Recently, there is a sharp rise in demand for portable electronic equipment, and a compact and accurate crystal oscillation device for generating a reference clock signal is indispensable to such electronic equipment.
The oscillation frequency of a crystal oscillator in a crystal oscillation device has a temperature characteristic including cubic and linear components derived from a quartz oscillator used in the crystal oscillator. Specifically, with the abscissa indicating the ambient temperature T.sub.a and the ordinate indicating the oscillation frequency f as is shown in FIG. 26(a), the oscillation frequency f of a crystal oscillator whose temperature characteristic is not compensated is represented as a substantially cubic curve 101 with a difference of approximately 10 ppm through 30 ppm between the maximum value and the minimum value. Herein, the ambient temperature T.sub.a, is assumed to be approximately -30.degree. C. through +80.degree. C. Accordingly, when an ideal control voltage curve 102 as is shown in FIG. 26(b), with the abscissa indicating the ambient temperature T.sub.a and the ordinate indicating a control voltage Vc, is generated and applied to the crystal oscillator, df/dT.sub.a can be zero and the oscillation frequency f can be substantially independent of the temperature as is shown in FIG. 26(c).
The temperature characteristic can be compensated, for example, as follows: A varactor diode (i.e.,a variable capacity diode) serving as a frequency adjustment device is connected with the crystal oscillator, and a control voltage having a cubic and linear temperature characteristic for compensating the temperature characteristic of the crystal oscillator is applied to the varactor diode. Thus, the temperature characteristic of the oscillation frequency can be stabilized.
Actually, it is technically very difficult to generate a control voltage Vc having the ideal temperature characteristic as is shown in FIG. 26(b). Therefore, in general, a control voltage having a pseudo cubic temperature characteristic is generated by any of various methods so as attain the temperature compensation of the oscillation frequency.
Now, a conventional temperature compensating crystal oscillation device disclosed in Japanese Laid-Open Patent Publication No. 8-288741 will be described with reference to accompanying drawings.
FIG. 27 shows the functional block configuration of the conventional temperature compensating crystal oscillation device. In the temperature compensation of this crystal oscillation device, the cubic and linear temperature characteristic of the crystal oscillator is divided into plural temperature regions, and voltages in the respective divided temperature regions represented as a function of the temperature are subjected to polygonal lines approximation to obtain temperature lines.
Specifically, a memory 111 of FIG. 27 stores each divided temperature region, a temperature coefficient (proportional coefficient) of the temperature line in the temperature region, and a voltage value at room temperature on the temperature line of each region of the voltage line. Voltage line data corresponding to the ambient temperature detected by a temperature sensor circuit 112 is selectively read from the memory 111, and a predetermined control voltage is generated in an amplifier circuit 113 on the basis of the read control voltage data. The thus generated control voltage is applied to a voltage control crystal oscillator 114, so that the oscillation frequency can be stabilized through the temperature compensation of the oscillation frequency.
Furthermore, as is shown in FIG. 28(a), the temperature sensor circuit 112 performs the polygonal lines approximation by using A/D conversion. Therefore, frequency skip, namely, temporary discontinuity of the voltage lines, is caused between temperature regions as is shown in FIG. 28(c). In order to avoid this frequency skip, a sample and hold circuit 115 is interposed between the amplifier circuit 113 and the voltage control crystal oscillator 114, so as to make the frequency vary smoothly with time.
However, since such a conventional temperature compensating crystal oscillation device uses the A/D conversion for the polygonal lines approximation for generating the control voltage to be used for the temperature compensation, a quantum noise is unavoidably caused, and the frequency skip cannot be avoided in principle. Moreover, a clock signal generator is indispensable, and hence, there arises a problem of a clock noise. In addition, it disadvantageously takes time to stabilize the oscillation frequency after the actuation due to a time constant of the sample and hold circuit 115.
Furthermore, in measurement and adjustment of the temperature characteristic, the temperature characteristic of the oscillation frequency of the crystal oscillation device is measured with the ambient temperature changed discretely in order to compensate the temperature characteristic. Therefore, an error can be caused in the adjustment itself. In order to reduce the error, it is necessary to increase the number of the divided temperature regions, which leads to another problem that the storage capacity of the memory 111 is increased.
The object of the invention is eliminating the frequency skip from the control voltage itself and easing the adjustment of the temperature compensation.