JPH10284941A - Piezoelectric oscillator provided with frequency correction function - Google Patents

Abstract

PROBLEM TO BE SOLVED: To drastically keep frequency stability even against a thermal shock and secular change by providing a control means and a storage means in a voltage control temp. compensating crystal oscillator, executing synchronization with an external reference frequency and storing the state in the storage means. SOLUTION: Frequency precision when a portable telephone terminal is synchronized with a base station frequency is provided with drastically high frequency stability being equivalent to that of base station carrier wave. The impression voltage VAFC-VADJ of a variable capacitance diode D1 in this state is converted into a digital voltage by an A/D converter with an operation amplifier U1 so at to be temporarily stored in a latch circuit 13. A temp. monitoring part 22 monitors the output of a temp. compensating part 20-1 and permits writing to EE-PROM 14 by a LOCK signal when temp. at that time is within a compensation range. Data which is written in a preceding time is read at the time of power supply, D/A-converted and impressed on the variable capacitance diode D1 so that a time till the portable telephone terminal is synchronized with a base station is shortened and communication is instantaneously started.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a crystal oscillator used in, for example, a network synchronizer or a portable telephone terminal, and more particularly to a piezoelectric oscillator with a frequency correction device having a frequency correction function added to stabilize the oscillation frequency.

[0002]

2. Description of the Related Art In recent years, there have been remarkable improvements in frequency stability, size reduction, price reduction, and the like of piezoelectric oscillators such as crystal oscillators, which have greatly contributed to the spread of network synchronization devices and portable telephone terminals. The output frequency of a crystal oscillator used as a reference frequency source for communication equipment varies depending on various factors.However, for frequency variations due to changes in conditions such as ambient temperature, power supply voltage, and output load, measures to deal with these variations may be taken. it can. For example, with respect to a temperature change, if a temperature compensation circuit is added to the crystal oscillator, a frequency change due to the temperature change can be kept within a desired value range.

A temperature-compensated crystal oscillator (hereinafter referred to as TCXO) cancels out the temperature-frequency characteristic fluctuation inherent in a crystal resonator by utilizing the well-known phenomenon that the oscillation frequency fluctuates when the load capacitance of the oscillation loop changes. As described above, the load capacitance is controlled with respect to a change in temperature, and there are roughly three compensation methods. The first is a method called direct compensation, in which a compensation circuit is connected in series with a crystal oscillator as shown in FIG. In general, a compensation circuit is a simple combination of a high-temperature section compensation circuit and a low-temperature section compensation circuit connected in series, which is basically composed of a temperature sensor (such as a thermistor) and a capacitor connected in parallel. It is widely used in the field of mobile phones and the like because of its easy implementation. The second is a method called indirect compensation, in which a variable capacitance diode is connected in series with the crystal oscillator as shown in FIG. It is connected to both ends of the diode. In this method, a DC voltage generated in a compensation circuit composed of a thermistor and a resistor is applied to the variable capacitance diode D via the high-frequency blocking resistor R, so that the frequency change of the circuit is limited to the temperature of only the crystal resonator. The temperature characteristic of the crystal oscillator is compensated by making the characteristic reverse to the characteristic. The third is a method called digital type compensation. As shown in FIG. 5, the compensation circuit shown by the second compensation method includes a temperature sensor, a semiconductor memory, an A / D converter, a D / A converter, and the like. This is a compensation method that uses digital processing. These TCs
The frequency stability required for the reference frequency source for the terminal of the current mobile phone system (temperature range -25
± 2 to 2.5 ppm at ~ 75 ° C).

On the other hand, recent portable telephone systems require a higher frequency stability (for example, ± 0.2 ppm) as a reference frequency source for a terminal. In order to satisfy this demand, a conventional TCXO having a voltage control function (referred to as a VC-TCXO) is used as a reference frequency source of a mobile phone terminal, and the frequency of a highly stable signal emitted from a base station is changed to the VC-TCXO. The method of keeping the frequency stability of the reference frequency source of the mobile phone terminal at the time of communication extremely high by synchronizing the outputs of the mobile phone terminals is used. Here, VC-TCX
Before describing O, let's talk a little about the voltage controlled crystal oscillator (VCXO). The oscillation circuit shown in FIG. 6 is an example of a voltage-controlled crystal oscillator. A variable capacitance diode _CD is inserted into an oscillation loop between a crystal oscillator and an amplifier, and one end of the variable capacitance diode CD is connected to a terminal a via a high-frequency blocking resistor R. One end is connected to the terminal b. Control input terminals a, utilized to circuit oscillation that the oscillation frequency load capacitance changes things and oscillation loop capacity appearing across the variable capacitance diode C _D by the voltage V applied between b changes varies This is a crystal oscillator that controls the frequency. VC-TCXO is TC
FIG. 7 is a configuration diagram showing an example of a conventional VC-TCXO in which a function of VCXO is added to XO. That is, the VC-TCXO shown in this example includes a temperature compensation circuit Tem including a temperature sensing element section 31, a temperature compensation voltage generation section 32, and a variable capacitance diode D1, a crystal oscillator Y1,
And a variable capacitance diode D2 for applying an external signal voltage.

In the VC-TCXO shown in FIG. 7, a temperature sensing element section 31 senses an ambient temperature, and a voltage generated from a temperature compensation voltage generating section 32 is applied to a variable capacitance diode D1 in accordance with the temperature at that time. It always cancels out the temperature of the child and generates an almost constant frequency. Further, by applying a voltage V _AFC to a variable capacitance diode D2 connected in series with the crystal unit so as to synchronize with an external reference signal, VC-T
As for the oscillation frequency of the CXO, frequency stability with the same accuracy as that of the external reference signal can be obtained. Further, the voltage V _AFC is often used for the purpose of switching the channel frequency.

