piezoelectric vibrator, and utilizes the above-described shift of the resonance frequency in a mechanism for changing the oscillation frequency. I have. That is, by applying a control voltage from the external control voltage input terminal 205, the capacitance value of the voltage variable capacitance element 203 is changed, and the frequency of the output clock signal changes accordingly. Control voltage input terminal 205 and voltage variable capacitance element 203
The resistance element 204 in between has a high resistance of about several hundred kilo-ohms so that the control voltage acts only as a bias voltage and does not affect the AC component of the signal.

[0006]

The frequency variable range of a voltage controlled piezoelectric oscillator is generally required to be as wide as possible. For this reason, conventionally, a voltage variable capacitive element having a wide capacitance variable range has been used, and a γ value (= C0 / C1) has been conventionally used. Although the use of a piezoelectric vibrator having a small size is performed, all of these characteristics have limitations. In particular, as shown in FIG. 3, there is a problem that the extension of the frequency becomes slow in a high frequency region in the control voltage versus frequency characteristic of the voltage controlled piezoelectric oscillator, and a wide variable range cannot be obtained. This is mainly due to the fact that the current voltage variable capacitance element saturates the capacitance value in a small capacitance region as shown in FIG. 4, and in actual use, parasitic capacitance such as terminals and wiring is added. The trend becomes even more pronounced.

SUMMARY OF THE INVENTION An object of the present invention is to provide a voltage controlled piezoelectric oscillator having a wide frequency variable range by eliminating the above-mentioned slowdown of frequency expansion with respect to a control voltage in a high frequency range.

[0008]

In order to achieve the above object, a voltage controlled piezoelectric oscillator according to the present invention comprises a series connection of a resistance element and a capacitance element (hereinafter referred to as a phase control element).
It is characterized by being inserted in parallel to a voltage variable capacitance element.

[0009]

(Embodiment 1) Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows the basic configuration of the present invention. By connecting a phase control element in which a capacitance element 106 and a resistance element 107 are connected in series to the voltage variable capacitance element 103 in parallel, the frequency variable range of the voltage controlled piezoelectric oscillator is expanded. The reason why the frequency variable range is expanded by this configuration will be described below.

[0010]

(Equation 2)

Assuming that the capacitance value of the voltage variable capacitance element is Cv, and the resistance and capacitance values connected in parallel thereto are Rp and Cp, respectively, their combined impedance is expressed by the above equation (2). It can be seen that a resistance component (real number term) occurs in series with the piezoelectric vibrator in addition to the capacitance component (imaginary number term) as the combined impedance.

A Bode diagram showing the oscillation conditions of the piezoelectric oscillation circuit is generally represented as shown in FIG. In FIG. 5, the upper diagram shows the phase phase diagram, and the lower diagram shows the gain phase diagram. If the gain is 0 dB or more at the point where the phase becomes 0 °, the oscillation condition is satisfied and oscillation starts. As shown in FIG. 5, the oscillation point of the piezoelectric oscillation circuit generally has a frequency slightly higher than the series resonance frequency of the vibrator.

When a resistance component having a certain magnitude or more is connected in series to the piezoelectric vibrator in the piezoelectric oscillation circuit, the phase condition of the oscillation circuit changes as shown in FIG. The broken line in FIG. 6 shows the phase and gain when the resistance components shown in FIG. 5 are not connected, while the solid line shows the phase and gain when the resistance components are connected in series. The resonance point does not move and the phase curve becomes slightly gentle, so that the point crossing 0 ° moves and the oscillation frequency is shifted to a higher one. At this time, the gain hardly changes except that the peak of the resonance point slightly decreases.

As described above, when a resistor having an appropriate size is inserted in series with the voltage variable capacitor, it can be seen that the oscillation frequency shifts to a higher range. However, in the piezoelectric oscillation circuit, the resistor is directly connected to the vibrator. When connected, only the entire control voltage vs. frequency characteristic is shifted upward as shown in FIG. 7, and it is difficult to greatly expand the frequency variable range.

On the other hand, in the configuration of the present invention, as can be seen from Equation 2, the resistance component is almost inversely proportional to the square of the capacitance Cv of the voltage variable capacitance element. Therefore, by appropriately selecting the element value, In the low frequency range where Cv is large, the frequency in the high frequency range where Cv is small can be shifted to a higher value with little change in frequency. As a result, as shown by the solid line in FIG. 8, the frequency variable range of the voltage controlled piezoelectric oscillator can be greatly expanded.

