JPH05218854A - Digital temperature compensating oscillator - Google Patents

Abstract

PURPOSE:To provide the subject oscillator which can work with a power supply of low voltage like 3V or so and can secure the oscillation output of high purity. CONSTITUTION:A time constant circuit 14 contains a capacitor 14b which is connected in series to a variable impedance 13 that varies in response to the temperature detected by a temperature sensor 12 connected thermally to a crystal resonator 11a of a crystal oscillator 11. The charging voltage of the capacitor 14b is applied to the input of a 1st or 2nd comparator 15 or 16, and at the same time the different levels of voltage divided by a resistance voltage dividing circuit 17 ere applied to each other input of both comparison. Then a flip-flop 18 is set and reset with the output of comparison. The output of the flip-flop 18 is applied to a counter 20 which counts the oscillation frequency of the oscillator 11 as a gate signal. The count value of the counter 20 is given to a temperature compensating circuit 21 which stores the compensation data to compensate the change of the oscillation frequency against the temperature change of the oscillator 11. Then the temperature compensation is applied to the oscillation frequency based on the compensation data corresponding to the count value of the counter 20.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital temperature-compensated crystal oscillator suitable for low voltage operation and capable of obtaining an oscillation output with a small noise component.

[0002]

2. Description of the Related Art Generally, a crystal oscillator using a crystal oscillator is known as an oscillator whose frequency is stable with respect to temperature. The crystal oscillator has extremely excellent chemical and physical characteristics as a piezoelectric body, has a remarkably high Q as the oscillator, and has good frequency stability. However, in recent years, stricter frequency accuracy and stability are required in various electronic devices, and for example, a crystal oscillator used as a frequency standard for a mobile station of a car telephone, which is currently planned, has a temperature range of -30 ° C to 80 ° C. It is required that the deviation is within ± 2ppm with respect to the temperature change. On the other hand, in crystal oscillators, the largest factor that changes the frequency when the oscillation conditions of the oscillation circuit are constant is temperature change. For example, in the case of the most commonly used thickness slip crystal oscillator, − It shows a substantially cubic curve-shaped frequency change with respect to a temperature change of 30 ° C. to 80 ° C. As a result, a frequency error of about 40 ppm occurs. Conventionally, as an oscillator with a good rate of frequency fluctuation with respect to temperature change, that is, a frequency-temperature characteristic, an oven-type crystal oscillator in which a crystal unit is housed in a constant temperature oven, a thermistor and varicap circuit network are connected in series There were oscillators by a connected indirect temperature compensation system, oscillators by a direct temperature compensation system in which a parallel circuit of a capacitor and a thermistor was connected in series to a crystal unit. However, the oven-type crystal oscillator consumes a large amount of power and has a problem that it takes a long time to stabilize the temperature in the constant temperature bath after the power is turned on. Further, both the indirect compensation method and the direct compensation method using the thermistor are approximate compensation, and there is a problem that even if the adjustment is sufficiently performed, a considerable frequency error remains uncompensated.