To facilitate understanding of the present invention, a description will be given of a frequency correction function of a temperature-compensated crystal oscillator with a voltage control function used in a mobile phone terminal of a mobile phone system operated in Japan. In this mobile phone system, an active TCXO is used for communication between a base station and a mobile phone terminal.
Frequency stability (± 2.0 to ± 2.5p at temperature range -20 to + 75 ° C)
pm), the use of a TCXO with a voltage control function as the reference frequency source in the mobile phone terminal allows the use of a mobile phone terminal with a reference frequency (approximately ± 0.2 ppm). Therefore, the carrier frequency is also synchronized with the frequency emitted from the base station. Generally, the function of synchronizing the reference frequency of the terminal with the frequency of the base station is A
This is called the FC function, and the accuracy of the reference frequency of the mobile phone terminal during communication is kept extremely high.

[0007]

However, in a conventional crystal oscillator, a change in the frequency due to thermal shock or aging of the crystal oscillator due to reflow when mounting the oscillator in a device is inevitable. The above-described TCXO, VCXO, VC-T incorporating such a crystal unit
The oscillation frequency of CXO and the like is shifted. Therefore, in order to correct the frequency deviation due to thermal shock, a frequency adjustment circuit, for example, a trimmer capacitor or the like was inserted into the oscillation loop and readjusted.However, an adjustment process of turning the trimmer is required, which increases the manufacturing cost. There was a problem. Further, there is a drawback that the aging of the frequency due to aging is inevitably performed by adjustment at the time of manufacturing, and maintenance by a user is required. The present invention has been made to solve the above-mentioned problem, and by adding an automatic frequency correction function to the crystal oscillator itself, it is possible to reduce the need for readjustment after assembly and to eliminate the need for maintenance by the user. Another object of the present invention is to provide a VCXO with a frequency correction device or a VC-TCXO with a frequency correction device having extremely high frequency stability against aging.

[0008]

According to a first aspect of the present invention, there is provided a temperature-compensated crystal oscillator with a frequency compensating device according to the present invention, the frequency of which varies according to a control voltage. In the oscillator, storage means for storing the information on the supply voltage is provided, and when the oscillation is stopped and restarted, the information stored in the storage means is read and the supply voltage is controlled so as to correspond to the information. This is a piezoelectric oscillator having a frequency correction function. Claim 2
According to a third aspect of the present invention, there is provided a piezoelectric oscillator having a frequency correction function, wherein the piezoelectric oscillator is synchronized with an external reference oscillation frequency and the state is stored in the storage means.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail based on an embodiment shown in the drawings. FIG. 1 is a block diagram for generally explaining the configuration principle and operation of a VCTCXO in which the present invention is applied to a temperature compensated VCXO. As shown in FIG. 3, the piezoelectric oscillator having the frequency correction function of the present invention has a piezoelectric oscillation unit 20-1, a temperature compensation unit 20-2, a voltage supply unit 21, and a monitoring unit. It comprises a monitoring unit 22 for monitoring whether it is within the compensation range and an initial voltage setting unit 23. The basic function included in each block will be described. The oscillation unit 20-1 includes an amplifier, a crystal oscillator, and a variable capacitance diode, and controls the voltage between both ends of the variable capacitance diode as is well known. By doing so, the oscillation frequency is controlled. The temperature compensator 20-2 includes a temperature-sensitive element such as a thermistor as described with reference to FIGS. 5 and 6, and generates a compensation voltage to be applied to the variable capacitance diode of the oscillator 20-1. It is. Voltage supply unit 21, the oscillating unit by combining the frequency correction voltage V _{A DJ} for outputting from the initial voltage setting unit 23 to be described later AFC signal V _AFC supplied from, for example portable radio or the like equipped with the oscillator It generates a control voltage at which the oscillation frequency of the oscillator 20-1 becomes a desired one and supplies the control voltage to the variable capacitance diode of the oscillator 20-1. The monitoring unit 22 determines whether or not the temperature at that time is within a predetermined temperature range based on the temperature information generated by the temperature compensating unit 20-2. Generate a signal. The remaining initial voltage setting unit 23 includes a writable and readable memory, for example, an EE-PROM.
And a function of storing the voltage between both ends of the variable capacitance diode after converting it into a digital signal in the voltage supply unit 21.

FIG. 2 is a detailed functional diagram (FIG. 1) of the VCTCXO according to the present invention, which is composed of four blocks 20 to 23 surrounded by chain lines. The configuration of each block will be described in order. The oscillation unit indicated by a chain line 20 includes a crystal oscillator Y1, an amplifier 10, two variable capacitance diodes D1 and D2, and two capacitances C1 and C2 (DC blocking,
An oscillation unit including an AC bypass capacitor) and a temperature compensation unit including a temperature sensor unit 11 and a temperature compensation voltage generation circuit 12 operate as a voltage-controlled temperature-compensated oscillator. A chain line 21 is a control voltage supply unit. The voltage V _AFC -V _ADJ across the variable capacitance diode D1 is applied to the operational amplifier U1 to obtain a voltage V _AD , which is analog / digital. The data is converted into digital data by a converter (hereinafter, referred to as an A / D converter) A / D and is temporarily stored in the latch circuit 13.

A chain line 22 is a monitoring unit for monitoring the output voltage of the temperature sensor 11 of the temperature compensating unit 20-1, and includes voltage comparators U3 and U4 and a NAND gate U5.
When the output voltage is a value between V _SENS1 and V _SENS2 , it is determined whether or not the temperature at that time is within the compensation range based on the information of the temperature sensor.
This signal permits writing of data to the EE-PROM 14 by the CK signal. A chain line 23 is an initial voltage setting unit, which stores data in the EE-PROM 14.
, A read / write enabled latch circuit 15 and a storage circuit RA
M, a digital / analog converter D / A and an operational amplifier U2. EE-
Digital data information written in advance is read from the PROM 14 (electrically writable and readable memory) and written to the readable / writable memory RAM.
This data is converted into an analog DC voltage value V _DA by a D / A converter. This voltage is referred to as gain G2 and reference voltage V
_The output voltage V _ADJ is converted by the operational amplifier U2 set to _{AFC (0)} , and is applied to the anode of the AFC / frequency correcting variable capacitance diode D1 via the AC signal blocking / DC voltage biasing resistor R2.