[0016]

(Equation 3)

The capacitance component obtained from the imaginary term of Equation 2 is expressed by Equation 3 above, and increases from the original capacitance value of the voltage variable capacitance element by the term added to Cv. This qualitatively works in the direction of lowering the resonance frequency, but in the frequency band above megahertz actually used for voltage-controlled oscillators, this increase becomes a very small value, and it has almost no effect quantitatively. Do not give.

The capacitance element 106 of the phase control element shown in FIG. 1 is used for the purpose of preventing the resistive voltage division by the high resistance element 104 and the resistance element 107 between the control voltage input terminal 105 and the voltage variable capacitance element 103. If a capacitor having a value equal to or greater than the capacitance value Cv of the voltage variable capacitance element is used, it hardly affects both the resistance component and the capacitance component in the combined impedance.

Examples of calculation results when actual values are substituted into Expressions 2 and 3 are shown below. The resonance frequency of the piezoelectric vibrator 101 is 10 MHz, and the voltage variable capacitance element 10
Considering the case where the capacitance value of 3 changes from 3 picofarads to 30 picofarads, when the resistance value of the resistive element 107 is 100 kohm and the capacitance value of the capacitive element 106 is 10 picofarads among the phase control elements. , Cv
= The resistance component (real term) of the combined impedance at 3 picofarads is 280.1 ohms, whereas Cv =
The resistance component at 30 picofarads is only 2.81 ohms. It can also be seen that the increase in capacity is 0.01 picofarad when Cv = 3 picofarads, and 0.003 picofarad when Cv = 30 picofarads, which is a level that hardly affects both.

As a result of calculating how much frequency shift occurs under the above conditions by circuit simulation, when Cv = 3 picofarads, the oscillation frequency becomes R
In contrast to the case where p and Cp are not inserted, the shift to the positive side of 406 Hertz is performed, whereas the frequency shift is only 4.1 Hertz at Cv = 30 picofarads, and the frequency variable range is 403 Hertz under the above conditions. It can be seen that the ratio is enlarged by 40.3 ppm.

As can be seen from the above calculation, in order to increase the frequency variable width, when the capacitance value of the voltage variable capacitance element 103 is minimized within the control voltage range, the voltage variable capacitance element 103 and the phase control element It is necessary to set the resistance value and the capacitance value of the phase control element so that the real number (resistance) component of the combined impedance (Equation 2) has a certain value or more. Specifically, unless the value of the phase control element is selected so that the real number (resistance) component of the combined impedance becomes at least 10 ohms or more, an effective expansion of the frequency variable width cannot be expected.

The larger the real number (resistance) component of the combined impedance (Equation 2) of the voltage variable capacitance element 103 and the phase control element when the capacitance value of the voltage variable capacitance element 103 is minimized within the control voltage range. Although the amount of expansion of the frequency variable width also increases, the real number (resistance) component acts in a direction to cancel the negative resistance which is a measure of the oscillation margin of the circuit. Etc. cause problems. When the capacitance value of the voltage variable capacitance element 103 is minimized within the control voltage range, the real (resistance) component of the combined impedance (Equation 2) of the voltage variable capacitance element 103 and the phase control element is at least the negative resistance of the oscillation circuit. It is necessary to set the resistance value of the resistance element 107 of the phase control element and the capacitance value of the capacitance element 106 so as to be equal to or less than the absolute value of the phase control element.

The present invention relates to a voltage controlled piezoelectric oscillator (VCX).
In addition to O), the present invention can be applied to a temperature controlled oscillator circuit (commonly called TCXO) having a frequency variable function. Further, the voltage controlled piezoelectric oscillator (VCXO) according to the present invention has a C
Not only the oscillation circuit using the MOS inverter 902,
It is also effective in a Colpitts oscillation circuit using transistors.

(Embodiment 2) As an actual circuit diagram of the present invention shown in the block diagram of FIG.
A configuration is conceivable in which the input / output terminals of the MOS inverter 902 are grounded via capacitors, and a combination of a parallel connection of the voltage variable capacitance element 903 and the phase control element 904 is arranged on at least one of them. In the circuit diagram, a DC cut capacitor 905 is provided for cutting a DC voltage component of a feedback signal to the inverter 902. An oscillation circuit using a CMOS inverter has a large oscillation margin compared to a Colpitts oscillation circuit using transistors, and has an advantage that it can be easily integrated into an integrated circuit because a large-capacity capacitor is not required. In the circuit of FIG.
The same effect is obtained even if the positional relationship between the resistive element and the capacitive element in No. 4 is reversed. In FIG. 9, the voltage variable capacitance element 903
Although the configuration in which the combination of the phase control element 904 and the phase control element 904 are arranged on the gate side of the CMOS inverter 902 is shown, the position where these combinations are arranged may be on the drain side.

