1. Field of the Invention
The present invention relates to an oven controlled crystal oscillator for high stability, using a thermostat (hereunder, “highly stable oscillator”), and particularly relates to a highly stable oscillator which is effective in heat utilization.
2. Description of the Related Art
In this kind of crystal oscillator, the operating temperature of the crystal resonator is maintained constant by an oven controlled using a thermostat so as to increase the frequency stability. For example, it has been used for base stations for optical communication. Recently, miniaturization has spread even to industrial crystal oscillators, requiring corresponding miniaturization.
FIG. 7 is diagrams for explaining an example of a conventional crystal oscillator, FIG. 7A being a fragmentary vertical sectional view of a highly stable oscillator, and FIG. 7B being a conceptual diagram showing a procedure for inserting a crystal resonator into a thermostat.
As shown in FIG. 7A, this highly stable oscillator comprises a crystal resonator 2, an oven controlled by a thermostat (hereunder, “thermostat”) 3 which accommodates the crystal resonator 2, and oscillating elements 4 and temperature control elements 5, which are all mounted on a first circuit board 1a and a second circuit board 1b. The first circuit board 1a is supported by metallic pins 7a serving as external terminals which are insulated from and pierce a metallic base 6. The second circuit board 1b is supported by metallic pins 7b placed on the first circuit board 1a. In addition, thermostat 3 is supported by a leg 7c with the second circuit board 1b. 
As shown in FIG. 7B, the crystal resonator 2 is sealed in a resonator container 9 composed of a metallic case out of which is led a pair of lead wires 8 for, for example, an AT cut or an SC cut crystal piece. The thermostat 3 comprises a heating coil 11 coiled around the periphery of a thermostat mainframe (metallic cylinder) 10 which accommodates the crystal resonator 2 therein. Alternatively, the heating coil 11 may be coiled directly around the resonator container 9 of the crystal resonator 2. The thermostat 3 is arranged in the center of one principal plane of the second circuit board 1b and adhered to the second circuit board 1b by an adhesive 12.
Particularly in the case where the crystal resonator 2 is vacuum sealed, the temperature of the crystal piece is determined by the radiant heat from the resonator container 9. Therefore the heat capacity is reduced compared to the case where a gas is additionally sealed in. On the other hand, if the thermostat mainframe 10 is used, the heat capacity is increased by the thermostat mainframe 10 itself. Therefore there is no oversensitive response with respect to rapid temperature variations, enabling prevention of momentary fluctuations in the oscillating frequency, and an increase in the stability. However, the starting characteristics of the crystal resonator are reduced. Moreover, in the case where the heating coil is coiled directly around the resonator container 9, then for example if the subsequently sealed crystal resonator 2 has some defect, the heating coil 11 and the like must also be discarded. However, if the thermostat mainframe 10 is used, only the defective crystal resonator 2 need be exchanged, which is economically convenient.
The oscillating elements 4 constitute an oscillation circuit together with the crystal resonator 2, and are arranged on the other principal plane of the second circuit board 1b. The temperature control elements 5 also contain at least a thermistor 5a as the temperature sensitive element, and together with a transistor constitutes a temperature control circuit which controls the temperature of the thermostat 3. The members except for the thermistor 5a are arranged on the outside surface of the first circuit board 1a. The temperature control circuit detects the temperature inside the thermostat 3 by joining the thermistor 5a to the thermostat 3 for example. Then, based on the detected temperature, the power to be supplied to the heating coil 11 is controlled to maintain the temperature inside the thermostat 3 constant. The first and the second circuit boards 1a and 1b, the crystal resonator 2 and the like are covered with a metallic cover 17.
According to such a construction, the operating temperature of the crystal resonator 2 can be kept constant by the thermostat 3, enabling prevention of frequency fluctuations of the oscillation frequency due to temperature variations. In other words, fluctuations in the oscillating frequency based on the frequency temperature characteristics of the crystal resonator 2 can be prevented. Moreover, since the second circuit board 1b mounted with the oscillating elements 4 is arranged on the thermostat 3, frequency fluctuations due to the temperature characteristic of the circuit element itself can be prevented.
Therefore, such a crystal oscillator is employed particularly for industrial crystal oscillators where, by increasing the frequency stability, for example the frequency deviation can be made 0.05 ppm or less.
Moreover, in the conventional crystal oscillator as shown in FIG. 7A, the second circuit board 1b is mounted with the oscillating elements 4 and electrically connected to the first circuit board 1a by the metallic pins 7b, and the metallic pins 7a of the first circuit board 1a are led out as the external terminals. Therefore, the metallic pins 7b of the second circuit board 1b are not led out directly to the outside, enabling prevention of heat release to the outside.
However, in the conventional highly stable oscillator having the above construction, the second circuit board 1b is jointed onto the thermostat 3 by the adhesive 12. Accordingly, the heat from the heating coil 11 of the thermostat 3 is blocked by this adhesive 12, so that the thermal efficiency with respect to the oscillating elements 4 on the second circuit board 1b is impaired. The general adhesive 12 is poor in pyroconductivity.
Moreover, even if a thermo-conductive adhesive 12 is applied, it is difficult to control the thickness of the adhesive 12, thus generating an uneven coating, so that the heat distribution of the second circuit board 1b becomes unstable. Furthermore, after the metallic cover 17 is covered over the thermostat 3, convection occurs inside the cover 17, which generates temperature variations in the oscillating elements 4 on the second circuit board 1b. Particularly, together with the thermo-sensor (thermistor 5a), for example a voltage variable capacitative element as the highly temperature dependent highly heat sensitive element is directly affected, so that its characteristics are reduced. Furthermore, the thermostat 3 is manufactured by coiling the heating coil 11 around the thermostat mainframe 10. Hence manufacturing costs are increased.