The overall operation of the above configuration will be described. First, the operation of the oscillating unit 20 will be described. The frequency synchronizing signal voltage V _AFC supplied from a mobile phone terminal or the like and the frequency correction voltage V _ADJ supplied from the operational amplifier U2 to the cathode of the diode D1 _produce D1 is the voltage difference V _AFC - becomes AC equivalent capacitance value corresponding to the V _ADJ, whereby the frequency of the oscillator 20 is controlled.

On the other hand, when the output of the temperature sensor 11 of the temperature compensating section 20 is obtained in accordance with the ambient temperature, a DC voltage V _COMP is generated in the temperature compensating voltage generating circuit 12. By applying this DC voltage to the variable capacitance diode D2 via the AC signal blocking and DC voltage application resistor R3,
The AC equivalent capacitance of the variable capacitance diode D2 changes, the oscillating frequency of the oscillating unit is controlled, and operation is performed so that the frequency of the oscillating unit is kept within a specified value with respect to a temperature change.

The operation of the monitoring unit 22 will be described. This circuit outputs the temperature signal of the temperature sensor 11 to the comparators U3 and U4.
And a function of prohibiting data writing to the EE-PROM when the temperature is lower than the minimum temperature or higher than the maximum temperature set in advance by these comparators. Here, for example, if a diode exhibiting a semiconductor PN junction band gap voltage change (about -2 mV / ° C.) is used as the temperature sensor 11, the output voltage of the temperature sensor 11 changes at a constant rate as the temperature rises as shown in FIG. , The characteristic is reduced.
A voltage V _SENS1 corresponding to the lowest temperature T1 of the operating temperature range of the voltage-controlled temperature-compensated crystal oscillator is generated using a regulator or the like, and supplied to the negative input terminal of the comparator U3.
If the temperature sensor voltage is higher than _VSENS1 , the output of U3 is set to the "HIGH" level. Similarly, in addition to the positive input terminal of the voltage V _{SENS2 wo} voltage comparator U4, which corresponds to the maximum temperature T2 of the movable temperature range, the output of if the sensor output voltage is lower than V _SENS2 U4 is "HIGH"
Set to be level. Therefore, the output of the NAND gate U5 becomes the “HIGH” level only when the temperature sensor output voltage is in the range between V _SENS1 and V _SENS2, and the output becomes EE.
-Give output permission to the PROM.

Next, an example of an operation of synchronizing a carrier frequency with a base station frequency in a portable telephone terminal using the present VCTCXO will be described. The output of the oscillating unit of the VCTCXO is applied as a reference frequency signal to a synthesizer provided in the mobile phone terminal. When the carrier frequency of the terminal is multiplied by a required frequency to generate the carrier frequency of the terminal, the frequency is emitted from the base station and the frequency is emitted from the base station. The V _AFC is generated so that the phase difference becomes zero by comparing with the frequency information received by the reception unit, and is fixed to the voltage V _AFC when synchronized with the base station frequency. In this state, since the reference frequency of the mobile phone terminal is synchronized with the base station, the frequency accuracy can be extremely high, equivalent to the carrier of the base station.

Next, the operation of holding the synchronized state in the memory EE-PROM of the VCTCXO will be described. The voltage V _AFC -V _ADJ applied between the anode and the cathode of the variable capacitance diode D 1 is converted into a voltage V _AD by an operational amplifier U 1, and converted into digital data D _AD by an A / D converter. Is done. On the other hand, after the power is turned on, the memory EE-PROM 14 transfers the digital data stored therein to the RAM,
DISABLE mode - is separated from the RAM in de writing mode - set to de, digital de described above by synchronizing the signal LOCK from the mobile telephone terminal - data D _AD is written through the latch circuit 13. Through this series of operations, control voltage information necessary to obtain an oscillation frequency that matches the frequency of the base station by the VCTCXO at that time is stored in the memory EE-PROM 14. This information is based on the voltage V _AFC − applied to the variable capacitance diode D1.
It will be stored as information associated with V _ADJ . That is, at a certain temperature, the control of the VCTCXO is set so that the oscillation frequency of the VCTCXO is synchronized with the high-stability frequency signal transmitted from the base station. The voltage to be applied to the variable capacitance diode D1 has been corrected so that the VCTCXO outputs the desired oscillation frequency even if the reactance of the other peripheral components becomes different from that at the time of the initial shipment adjustment due to aging. Become.

Thereafter, when the portable telephone terminal is turned on,
The data stored in the memory EE-PROM 14 is read from the EE-PROM to the RAM as described above,
By performing the / A conversion and applying it to the variable capacitance diode D1 as the frequency correction voltage V _ADJ , the oscillation frequency of the VCTCXO is maintained with high accuracy. As described above, if the control voltage information stored in the EE-PROM is updated by the AFC function, the frequency accuracy of the VCTCXO can be maintained within a constant value regardless of aging. Even if the power of the mobile phone terminal is turned off, the EE-PROM
Needless to say, this data is retained. FIG. 2 is a functional block diagram showing the entire VCTCXO circuit described above, which helps to understand the overall operation.

Next, the above-mentioned operation will be described with reference to a flowchart shown in FIG. 4 and specific numerical values in order to better understand the present invention. First, the voltage range and the center value V _{AFC (0)} of the frequency synchronization voltage V _AFC are generally determined. For example, the voltage range is set to +0.5 V ≦ V _AFC ≦ + 2.5 V,
The center value is V _{AFC (0)} = + 1.5V. The voltage gains of the operational amplifiers U1 and U2 are both 1, that is, G1 = G2
= 1, the input voltage V _{AD of the} A / D converter
The relationship between V _{AFC and} V _ADJ applied to the variable capacitance diode D1 is V _AD = V _AFC -V _ADJ (1). Further, the output voltage V _{DA of} the D / A converter and the output voltage of the operational amplifier U2, that is, the variable capacitance diode D
The relationship between 1 and the anode applied voltage V _ADJ is as follows: V _ADJ = V _{AFC (0)} -V _DA (2) Initially, VCTCXO is centered on V _{AFC
With AFC (0)} = + 1.5V, the oscillation frequency is adjusted to the standard frequency with high accuracy. This frequency adjustment is F
_{This is performed} by inputting digital data from the _ADJ terminal and writing the digital data into the EE-PROM 14 via the latch circuit 13 when the oscillation frequency reaches the desired frequency. The output voltage of the D / A converter at this time is V
Assuming that _DA = + 1.0 V, V _ADJ = + 0.
5 V, V _AD = + 1.0 V from equation (1). The above-described adjustment is easier to automate and improve the accuracy than the operation of turning the trimmer capacitor.