FIGS. 10 and 1 show another example of the circuit diagram of the present invention.
It is shown in FIG. The circuit in FIG. 10 has a configuration in which a capacitive element 1006 of the phase control elements is arranged in a resonance loop. The circuit of FIG. 11 has a configuration in which a DC cut capacitor 1105 is provided in a resonance loop. In these configurations, the capacitance element 10
Since the capacitor 06 and the DC cut capacitor 1105 serve as a limiting capacitor for the voltage variable capacitor 1103, it is necessary to use a capacitor having a relatively large value to secure a large frequency variable width.

In the circuit diagrams shown in FIGS. 9 to 11, the combination of the voltage variable capacitance element and the phase control element is arranged on either the inverter gate side or the drain side, but as shown in FIG. This combination to C
A configuration in which the MOS inverter 1202 is arranged on both sides of the gate and the drain is also possible. In this circuit configuration, compared to the configuration arranged on one side, the number of voltage variable capacitance elements is plural, so that the frequency variable width can be expanded, and in addition, the balance of the capacitance ratio between the gate side and the drain side is not largely broken. Positive effects are also seen on negative resistance. However, the configuration in which the voltage variable capacitance elements are arranged on both sides has a disadvantage that the number of components increases unless the circuit is integrated.

Consider integrating the invention into an integrated circuit. As a voltage variable capacitance element for a voltage controlled piezoelectric oscillator, a semiconductor pn junction capacitance (commonly called a varicap) is often used.
-C) and the like can also be used. When these are formed in an integrated circuit, it is difficult to impart a concentration gradient of impurities to a part of the integrated circuit, unlike the case of a single unit, so that the capacitance variable ratio is inevitably smaller than that of a single unit. That is, when the voltage-controlled oscillation circuit is integrated, whether to incorporate the voltage variable capacitance element into the integrated circuit depends on which of the degree of integration and the frequency variable width is important.

FIGS. 13 and 14 show integrated circuits of the present invention. FIG. 13 shows a case where a voltage-converting capacitor element 1303 is built-in, and FIG. Is the case. In the figure, portions 1300 and 1400 surrounded by dotted lines represent integrated circuits, and rectangles 1308, 1309 and 1408 drawn on dotted lines,
Reference numerals 1409 and 1410 represent input / output terminals of the integrated circuit. In each case, the voltage variable capacitance elements 1303 and 1
Phase control elements 1304 and 1404 connected in parallel to 403 can be incorporated in integrated circuits 1300 and 1400, and an increase in the number of components can be suppressed. In an actual oscillator integrated circuit, a waveform shaping buffer, a frequency dividing circuit, an output buffer, and the like are provided after the oscillation stage, but these are omitted from FIGS. 13 and 14.

(Embodiment 3) As shown in FIG. 15, a part or the whole of a resistance element of a phase control element connected in parallel to a voltage variable capacitance element 1503 is constituted by a variable resistance element 1508 which can be adjusted from the outside. By doing so, the center frequency, frequency variable width,
It is possible to adjust the linearity of the frequency change with respect to the control voltage, and it is possible to correct a frequency error caused by heat, stress, or the like at the time of manufacturing the oscillator. As the variable resistance element 1508, a variable resistance element of a type in which the resistance value of a chip resistor is trimmed by laser light or a voltage-controlled variable resistance element can be used in addition to a mechanical trimmer type variable resistance that is generally used.

When a voltage control resistor is used as the variable resistance element, when the oscillation circuit is integrated, the variable resistance element can be built in the integrated circuit. FIG. 16 shows an example of a circuit diagram in which a variable resistor is incorporated. As the voltage variable resistor, for example, as shown in FIG. 16, a change in the ON resistance value of the MOS transistor 1614 due to the gate application voltage can be used. A storage element 1612 such as a ROM or a RAM for storing the gate applied voltage value is provided in the integrated circuit 1600, and a resistance signal input terminal 161 is externally provided.
The resistance value can be adjusted by electrically writing the set value to the storage element 1612 via the line 1. D /
The A converter 1613 is for converting a digital signal stored in the storage element 1612 into an analog voltage. In recent years, downsizing of oscillators has been remarkable, but an oscillation circuit having such a configuration does not require an external variable resistance element having a large outer shape, so that an oscillator having a very small size can be supported. Although FIG. 16 illustrates a case where the variable capacitance element is externally attached, a configuration incorporating the variable capacitance element is naturally possible as described in the second embodiment.

