JPH02149106A - Digital temperature compensated oscillator - Google Patents

Abstract

PURPOSE:To reduce the quantity of digital compensation and to reduce the capacity of a memory by performing temperature compensation based on the combination of crystal resonators in advance at the time of performing the temperature compensation in digital manner. CONSTITUTION:The temperature compensation is performed by selecting the address of a storage element 13 in which the data of temperature compensation is stored after applying digital conversion on the output of a temperature sensor 11 by an analog-digital converter 12, and controlling a voltage capacity conversion element 15 connected to the crystal resonator 16 after applying analog conversion on output data by a digital-analog converter 14. Here, the crystal resonator 16 is used by combining the crystal resonator of AT cut having cubic temperature characteristic and the crystal resonator of BT cut having quadratic temperature characteristic. In such a way, it is possible to perform the temperature compensation with a few of memory capacity and with high accuracy.

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field of the Invention) The present invention relates to a digital temperature compensated oscillator that digitally compensates for changes in oscillation frequency due to temperature changes.

(Technical background of the invention and its problems) Recently, crystal oscillators have been widely used as standards for frequency, time, etc. Incidentally, a crystal resonator used in a crystal oscillator generally has a temperature coefficient, and its frequency changes with changes in temperature. For example, AT-cut crystal resonators, which are common at frequencies of several MHz to tens of MHz, are
In the figure, the temperature coefficient has a cubic curve shape as shown by curve A, and its inflection point is about 26°C.

On the other hand, as electronic equipment becomes more precise, requirements for frequency stability tend to become stricter even for crystal oscillators. A crystal oscillator that meets these requirements has an oscillation circuit housed in a thermostatic chamber, but the use of a thermostatic chamber increases the size and power consumption, and it takes a long time until the frequency stabilizes when the power is turned on. It takes a long time to process, and there is also a reliability problem because the parts are exposed to a relatively high temperature of about 70°C.

For this purpose, some devices connect a temperature detection element such as a thermistor to a crystal resonator and perform temperature compensation by changing the reactance of the element. However, in such a device, the frequency stability is 10 times worse than that in the above-mentioned device using a constant temperature bath.

For this purpose, a digital temperature compensated oscillator having a configuration as shown in FIG. 4, for example, is known.

In this oscillator, the detection output of the temperature sensor 1 is digitally converted by an analog-to-digital converter 2, and an address of a storage element, that is, a memory 3 is selected and accessed based on this digital output.

Data for compensating for changes in the oscillation frequency of the oscillator due to temperature changes is written in advance in this memory 3.

Then, the output data of the memory 3 is given to a digital-to-analog converter 4 to be converted into an analog signal.

This analog signal is then applied to the voltage capacitance conversion element 5, for example a varicap, to control its capacitance. The voltage capacitance conversion element 5 is connected to the crystal resonator 6 of the oscillation circuit 7, and the oscillation frequency is minutely varied to perform temperature compensation.

By the way, with something like this, for example, curve A in Figure 3
When temperature compensation is performed on an AT-cut crystal resonator that has a cubic temperature characteristic as shown in , the amount of compensation increases especially in the low temperature region below 0°C and the high temperature region above 40°C, and the amount of compensation increases. In order to perform temperature compensation smoothly in a state where the temperature is large, it is necessary to make the temperature steps fine, and as a result, there is a problem that a large capacity memory is required.

(Object of the Invention) The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a digital temperature compensation oscillator that can perform temperature compensation with high accuracy with a small memory capacity.

(Summary of the Invention) The present invention converts the output of a temperature sensor into digital data using an analog-to-digital converter, selects the address of a memory element that stores temperature compensation data, and converts this output data into analog data using a digital-to-analog converter. In this device, temperature compensation is performed by controlling a voltage-capacitance conversion element connected to a crystal resonator, in which the crystal resonator is an AT-cut crystal resonator having a cubic curve-shaped temperature characteristic and a quadratic curve-shaped crystal resonator. It is characterized by a combination of a BT-cut crystal resonator having temperature characteristics similar to the above.

(Example) Hereinafter, an example of the present invention will be described in detail with reference to the block diagram shown in FIG.

In the figure, 11 is a temperature sensor, for example, a thermistor whose resistance value changes according to temperature changes. Reference numeral 12 denotes an analog-to-digital converter which is supplied with analog signal temperature data corresponding to the detected temperature from the temperature sensor 11, and converts the analog signal into a predetermined number of digital signal temperature data.

Then, in step 7, the address of the storage element 13, such as an EP-ROM, is selected based on the digitized temperature data.