Thereafter, if the VCTCXO is not subjected to thermal shock or the like, does not undergo a frequency change due to aging, and if the VCTCXO reproduces the oscillation frequency at the time of its adjustment even after being mounted on a mobile phone terminal, the AFC Voltage is V
Synchronized at _AFC = + 1.5 V, and digital data corresponding to the input voltage V _AD = + 1.0 V of the A / D converter, which is the same as the above value, is written into the memory EE-PROM 13 by the synchronization signal from the portable telephone terminal. It is. In this case V _AFC
Is the frequency adjustment condition of VCTCXO, that is, V _AFC = V _{A
Since FC (0)} is the same value as +1.5 V, EE−
The data in PROM 14 does not change.

On the other hand, if the oscillation frequency of the VCTCO changes due to a change in the characteristics of a built-in crystal unit or peripheral components due to thermal shock or aging, the synchronization voltage V
_AFC changes. Assuming that the AFC synchronization voltage for matching the base station frequency becomes V _AFC = + 1.7 V, from equation (1), V _AD = + 1.7 V −0.5 V = +
V _AD = 1 1.2V, and the on behalf before the V _AD = 1V.
The value of 2V is converted into digital data and converted to EE-PR
Written to OM14. Therefore, in order to obtain frequency synchronization at V _AFC = V _{AFC (0)} = + 1.5 V when the power is turned on thereafter, from the equation (2), V _ADJ = 1.5 V−1.
2V = + 0.3V. By substituting this value into equation (1), V _AD = 1.5 V-0.3 V = 1.2 V, and synchronization is obtained. Therefore, the center value V _AFC = V _{AFC (0)} = + 1.5V
The frequency synchronization is obtained with the synchronization voltage of, and the frequency is automatically corrected.

The VCTCXO according to the present invention is in a state in which it is synchronized with an external reference frequency even if the crystal oscillator or other parts change due to thermal shock or aging, and as a result, the oscillation frequency of the VCTCXO shifts. Has a function of calibrating the self-oscillation frequency and storing the state of the self-oscillation frequency, so that there is no need to stop the oscillator and adjust the oscillation frequency or replace the crystal oscillator as in the conventional case. In addition, since information for synchronizing with the external reference frequency is stored, the communication terminal using the VCTCXO of the present invention can reduce the time until synchronization after turning on the power, and can immediately start communication. The start-up time is much faster than in a system that starts communication after synchronizing with a reference frequency coming from a base station like a system.

As described above, VCT used for mobile phone terminals and the like
Although the present invention has been described based on an example of CXO, the present invention can be similarly applied to other applications, for example, a simple voltage-controlled crystal oscillator (VCXO) having no temperature compensation function. is there. For example, by synchronizing the frequency of a crystal oscillator with an external signal, writing information such as the state at the time of synchronization and control voltage to a semiconductor memory by using a synchronization signal, reading the information when power is turned on, and rewriting this intermittently over time. Even if there is a change, the frequency of the crystal oscillator can be kept within a certain value.

[0023]

Since the present invention is constructed as described above, the VCT which becomes the present invention due to thermal shock, aging, etc.
When the frequency of the oscillating section of the CXO changes, the change can be automatically corrected by a frequency compensation circuit and the state can be stored and updated. It is possible to provide a high crystal oscillator, and it is possible to obtain an extremely effective effect that frequency stability that cannot be achieved by a conventional crystal oscillator can be realized. Further, by constituting the circuit according to the present invention with an IC or the like, there is an excellent effect that a small-sized crystal oscillator having extremely high stability can be provided at low cost.

[Brief description of the drawings]

FIG. 1 is a diagram showing an embodiment of a piezoelectric oscillator having a frequency correction function according to the present invention.

FIG. 2 is a functional block diagram illustrating the entire circuit of FIG. 1;

FIG. 3 is a diagram illustrating an output voltage of the temperature sensor of FIG. 1;

FIG. 4 (a) is a conventional direct temperature-compensated crystal oscillator,
(B) is a circuit diagram of the indirect-type temperature-compensated crystal oscillator.

FIG. 5 is a circuit diagram of a digital temperature-compensated crystal oscillator.

FIG. 6 is a circuit diagram of a general voltage-controlled crystal oscillator.

FIG. 7 is a circuit diagram showing a conventional VC-TCXO.

[Explanation of symbols]

Y1 crystal oscillator D1, D2 variable capacitance diode C1, C2 capacitor R1, R2, R3, R4, R5, R6, R7, R8, R
9, R10, R11 ··· resistor U1, U2 ··· operational amplifier U3, U4 ··· voltage comparator U5 · · · NAND gate 10 · · · amplifier 11 · · · temperature sensor 12 · · · temperature compensation voltage generation circuit 13, 15 ..Latch circuit (digital signal path switching and disconnection circuit) 14.EE-PROM (electrically writable and readable semiconductor memory) A / D A.D / A converter (analog signal-digital signal converter) D / A ··· D / A converter (digital signal-analog signal converter) RAM · · · temporary data writable and readable memory V _COMP · · · temperature compensation voltage V _AFC · · frequency control voltage V _{AFC ( 0)} Frequency control center voltage V _ADJ Frequency correction voltage V _SENS1 Temperature sensor output voltage low temperature setting V _SENS2 Temperature sensor output voltage high temperature setting INPUT A / D / RA M..EE-PROM writing data
Data, A / D, RAM switching LOCK: EE-PROM write command signal at frequency synchronization MODE READ / WRITE: EE-PROM read, write mode selection READ ABLE / DISABLE: RAM data can be read, Disable selection F _ADJ DATA ・ ・ Frequency adjustment data input WRITE ABLE / DISABLE ・ ・ Write enable / disable signal

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[Procedure amendment]

[Submission date] May 26, 1997

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] Full text

[Correction method] Change

[Correction contents]

[Document Name] Statement

Patent application title: Piezoelectric oscillator having frequency correction function

[Claims]

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a crystal oscillator used in, for example, a network synchronizer or a portable telephone terminal, and more particularly to a piezoelectric oscillator with a frequency correction device having a frequency correction function added to stabilize the oscillation frequency.