[0031]

As described above, the voltage controlled piezoelectric oscillator of the present invention does not slow down the frequency expansion even in the high frequency region of the control voltage versus frequency characteristic, and obtains a wider frequency variable range than the conventional product. It is possible.

[Brief description of the drawings]

FIG. 1 is a basic configuration diagram of a voltage controlled piezoelectric oscillator of the present invention.

FIG. 2 is a basic configuration diagram of a conventional voltage controlled piezoelectric oscillator.

FIG. 3 is an example of a control voltage versus frequency characteristic of a voltage controlled piezoelectric oscillator.

FIG. 4 is an example of an applied voltage vs. terminal-to-terminal capacitance characteristic of a voltage variable capacitance element.

FIG. 5 is an example of a Bode diagram of an oscillation circuit.

FIG. 6 is an example showing a change in a Bode diagram when a resistance component is inserted in series into a piezoelectric vibrator.

FIG. 7 is an example showing a change in voltage frequency characteristics when a resistance element is inserted in series with a piezoelectric vibrator.

FIG. 8 is an example showing a change in a voltage frequency characteristic according to the present invention.

FIG. 9 is an example of an actual circuit diagram of the present invention.

FIG. 10 is an example of an actual circuit diagram of the present invention.

FIG. 11 is an example of an actual circuit diagram of the present invention.

FIG. 12 is an example of an actual circuit diagram of the present invention.

FIG. 13 is a configuration diagram when the present invention is integrated.

FIG. 14 is a configuration diagram when the present invention is integrated.

FIG. 15 is another configuration diagram of the voltage controlled piezoelectric oscillator of the present invention.

FIG. 16 is a configuration diagram when the configuration of FIG. 15 is integrated;

[Explanation of symbols]

 101, 201, 901 Piezoelectric vibrator 102, 202 Oscillation circuit 103, 203, 903 Voltage variable capacitance element 106 Capacitance element constituting phase control element 107 Resistance element constituting phase control element 902 CMOS inverter 1300, 1400, 1600 Integrated circuit 1508 Variable resistance element 1614 MOS transistor

Claims (9)

[Claims] 1. A voltage controlled piezoelectric oscillator comprising a piezoelectric vibrator, a voltage variable capacitance element connected in series to the piezoelectric vibrator, and an oscillation circuit for exciting the piezoelectric vibrator, further connected in series. A voltage-controlled piezoelectric oscillator, wherein the resistance element and the capacitance element are connected in parallel to the voltage-variable capacitance element. 2. A real component of a combined impedance of a combination of the resistance element, the capacitance element, and the voltage variable capacitance element when a capacitance value of the voltage variable capacitance element is minimized within a control voltage range is 10 ohms. 2. The voltage controlled piezoelectric oscillator according to claim 1, wherein the values of the resistance element and the capacitance element are set to be equal to or less than the absolute value of the negative resistance of the oscillation circuit. 3. The oscillation circuit according to claim 1, wherein at least one of a ground capacitance at an input / output terminal of a CMOS inverter, which is one of the constituent elements, is a combination combination of the resistance element, the capacitance element, and the voltage variable capacitance element. The voltage controlled piezoelectric oscillator according to claim 1, wherein the voltage controlled piezoelectric oscillator is grounded as a ground capacitance. 4. The voltage controlled oscillator according to claim 1, wherein all the elements except the piezoelectric vibrator and the voltage variable capacitance element are integrated in a single integrated circuit. The voltage controlled piezoelectric oscillator according to claim 2. 5. The voltage controlled oscillator according to claim 1, wherein all the elements except for the piezoelectric vibrator are integrated in a single integrated circuit. 3. The voltage controlled piezoelectric oscillator according to 3. 6. The voltage controlled piezoelectric oscillator according to claim 1, wherein part or all of said resistance element is constituted by a variable resistance element. 7. The voltage controlled piezoelectric oscillator according to claim 6, wherein said variable resistance element is a voltage controlled variable resistance element whose resistance value changes according to an applied voltage. 8. A voltage controlled oscillator in which all elements except for a piezoelectric vibrator and a voltage variable capacitance element are integrated, and further comprising a storage element for storing a voltage value applied to the voltage controlled variable resistance element. The voltage controlled piezoelectric oscillator according to claim 7, wherein: 9. A voltage-controlled oscillator, in which all the elements except for the piezoelectric vibrator are integrated, and further comprising a storage element for storing a voltage value applied to the voltage-controlled variable resistance element. A voltage controlled piezoelectric oscillator according to claim 7.