Temperature compensation data corresponding to the above-mentioned temperature data is stored in each address of this EP-ROM 13 in advance. The temperature compensation data read from the selected address is supplied to the digital-to-analog converter 14 for analog conversion and applied to the voltage capacitance conversion element 15 to control its capacitance. This voltage capacitance conversion element 15 is connected to the crystal resonator 16 of the crystal oscillator 17 to minutely vary its oscillation frequency, thereby compensating for changes in the oscillation frequency due to temperature changes.

Note that the crystal resonator 16 has a curve A in FIG. 3, for example.
The AT plate 18a, which is an AT-cut crystal resonator, has a frequency-temperature characteristic as shown in FIG.
The BT plate 16b, which is a crystal oscillator, is connected in parallel, and the peak temperature of the BT plate is approximately -35°C, and the inductance of the BT plate 16b is approximately twice that of the AT plate 16a, so that the temperature is low. The transition from the resonant frequency of the AT plate 26a to the resonant frequency of the BT plate is made smooth in the region.

In addition, in FIG. 3, curve C shows the frequency temperature characteristic when the AT board 16a is connected to an actual oscillation circuit, and the curve B
It shows the frequency-temperature characteristics when the T-plate 16b is connected to an actual oscillation circuit.

Further, in FIG. 3, the curve E indicates the AT plate 16a,
The frequency-temperature characteristics of the oscillation circuit connected in parallel with the BT board 16h are shown, and region F shows the frequency-temperature characteristics of the digital temperature-compensated oscillator of the above embodiment in which the voltage-capacitance conversion element 15 is controlled by the output of the analog-to-digital converter 14. It shows the frequency fluctuation range of frequency temperature characteristics.

With this configuration, the crystal oscillator 16 has an AT plate 16a, which is an AT-cut crystal oscillator, and a BT plate 16b, which is a BT-cut crystal oscillator, connected in parallel, and the peak temperature of the BT plate is approximately -35°C, and the inductance of the BT plate 16b is approximately twice the inductance of the AT plate 16a to ensure a smooth transition from the resonance frequency of the AT plate 16a to the resonance frequency of the BT plate in the low temperature region. For example, as shown in curve C shown in Fig. 3, relatively good temperature characteristics can be obtained essentially in the medium and low temperature ranges below 40°C, and for example, ±2 in the range of -40°C to 60°C.
Good frequency-temperature characteristics on the order of ppm can be obtained.

Then, by roughly performing temperature compensation with the output of the digital-to-analog converter for the medium temperature range and low temperature range where relatively good frequency-temperature characteristics can be obtained, and in the high-temperature range where frequency-temperature characteristics are deteriorated, finely. For example, as shown by F in Figure 3, ±
0. Good frequency-temperature characteristics of 5 ppm or less can be obtained, and power consumption can be significantly reduced compared to those using a constant temperature bath or the like. Note that the present invention is not limited to the above embodiments, and in the above embodiments, A
The T plate and the BT plate were combined to compensate for the frequency temperature characteristics in the medium and low temperature ranges, but for example, as shown in Fig. It is also possible to flatten the frequency-temperature characteristics in advance by combining a crystal oscillator, and then perform temperature compensation using the output of the digital memory.

(Effects of the Invention) As detailed above, according to the present invention, when temperature compensation is performed digitally, the temperature compensation is performed in advance by aligning the crystal oscillator, thereby reducing the amount of digital compensation. It is possible to provide a digital temperature compensated oscillator that can reduce memory capacity.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing one embodiment of the present invention, FIG. 2 is a diagram of frequency-temperature characteristics explaining another embodiment of the present invention, and FIG. 3 is a diagram explaining frequency-temperature characteristics of the present invention. FIG. 4 is a block diagram of a conventional digital temperature compensated oscillator. l 3 φ 14 ・ 15 φ l 6 ・ 6a 6b l 7 ・Storage element Digital-analog converter Voltage capacitance conversion element Crystal resonator AT board BT board Oscillation circuit 11...Temperature sensor 12...Analog-digital Converter No. 301

Claims (1)

[Claims] A temperature sensor that detects temperature; an analog-digital converter that converts the output of the temperature sensor into a digital signal having a value corresponding to the detected value; and a digital output of the analog-digital converter. a memory element that selects an address and outputs temperature compensation data stored in advance; a digital-to-analog converter that converts the output data of this memory element into an analog signal; a voltage-capacitance conversion element that is controlled; a crystal resonator connected to the voltage-capacitance conversion element; and an oscillation circuit that constitutes an oscillator together with the crystal resonator, wherein the crystal resonator has a cubic curve shape. An AT-cut crystal resonator with temperature characteristics of , and a BT with quadratic temperature characteristics.
A digital temperature compensated oscillator featuring a combination of cut crystal resonators.