[0002]

2. Description of the Related Art In recent years, there have been remarkable improvements in frequency stability, size reduction, price reduction, and the like of piezoelectric oscillators such as crystal oscillators, which have greatly contributed to the spread of network synchronization devices and portable telephone terminals. The output frequency of a crystal oscillator used as a reference frequency source for communication equipment varies depending on various factors.However, for frequency variations due to changes in conditions such as ambient temperature, power supply voltage, and output load, measures to deal with these variations may be taken. it can. For example, with respect to a temperature change, if a temperature compensation circuit is added to the crystal oscillator, a frequency change due to the temperature change can be kept within a desired value range.

A temperature-compensated crystal oscillator (hereinafter referred to as TCXO) cancels out the temperature-frequency characteristic fluctuation inherent in a crystal resonator by utilizing the well-known phenomenon that the oscillation frequency fluctuates when the load capacitance of the oscillation loop changes. As described above, the load capacitance is controlled with respect to a change in temperature, and there are roughly three compensation methods. The first is a method called direct compensation, in which a compensation circuit is connected in series with a crystal oscillator as shown in FIG. In general, a compensation circuit is a simple combination of a high-temperature section compensation circuit and a low-temperature section compensation circuit connected in series, which is basically composed of a temperature sensor (such as a thermistor) and a capacitor connected in parallel. It is widely used in the field of mobile phones and the like because of its easy implementation. The second is a method called indirect compensation, in which a variable capacitance diode is connected in series with the crystal oscillator as shown in FIG. It is connected to both ends of the diode. In this method, a DC voltage generated in a compensation circuit composed of a thermistor and a resistor is applied to the variable capacitance diode D via the high-frequency blocking resistor R, so that the frequency change of the circuit is limited to the temperature of only the crystal resonator. The temperature characteristic of the crystal oscillator is compensated by making the characteristic reverse to the characteristic. The third is a method called digital type compensation. As shown in FIG. 6, the compensation circuit shown by the second compensation method includes a temperature sensor, a semiconductor memory, an A / D converter, a D / A converter, and the like. This is a compensation method that uses digital processing. These TCs
According to XO, the frequency stability required for the reference frequency source for the terminal of the current mobile phone system (temperature range-2)
(± 2 to 2.5 ppm at 5 to 75 ° C.).

On the other hand, in recent mobile phone systems, higher frequency stability (for example, ± 0.2 ppm) is required as a reference frequency source for a terminal. In order to satisfy this demand, a conventional TCXO having a voltage control function (referred to as a VC-TCXO) is used as a reference frequency source of a mobile phone terminal, and the frequency of a highly stable signal emitted from a base station is changed to the VC-TCXO. The method of keeping the frequency stability of the reference frequency source of the mobile phone terminal at the time of communication extremely high by synchronizing the outputs of the mobile phone terminals is used. Here, VC-TC
Before describing XO, let's talk a little about voltage controlled crystal oscillator (VCXO). The oscillation circuit shown in FIG. 7 is an example of a voltage-controlled crystal oscillator. A variable capacitance diode _CD is inserted into an oscillation loop of a crystal oscillator and an amplifier, and one end of the variable capacitance diode CD is connected to a terminal a via a high-frequency blocking resistor R. One end is connected to the terminal b. Control input terminals a, utilized to circuit oscillation that the oscillation frequency load capacitance changes things and oscillation loop capacity appearing across the variable capacitance diode C _D by the voltage V applied between b changes varies This is a crystal oscillator that controls the frequency. VC-TCXO is T
FIG. 8 is a diagram in which VCXO functions are added to CXO.
FIG. 1 is a configuration diagram showing an example of a conventional VC-TCXO. That is, the VC-TCXO shown in this example includes a temperature compensation circuit Tem including a temperature sensing element section 31, a temperature compensation voltage generation section 32, and a variable capacitance diode D1, a crystal oscillator Y1,
And a variable capacitance diode D2 for applying an external signal voltage.

In a VC-TCXO shown in FIG. 8, a temperature sensing element section 31 senses an ambient temperature, and a voltage generated from a temperature compensation voltage generating section 32 is applied to a variable capacitance diode D1 in accordance with the temperature at that time, and a crystal oscillation is performed. It always cancels out the temperature of the child and generates an almost constant frequency. Further to the variable capacitance diode D2 connected to the crystal oscillator in series, by applying a voltage V _AFC in synchronization with the external reference signal VC-
As for the oscillation frequency of the TCXO, frequency stability with the same accuracy as that of the external reference signal can be obtained. Further, the voltage _VAFC is often used for the purpose of switching the channel frequency.

To facilitate understanding of the present invention, a description will be given of a frequency correction function of a temperature-compensated crystal oscillator with a voltage control function used in a mobile phone terminal of a mobile phone system operated in Japan. In this mobile phone system, an active TCXO is used for communication between a base station and a mobile phone terminal.
Frequency stability (± 2.0 at temperature range -20 to + 75 ° C)
Since a high-precision frequency stability (approximately ± 0.2 ppm) exceeding ± 2.5 ppm) is required, the use of a TCXO having a voltage control function as a reference frequency source in a mobile phone terminal allows a mobile phone to be used. The reference frequency of the terminal (and thus also the carrier frequency) is synchronized with the frequency emitted from the base station. The function of synchronizing the reference frequency of the terminal with the frequency of the base station is generally called an AFC function, and the accuracy of the reference frequency of the mobile phone terminal during communication is kept extremely high.