Further, there has been proposed a digital temperature compensation oscillator in which the relationship between the ambient temperature of the crystal unit and the frequency deviation is measured in advance, the compensation data is stored in a digital memory, and the temperature is compensated according to the compensation data. .. According to such a digital temperature-compensated oscillator, the power-on starts up quickly, and if a memory with a large capacity is used for storing compensation data, short-term frequency stability can be significantly improved and accurate temperature compensation can be performed. It becomes possible. On the other hand, in such a digital temperature-compensated oscillator, it is desired that it can be operated at a low voltage of about 3 V and that the oscillation output has a high purity, for use in, for example, a portable electronic device. By the way, as a temperature sensor used in such a digital temperature compensation oscillator, a temperature sensor is used to convert the temperature into a voltage by utilizing the temperature dependence of the voltage drop of the PN junction of the semiconductor, or the oscillation frequency changes greatly according to the temperature. There is a device that converts the temperature into a frequency using a special crystal oscillator.
However, in the case of converting the temperature into a voltage like the former, a temperature change in the operating temperature range, for example, -30 ° to 80 °, corresponds to a voltage change of about 2V when the power supply voltage is 3V. The resolution is about 2 mV for a 10-bit digital signal. Therefore, such a system is easily affected by noise and is unrealistic.
Therefore, especially when operating with a low-voltage power supply, it is desirable to use a method that converts the temperature into a frequency in order to increase the temperature resolution. According to such a method, it is necessary to lengthen the gate time of the counter that measures the frequency. The measurement accuracy can be easily improved by. However, in such a device, it is necessary to thermally couple the oscillation element of the oscillation circuit for temperature detection and the crystal oscillator of the crystal oscillator whose temperature is to be controlled, and the oscillation output of the oscillation element causes noise in the output of the crystal oscillator. There is a problem that it is mixed as a component to reduce the purity of the oscillation output. Further, it is also considered that a change in temperature is detected as a change in oscillation frequency by using a ring oscillator composed of a temperature sensitive resistor whose resistance value greatly changes with temperature and an inverter. FIG. 5 is a block diagram showing an example of such a ring oscillator. That is, three inverters 1, 2 and 3 are connected in series and the output of the final stage is connected to the temperature sensing resistor 4
It returns to the input of the first-stage inverter 1 via. A resistor 5 is inserted in the input of the first-stage inverter 1, and the output of the intermediate inverter 2 is fed back to the input of the first-stage inverter 1 via a capacitor 6. However, such an oscillator has a high dependence on the operating voltage, and the oscillation frequency fluctuates greatly when the power supply voltage fluctuates.Also, even if the resistance value of the temperature sensitive resistor is constant, the oscillation frequency changes due to temperature changes. Therefore, there is a problem in using it for temperature detection. Furthermore, the temperature sensor must be thermally coupled to the crystal unit of the crystal oscillator so that the temperature of the crystal unit of the crystal oscillator can be accurately measured.To this end, the temperature sensor is installed in the airtight container of the crystal unit. Alternatively, the structure is such that it is brought into close contact with the airtight container. However, an alternating current is applied to the temperature sensor placed close to the crystal unit in this way,
Alternatively, when an alternating current such as a charging / discharging current flows, an alternating current flowing through the temperature sensor affects the oscillation output of the crystal unit, which increases a noise component, resulting in a decrease in the purity of the oscillation output. In order to reduce such noise components, it is desirable that a direct current be passed through the temperature sensor to detect a temperature change.

[0004]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances and is a digital temperature compensation oscillator which can be operated by a power source of a low voltage of about 3 V and which can improve the purity of oscillation output. It is intended to provide.

[0005]

According to the present invention, a variable impedance is changed according to a temperature detected by a temperature sensor thermally coupled to a crystal oscillator of a crystal oscillator, and a capacitor is connected in series to the variable impedance. The charging voltage of the capacitor of the time constant circuit is applied to one input of the first and second comparators, and different voltages divided by the resistance voltage dividing circuit are applied to the other inputs of these comparators, and The flip-flop is set and reset by the comparison outputs of the first and second comparators, and when the charge voltage of the time constant circuit reaches a predetermined maximum voltage by the output of this flip-flop, discharging is started and the predetermined minimum voltage is reached. Controls the charge / discharge switch to start charging when it reaches and gates the output of the flip-flop to the counter that counts the oscillation frequency of the crystal oscillator. Temperature compensation circuit for compensating the oscillation frequency of the crystal oscillator by the compensation data corresponding to the count value. It is characterized in that the temperature is compensated for.