[0007]

However, in the conventional crystal oscillator, a change in the frequency is unavoidable due to thermal shock or aging of the crystal oscillator due to reflow or the like when the oscillator is mounted on a device. TCXO, VCXO, VC-T incorporating the above quartz oscillator
The oscillation frequency of CXO and the like is shifted. Therefore, in order to correct the frequency deviation due to thermal shock, a frequency adjustment circuit, for example, a trimmer capacitor or the like was inserted into the oscillation loop and readjusted.However, an adjustment process of turning the trimmer is required, which increases the manufacturing cost. There was a problem. Further, there is a drawback that the aging of the frequency due to aging is inevitably performed by adjustment at the time of manufacturing, and maintenance by a user is required. The present invention has been made to solve the above-mentioned problem, and by adding an automatic frequency correction function to the crystal oscillator itself, it is possible to reduce the need for readjustment after assembly and to eliminate the need for maintenance by the user. Another object of the present invention is to provide a VCXO with a frequency correction device or a VC-TCXO with a frequency correction device having extremely high frequency stability against aging.

[0008]

According to a first aspect of the present invention, there is provided a temperature-compensated crystal oscillator with a frequency compensating device according to the present invention, the frequency of which varies according to a control voltage. In the oscillator, storage means for storing the information on the supply voltage is provided, and when the oscillation is stopped and restarted, the information stored in the storage means is read and the supply voltage is controlled so as to correspond to the information. This is a piezoelectric oscillator having a frequency correction function. Claim 2
According to a third aspect of the present invention, there is provided a piezoelectric oscillator having a frequency correction function, wherein the piezoelectric oscillator is synchronized with an external reference oscillation frequency and the state is stored in the storage means.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail based on an embodiment shown in the drawings. FIG. 1 is a block diagram for generally explaining the configuration principle and operation of a VCTCXO in which the present invention is applied to a temperature compensated VCXO. As shown in FIG. 1, a piezoelectric oscillator having a frequency correction function according to the present invention has a piezoelectric oscillation unit 20-1, a temperature compensation unit 20-2, a voltage supply unit 21, and a monitoring unit. It comprises a monitoring unit 22 for monitoring whether it is within the compensation range and an initial voltage setting unit 23. The basic function included in each block will be described. The oscillation unit 20-1 includes an amplifier, a crystal oscillator, and a variable capacitance diode, and controls the voltage between both ends of the variable capacitance diode as is well known. By doing so, the oscillation frequency is controlled. The temperature compensator 20-2 includes a temperature-sensitive element such as a thermistor as described with reference to FIG. 5A, for example, and generates a compensation voltage to be applied to the variable capacitance diode of the oscillator 20-1. It is. The voltage supply unit 21 synthesizes an AFC signal V _AFC supplied from, for example, a portable wireless device or the like equipped with the oscillator and a frequency correction voltage V _ADJ output from an initial voltage setting unit 23 which will be described later, to thereby generate the oscillation unit 20. A control voltage for generating a desired oscillation frequency of -1 is generated, and this control voltage is supplied to the variable capacitance diode of the oscillation section 20-1. The monitoring unit 22 is connected to the temperature compensating unit 20-2.
, It is determined whether or not the temperature at that time is within a predetermined temperature range. When the condition is satisfied, a write enable signal is generated. The remaining initial voltage setting unit 23 includes a writable and readable memory, for example, an EE-PROM.
-1 variable-capacitance diode across the voltage supply unit 21
Has the function of storing digitalized signals in

FIG. 2 is a detailed functional diagram (FIG. 1) of the VCTCXO according to the present invention, which is composed of four blocks 20 to 23 surrounded by chain lines. The configuration of each block will be described in order. The oscillation unit indicated by a chain line 20 includes a crystal oscillator Y1, an amplifier 10, two variable capacitance diodes D1 and D2, and two capacitances C1 and C2 (DC blocking,
An oscillation unit including an AC bypass capacitor) and a temperature compensation unit including a temperature sensor unit 11 and a temperature compensation voltage generation circuit 12 operate as a voltage-controlled temperature-compensated oscillator. A chain line 21 is a control voltage supply unit, and a voltage V _AFC -V _ADJ across the variable capacitance diode D1 is applied to the operational amplifier U1 to obtain a voltage V _AD , which is analogized.・ Digital converter (hereinafter referred to as A / D converter) A
The data is converted into digital data by / D and is temporarily stored in the latch circuit 13.

A chain line 22 is a monitoring unit for monitoring the output voltage of the temperature sensor 11 of the temperature compensating unit 20-1, and includes voltage comparators U3 and U4 and a NAND gate U5.
And the output voltages are V _SENS1 and V _SENS1
If the value is between _SENS2 and _SENS2 , it is a signal that determines whether or not the temperature at that time is within the compensation range based on the information of the temperature sensor and permits data writing to the EE-PROM 14 by a LOCK signal described later. In addition, chain line 2
Reference numeral 3 denotes an initial voltage setting unit which stores data.
E-PROM 14 and latch circuit 15 with readability
, Storage circuit RAM, digital / analog converter D /
A and an operational amplifier U2. At the same time as the power is turned on, digital data information written in advance is read from the EE-PROM 14 (electrically readable and readable memory), and read / write memory R
Write to AM, converts the data by D / A converter to an analog DC voltage value _{V DA.} This voltage is converted to an output voltage V _ADJ by an operational amplifier U2 set to a gain G2 and a reference voltage _{VAFC (0)} , and the AFC / frequency correcting variable capacitor is connected via an AC signal blocking / DC voltage bias resistor R2. Applied to the anode of diode D1.

The overall operation of the above configuration will be described. First, the operation of the oscillating unit 20 will be described. The signal voltage V _AFC for frequency synchronization supplied from a mobile phone terminal or the like is used.
And the operational amplifier U2 and the frequency correction voltage _{V ADJ} supplied to the cathode of the diode D1, the diode D1 becomes AC equivalent capacitance value corresponding to the voltage difference _V AFC _{-V ADJ,} is thereby frequency of the oscillator 20 Controlled.

On the other hand, when the output of the temperature sensor 11 of the temperature compensating section 20 is obtained according to the ambient temperature, a DC voltage V _COMP is generated in the temperature compensating voltage generating circuit 12. By applying this DC voltage to the variable capacitance diode D2 via the AC signal blocking / DC voltage application resistor R3, the AC equivalent capacitance of the variable capacitance diode D2 changes, and the oscillation frequency of the oscillation unit is controlled, It operates to keep the frequency of the oscillating unit within the specified value in response to the change.

The operation of the monitoring unit 22 will be described. This circuit outputs the temperature signal of the temperature sensor 11 to the comparators U3 and U4.
And a function of prohibiting data writing to the EE-PROM when the temperature is lower than the minimum temperature or higher than the maximum temperature set in advance by these comparators. Here, for example, if a diode exhibiting a semiconductor PN junction band gap voltage change (about -2 mV / ° C.) is used as the temperature sensor 11, the output voltage of the temperature sensor 11 changes at a constant rate as the temperature rises as shown in FIG. , The characteristic is reduced.
A voltage V _SENS1 corresponding to the lowest temperature T1 of the operating temperature range of the voltage-controlled temperature-compensated crystal oscillator is generated using a regulator or the like, and supplied to the negative input terminal of the comparator U3. If the temperature sensor voltage is higher than V _SENS1 The output of U3 is set to be "HIGH" level. Similarly, the voltage V corresponding to the maximum temperature T2 of the movable temperature range
_In addition to the positive input terminal of the _SENS2 voltage comparator U4, if the sensor output voltage is lower than V _SENS2 , the output of U4 is set to the “HIGH” level. Therefore, the output of the NAND gate U5 becomes “HIGH” level only when the temperature sensor output voltage is in the range between V _SENS1 and V _SENS2, and gives output permission to the EE-PROM.

Next, an example of an operation of synchronizing a carrier frequency with a base station frequency in a portable telephone terminal using the present VCTCXO will be described. The output of the oscillating unit of the VCTCXO is applied as a reference frequency signal to a synthesizer provided in the mobile phone terminal. When the carrier frequency of the terminal is multiplied by a required frequency to generate the carrier frequency of the terminal, the frequency is emitted from the base station and the frequency is emitted from the base station. The V _AFC is generated so that the phase difference becomes zero by comparing the received frequency information with the reception unit, and is fixed to the voltage V _AFC synchronized with the base station frequency. In this state, since the reference frequency of the mobile phone terminal is synchronized with the base station, the frequency accuracy can be extremely high, equivalent to the carrier of the base station.

Next, the operation of holding the synchronized state in the memory EE-PROM of the VCTCXO will be described. The anode of the variable capacitance diode D1, the voltage applied between the _cathode, V AFC _{-V ADJ} operational amplifier U1
By being converted to a voltage _{V AD,} it is converted by the A / D converter into digital data _{D AD.} On the other hand, after the power is turned on, the memory EE-PROM 14 transfers the digital data stored therein to the RAM, is separated from the RAM in the READ DISABLE mode of the latch 15, is set to the write mode, and is synchronized with the mobile phone terminal. The digital data D described above by the signal LOCK
_AD is written through the latch circuit 13. Through this series of operations, control voltage information necessary to obtain an oscillation frequency that matches the frequency of the base station by the VCTCXO at that time is stored in the memory EE-PROM 14. This information will be stored as information associated with the voltage _V AFC _{-V ADJ} applied to the variable capacitance diode D1. That is, at a certain temperature, the VC
The fact that the control of the VCTCXO is set so that the oscillation frequency of the TCXO is synchronized with the high-stability frequency signal transmitted from the base station means that the temperature-frequency characteristics of the crystal unit or the reactance of other peripheral components at that time point This means that the voltage to be applied to the variable capacitance diode D1 has been corrected so that the VCTCXO outputs the desired oscillation frequency even if the time and the like become different from those at the time of the first shipment adjustment due to aging.

Thereafter, when the portable telephone terminal is turned on,
The data stored in the memory EE-PROM 14 is read from the EE-PROM to the RAM as described above,
/ A conversion, by applying a frequency correction voltage _{V ADJ} to the variable capacitance diode D1, so that the oscillation frequency of the VCTCXO is maintained with high accuracy. As described above, if the control voltage information stored in the EE-PROM is updated by the AFC function, the frequency accuracy of the VCTCXO can be maintained within a constant value regardless of aging. Even if the power of the mobile phone terminal is turned off, the EE-PROM
Needless to say, this data is retained. FIG. 2 is a functional block diagram showing the entire VCTCXO circuit described above, which helps to understand the overall operation.

Next, the above-mentioned operation will be described with reference to a flowchart shown in FIG. 4 and specific numerical values in order to better understand the present invention. First, the voltage range and the center value _{VAFC (0)} of the frequency synchronizing voltage _VAFC are generally determined. For example, the voltage range is set to + 0.5V ≦ _VAFC ≦ +.
2.5 V, and the central value is _{VAFC (0)} = + 1.5 V. The voltage gains of the operational amplifiers U1 and U2 are both 1,
That is, when G1 = G2 = 1, the relationship between the input voltage V _AD of the A / D converter and the applied voltage V _AFC −V _ADJ of the variable capacitance diode D1 is V _AD = V _AFC −V _ADJ (1) . Also, the output voltage V _DA of the D / A converter,
Output voltage of the operational amplifier U2, i.e., the relationship between the anode voltage applied _{V ADJ} of the variable capacitance diode D1 becomes _{V ADJ = V AFC (0)} -V DA (2). Initially, VCTCXO sets VA _AFC to the center value voltage V
_{With AFC (0)} = + 1.5 V, the oscillation frequency is precisely adjusted to the standard frequency. The frequency adjustment inputs the digital data from the F _ADJ pin, when the oscillation frequency is a desired frequency, is carried out by writing the digital data into EE-PROM 14 through the latch circuit 13. Assuming that the output voltage of the D / A converter at this time is V _DA = + 1.0 V, from the equation (2),
_ADJ = + 0.5 V, V _AD = + 1.0 V from equation (1)
Becomes The above-described adjustment is easier to automate and improve the accuracy than the operation of turning the trimmer capacitor.