[0006]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below in detail with reference to the block diagram shown in FIG. Reference numeral 11 in the figure is a crystal oscillator, which is provided with a crystal oscillator 11a and an oscillation circuit 11b that oscillates using the resonance frequency of the crystal oscillator 11a as a frequency determining element. The crystal oscillator 11a can change the resonance frequency by changing the load capacitance thereof. A temperature sensor 12 is, for example, a polysilicon resistor whose resistance value changes according to temperature. With such a polysilicon resistor, for example, 150
A resistance having a temperature characteristic of about 0 ppm / ° C. can be obtained. Then, the temperature sensor 12 is connected to the variable impedance 13 whose impedance changes according to the detected temperature. The variable impedance 13 is, for example, a current mirror circuit as shown in FIG. In this current mirror circuit, the drains and gates of a pair of P-type FETs 13a and 13b are connected in common, and the gates and sources of the FETs are connected in common in the circuit support on the constant current side. A temperature sensor 12, for example, a temperature-sensitive resistor is inserted in series. Therefore, the impedance of the circuit support on the variable impedance side of the current mirror circuit, that is, the impedance between the drain and the source of the FET 13b changes according to the resistance value of the temperature-sensitive resistor which is the temperature sensor 12. A resistor 14a and a capacitor 14b are connected in series with the variable impedance 13.
Are connected to form a time constant circuit 14. That is, the time constant circuit 14 is connected to the power source Vd via the variable impedance 13 in series.
It is inserted between d and the reference potential. And the time constant circuit 14
The charging voltage of the capacitor 14b is applied to the non-inverting input of the first comparator 15 and the inverting input of the second comparator 16.
A plurality of, for example 3
A resistance voltage dividing circuit 17 in which individual resistors 17a, 17b, 17c are connected in series is inserted, and the first voltage dividing voltage V1 having a high voltage value obtained by the resistance voltage dividing circuit 17 is supplied to the first comparator 15. The second divided voltage V2 having a low voltage value is supplied to the inverting input to the second
To the non-inverting input of the comparator 16. And the first,
The respective comparison outputs of the second comparators 15 and 16 are applied to the flip-flop 18 as reset and set signals. The output of the flip-flop 18 causes the capacitor 14
The on / off control of the discharge switch 19 inserted between the charging side terminal of b and the reference potential is performed. And capacitor 14
When the charging voltage of b rises to the divided voltage V1, the discharge switch 19 is turned on to discharge the charging voltage of the capacitor 14b, and when it drops to the divided voltage V2, the discharge switch 19 is turned off to perform charging. I am trying. Reference numeral 20 is a counter for counting the oscillation frequency of the crystal oscillator 11, and the counter 20 is provided with the output of the flip-flop 18 as a gate signal to perform the counting operation. Then, when the temperature changes, the resistance value of the temperature sensor 12 changes, which changes the output signal of the flip-flop 18, that is, the frequency of the gate signal of the counter 20. The change in the output frequency of the crystal oscillator 11 is negligible with respect to the change in the frequency of the gate signal. Therefore, the count value of the counter 20 corresponds to the temperature detected by the temperature sensor 12. Reference numeral 21 denotes a temperature compensating circuit, which stores compensation data for compensating a change in the oscillation frequency of the crystal oscillator 11 with respect to a temperature change in a digital memory, and oscillates the crystal oscillator 11 according to the compensation data corresponding to the count value of the counter 20. It is designed to compensate for changes in frequency. FIG. 3 is a block diagram showing an example of the temperature compensating circuit 21. For example, an address signal generating circuit 23a that generates an address signal of a plurality of bits corresponding to the count value of the counter 20 and a selective address signal generating circuit 23a. A digital memory 23b to be accessed is provided. The digital memory 23b stores compensation data to be given to the crystal oscillator 11 in order to maintain the oscillation frequency of the crystal oscillator 11 at a constant frequency corresponding to the count value of the counter 20. The output data of the digital memory 23b is given to the decoder 23c, and a plurality of analog switches 23d are provided for selectively controlling ON / OFF by the decode output of the decoder 23c. By the analog switch 23d, a plurality of capacitors 23e are selectively connected to, for example, the crystal oscillator 11a of the crystal oscillator 11 to finely adjust the oscillation frequency and maintain a constant oscillation frequency.

With such a configuration, the temperature sensor 12
Detects the temperature of the crystal unit 11a and applies a detection signal to the variable impedance 13. When the impedance value of the variable impedance 13 changes, the charging current for the time constant circuit 14 also changes accordingly. When the charge voltage of the time constant circuit 14 reaches the predetermined voltage V1, the flip-flop 18 is reset by the output of the first comparator 15 to turn on the charge / discharge switch 19 to discharge the charge of the capacitor 14b. Let it start. When the charging voltage of the time constant circuit 14 drops to a predetermined voltage V2, the flip-flop 18 is set by the output of the second comparator 16 to turn off the charging / discharging switch 19, and the capacitor 1
Charging is started for 4b. Therefore capacitor 1
4b charge / discharge cycle, ie flip-flop 18
The inversion cycle of the output of corresponds to the charging current, and the value of this charging current is the impedance value of the variable impedance 13,
That is, it corresponds to the temperature detected by the temperature sensor 12. Then, the first to detect the predetermined voltages V1 and V2,
Since the second comparators 15 and 16 are supplied with the voltages V1 and V2 obtained by dividing the power supply voltage Vdd at their one input terminals, the fluctuation of the power supply voltage does not affect the charge / discharge cycle. Then, the oscillation output of the voltage controlled oscillator 11 is counted by the counter using the output of the flip-flop 18 as a gate signal, and the count value is obtained by the temperature compensation circuit 23.
I am trying to give it to. In this temperature compensation circuit 23,
A digital memory 23b storing compensation data to be given to the voltage controlled oscillator corresponding to the count value of the counter, that is, the temperature detected by the temperature sensor is provided, and the oscillation of the crystal oscillator 11 is performed by the compensation data read from the digital memory 23b. The frequency is compensated to control so as to maintain a constant oscillation frequency. However, only a direct current flows through the temperature sensor 12 thermally coupled to the crystal oscillator 11a of the crystal oscillator 11, and the temperature sensor 1
A charging / discharging current flows through the variable impedance 13 whose impedance is changed by the change of the resistance value of 2. Therefore, the variable impedance 13 and the time constant circuit 14 that repeats charging / discharging at a frequency corresponding to the impedance change of the variable impedance 13 need not be thermally coupled to the crystal oscillator 11a, so that the crystal oscillator 11 can be electrostatically connected to the crystal oscillator 11a.
A crystal oscillator that can be electromagnetically shielded sufficiently 11
It is possible to increase the purity of the oscillation output of. Then, the first and second comparators 1 for controlling the charging and discharging of the time constant circuit 14
Since 5 and 16 are provided with the first and second divided voltages V1 and V2 divided by the resistance dividing circuit 17, even if the voltage of the power supply Vdd fluctuates, the charging / discharging cycle is affected. Don't give. Therefore, even if the power supply voltage fluctuates, the counter 2
The frequency of the gate signal applied to 0 can be maintained constant, and accurate counting operation can be performed.

The present invention is not limited to the above-described embodiment. For example, in the above-mentioned embodiment, the combination of a large number of analog switches and capacitors as the temperature compensating circuit 23 has been described, but the present invention is not limited to this. Of course. For example, as shown in FIG. 4, the compensation data provided from the digital memory 23b is converted into an analog signal by a digital-analog converter (DA converter) 23f, and this analog signal is connected in series to the crystal unit 11a. 23 g may be applied to vary the capacitance, and the load capacitance may be controlled to finely adjust the oscillation frequency. Further, although the polysilicon resistor is used as the temperature sensor 12, it is possible to use an appropriate electronic component whose resistance value changes according to the temperature, and the voltage changes according to the temperature like a semiconductor PN junction. The variable impedance may be controlled using an element for controlling the variable impedance.

[0009]

As described above in detail, according to the present invention, a low temperature operation is possible, a digital temperature which is not affected by the fluctuation of the power supply voltage, and which can obtain a high-purity oscillation output. A compensation oscillator can be provided.

[0010]

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention.

FIG. 2 is a circuit diagram showing an example of the variable impedance shown in FIG.

FIG. 3 is a block diagram showing a temperature compensation circuit of the embodiment shown in FIG.

FIG. 4 is a block diagram showing another embodiment of the temperature compensation circuit of the present invention.

FIG. 5 is a circuit diagram showing an example of a ring oscillator.

[Explanation of symbols]

 11 crystal oscillator 11a crystal oscillator 12 temperature sensor 13 variable impedance 14 time constant circuit 15, 16 comparator 17 voltage divider circuit 18 flip-flop 19 charge / discharge switch 20 counter 21 temperature compensation circuit

Claims (1)

[Claims] 1. A crystal oscillator, a temperature sensor thermally coupled to a crystal oscillator of the crystal oscillator, a variable impedance whose impedance changes according to a temperature detected by the temperature sensor, and a capacitor connected in series with the variable impedance. And a first constant circuit and a second constant circuit to which the charging voltage of the capacitor of this time constant circuit is applied to one input and different voltages divided by a resistance voltage dividing circuit are applied to the other input. And a flip-flop that is set and reset by the comparison output of the first and second comparators, and discharged when the charging voltage of the time constant circuit controlled by the output of the flip-flop reaches a predetermined maximum voltage. The output of the flip-flop and the charge / discharge switch that starts charging when the specified minimum voltage is reached A counter for counting the oscillation frequency of the crystal oscillator, which is provided as a clock signal, and compensation data for compensating the change of the oscillation frequency with respect to the temperature change of the crystal oscillator, and storing the compensation data corresponding to the count value of the counter. A digital temperature compensation oscillator, comprising: a temperature compensation circuit that compensates for the oscillation frequency of the oscillator.