Thereafter, if the VCTCXO is not subjected to thermal shock or the like, does not undergo a frequency change due to aging, and if the VCTCXO reproduces the oscillation frequency at the time of its adjustment even after being mounted on a mobile phone terminal, the AFC Voltage is V
Synchronization is performed at _AFC = + 1.5 V, and digital data corresponding to the input voltage V _AD = + 1.0 V of the A / D converter, which is the same as the above value, is written into the memory EE-PROM 13 by a synchronization signal from the mobile phone terminal. In this case V
_AFC is the frequency adjustment condition of VCTCXO, that is, V _AFC
= V _{AFC (0} ) = + 1.5V, so that the data in the EE-PROM 14 does not change as a result.

On the other hand, if the oscillation frequency of the VCTCO changes due to a change in the characteristics of a built-in crystal unit or peripheral components due to thermal shock or aging, the synchronization voltage V
_AFC changes. Assuming that the AFC synchronization voltage for matching the base station frequency becomes V _AFC = + 1.7 V, from equation (1), V _AD = + 1.7 V-0.5 V
= + 1.2V next, _{V AD} in place in front of the _V AD = 1V
= 1.2V is converted to digital data and EE
-Written into PROM 14; Therefore, when the power is turned on thereafter, V _AFC = V _{AFC (0)} = + 1.5V
In order to obtain frequency synchronization by using the equation (2), V _ADJ
= 1.5V-1.2V = + 0.3V. By substituting this value into equation (1), V _AD = 1.5V−0.3V =
It becomes 1.2 V and synchronization is obtained. Therefore, the center value V _AFC
= V _{AFC (0)} = Frequency synchronization is obtained with a synchronization voltage of +1.5 V, and the frequency is automatically corrected.

The VCTCXO according to the present invention is in a state in which it is synchronized with an external reference frequency even if the crystal oscillator or other parts change due to thermal shock or aging, and as a result, the oscillation frequency of the VCTCXO shifts. Has a function of calibrating the self-oscillation frequency and storing the state of the self-oscillation frequency, so that there is no need to stop the oscillator and adjust the oscillation frequency or replace the crystal oscillator as in the conventional case. In addition, since information for synchronizing with the external reference frequency is stored, the communication terminal using the VCTCXO of the present invention can reduce the time until synchronization after turning on the power, and can immediately start communication. The start-up time is much faster than in a system that starts communication after synchronizing with a reference frequency coming from a base station like a system.

As described above, VCT used for mobile phone terminals and the like
Although the present invention has been described based on an example of the CXO, the present invention can be similarly applied to other applications, for example, a simple voltage-controlled crystal oscillator (VCXO) having no temperature compensation function. For example, by synchronizing the frequency of a crystal oscillator with an external signal, writing information such as the state at the time of synchronization and control voltage to a semiconductor memory by using a synchronization signal, reading the information when power is turned on, and rewriting this intermittently over time. Even if there is a change, the frequency of the crystal oscillator can be kept within a certain value.

[0023]

Since the present invention is constructed as described above, the VCT which becomes the present invention due to thermal shock, aging, etc.
When the frequency of the oscillating section of the CXO changes, the change can be automatically corrected by a frequency compensation circuit and the state can be stored and updated. It is possible to provide a high crystal oscillator, and it is possible to obtain an extremely effective effect that frequency stability that cannot be achieved by a conventional crystal oscillator can be realized. Further, by constituting the circuit according to the present invention with an IC or the like, there is an excellent effect that a small-sized crystal oscillator having extremely high stability can be provided at low cost.

[Brief description of the drawings]

FIG. 1 is a diagram showing an entire circuit in a functional block diagram.

FIG. 2 is a diagram showing an embodiment of a piezoelectric oscillator having a frequency correction function according to the present invention.

FIG. 3 is a diagram illustrating a temperature sensor output voltage of FIG. 2;

FIG. 4 is a diagram showing an adjustment flow of a piezoelectric oscillator having a frequency correction function according to the present invention.

FIG. 5 (a) is a conventional direct temperature-compensated crystal oscillator,
(B) is a circuit diagram of the indirect-type temperature-compensated crystal oscillator.

FIG. 6 is a circuit diagram of a digital type temperature compensated crystal oscillator.

FIG. 7 is a circuit diagram of a general voltage-controlled crystal oscillator.

FIG. 8 is a circuit diagram showing a conventional VC-TCXO.

[Explanation of Signs] Y1 ··· Crystal Resonator D1, D2 ··· Variable Capacitance Diode C1, C2 ··· Capacitor R1, R2, R3, R4, R5, R6, R7, R8, R
9, R10, R11, resistance U1, U2, operational amplifier U3, U4, voltage comparator U5, NAND gate 10, amplifier 11, temperature sensor 12, temperature compensation voltage generation circuit 13, 15, latch circuit (Digital signal path switching and disconnection circuit) 14..EE-PROM (electrically writable and readable semiconductor memory) A / D..A / D converter (analog signal-digital signal converter) D / A..D / A converter (digital signal - analog signal converter) RAM · · temporary data writing, readable memory V _COMP · · temperature compensation voltage V _AFC · · frequency control voltage V _{AFC (0)} · · frequency control center voltage V _ADJ · frequency correction voltage V _SENS1 ... temperature sensor output voltage low temperature side set value V _SENS2 ... temperature sensor Input voltage high-temperature side set value INPUT A / D / RAM ··· EE-PROM write data, A / D, RAM switching LOCK ··· Frequency synchronous EE-PROM write command signal MODE READ / WRITE · · EE-PROM read and write Mode select READ ABLE / DISABLE ... Readable / unable to read data from RAM F _ADJ DATA ... Frequency adjustment data input WRITE ABLE / DISABLE ... Write enable / disable signal

Claims (2)

[Claims] 1. A voltage-controlled piezoelectric oscillator whose frequency changes in accordance with a control voltage, wherein a storage means for storing information about the supply voltage is provided, and the information is stored in the storage means when oscillation stops and restarts. A piezoelectric oscillator having a frequency correction function, which reads information and controls a supply voltage so as to correspond to the information. 2. A piezoelectric oscillator having a frequency correction function, wherein the piezoelectric oscillator according to claim 1 synchronizes with an external reference oscillation frequency and stores this state in said storage means.