1. Field of the Invention
The invention relates to a technical field of a constant-temperature type crystal oscillator (hereinafter called constant-temperature type oscillator) using a lead type crystal unit, and in particular, to a constant-temperature type oscillator which can reduce height size thereof.
2. Description of the Related Art
Constant-temperature type oscillators keep the operational temperatures of their crystal units constant and improve the frequency stability even when an ambient temperature is changed. Therefore, the constant-temperature type oscillators are applied to wireless devices in communication facilities which are, for example, base stations. In recent years, in place of a traditional constant-temperature bath on which heating coils are wound, heating resistors are used as a heat source, to simplify a constant-temperature structure. As one of these, there is a constant-temperature type oscillator using a lead type crystal unit.
FIGS. 2A and 2B are diagrams for explanation of a related art constant-temperature type oscillator. FIG. 2A is a cross-sectional view of the related art constant-temperature type oscillator, and FIG. 2B is a circuit diagram thereof. FIGS. 3A and 3B are diagrams for explanation of a crystal unit of the related art constant-temperature type oscillator. FIG. 3A is a cross-sectional view thereof, and FIG. 3B is a frequency-temperature characteristic diagram thereof.
The constant-temperature type oscillator shown in FIGS. 2A and 2B includes a lead type crystal unit 1, respective circuit elements 4 forming an oscillator output circuit 2 and a temperature control circuit 3, and a circuit substrate 5 on which the circuit elements 4 including the crystal unit 1 are installed thereon. Then, the constant-temperature type oscillator is configured such that the circuit substrate 5 is held with lead wires 8 (so-called airtight terminals) which is made airtight with glass 6 of a base for oscillator (metal base) 7, and those are covered with a cover for oscillator (a metal cover) 9 by resistance welding or the like. In this case, the base for oscillator 7 and a flange 10 projecting on the outer circumference of the cover for oscillator 9 are bonded to hermetically encapsulate it.
As shown in FIG. 3A, in the crystal unit 1, both end parts of a leading electrode 1y extended from an excitation electrode 1x of a crystal element 1A are held by supporters 12a connected to a pair of lead wires (airtight terminals) 12 of a metal base 11. Then, the metal base 11 and a flange 14 of a metal cover 13 are bonded by resistance welding in the same way as described above, to hermetically encapsulate the crystal element 1A. The crystal element 1A is formed as, for example, an SC-cut crystal element or an AT-cut crystal element, and has the frequency-temperature characteristic that approximately 80° C. is an extreme value. For example, in an AT-cut crystal element, the frequency-temperature characteristic shows a cubic curve (curve A in FIG. 3B), and in an SC-cut crystal element, the frequency-temperature characteristic shows a quadratic curve (curve B in FIG. 3B). Incidentally, frequency deviation Δf/f is plotted along the ordinate of the diagram, where f is a frequency at room temperature, and Δf is a frequency difference from the frequency f at room temperature.
The oscillator output circuit 2 is composed of an oscillating stage 2a serving as an oscillator circuit and a buffering stage 2b having a buffer amplifier or the like. The oscillating stage 2a is formed as a Colpitts type circuit having an unillustrated voltage dividing capacitor and transistor for oscillation, that form a resonance circuit along with the crystal unit 1. Here, the oscillating stage 2a is formed as a voltage control type circuit having a voltage-controlled capacitative element 4Cv in an oscillatory loop, for example. In the drawing, Vcc is a power source, Vout is an output, and Vc is a control voltage.
In the temperature control circuit 3, a temperature sensing voltage Vt by a temperature sensing element (for example, thermistor) 4th and a resistor 4R1 is applied to one input terminal of an operational amplifier 40A, and a reference voltage Vr by resistors 4R2 and 4R3 is applied to the other input terminal. Then, a differential voltage between the reference voltage Vr and the temperature sensing voltage Vt is applied to the base of a power transistor 4Tr, and electric power from the power source Vcc is supplied to the chip resistors 4h (one example of heating resistors) (hereinafter called heating resistors 4h) serving as heating elements. According thereto, the electric power to the heating resistors 4h is controlled with a temperature-dependent resistance value of the temperature sensing element 4th, to keep the operational temperature of the crystal unit 1 constant. An operational temperature is to be approximately 80° C. which is a minimum value or a maximum value at room temperature or more, for example (see FIG. 3B).
As shown in FIG. 2A, the circuit substrate 5 is composed of, for example, a glass epoxy substrate, and an unillustrated circuit pattern is formed thereon, and the respective circuit elements 4 including the crystal unit 1 are installed on both principal surfaces. Then, the circuit substrate 5 is held by the lead wires 8 of the base for oscillator. In this example, the crystal unit 1, and the heating resistors 4h, the power transistor 4Tr, and the temperature sensing element 4th in the temperature control circuit 3 are installed on one side board plane of the circuit substrate 5. The one side board plane (bottom face) of the circuit substrate 5 is set at the side of the base for oscillator 7, and the other side board plane (top face) thereof is set at the side of the cover for oscillator 9.
The principal surface of the crystal unit 1 faces the one side board plane of the circuit substrate 5, to touch, for example, two heating resistors 4h installed on the central area of the circuit substrate. Then, the pair of lead wires 12 are bent to penetrate through the circuit substrate 5, to be firmly fixed at the other side board plane with an unillustrated solder or the like. The power transistor 4Tr is disposed at the outer side of the crystal unit 1, and the temperature sensing element 4th is installed between the heating resistors 4h. These crystal unit 1 and circuit elements 4h, 4Tr, and 4th are covered with heat conducting resin 15. Here, the voltage-controlled capacitative element 4Cv which is temperature-dependent to greatly vary its characteristic, is installed between the heating resistors 4h. 
The other circuit elements 4 of the oscillator output circuit 2 and the temperature control circuit 3 are installed on the top face set as the other side board plane of the circuit substrate 5 and on the outer circumferential portion out of the area of the heat conducting resin 15 on the bottom face set as the one side board plane. In particular, the respective circuit elements 4 of the oscillating stage 2a having an affect on an oscillating frequency are disposed on the top face of the circuit substrate 5 facing the area covered with the heat conducting resin 15. Further, for example, an adjuster element such as a capacitor for adjusting oscillating frequency and the like is installed on the top face of the circuit substrate 5, which makes it easy to adjust oscillating frequency.
Then, for example, in the case in which an exchange work of a soldered capacitor or resistor serving as an adjuster element is needed, by composing the circuit substrate 5 of a glass epoxy substrate, the exchange work by soldering an adjuster element can be made easy. That is, since heat in soldering is accumulated by the glass epoxy, the soldering is made easy. For example, in the case in which the circuit substrate 5 is a ceramic substrate, heat is easily let out, which makes a soldering work in exchange of elements difficult.
Incidentally, JP-A-2000-315916, JP-A-2006-311496 and JP-A-2005-341191 each discloses a related art constant-temperature type oscillator.
However, in the constant-temperature type oscillator having the above-described configuration, the principal surface of the crystal unit 1 is disposed on the heating resistors 4h so as to touch those, which results in the problem that the oscillator is increased in height size. For this reason, as shown in FIG. 4, for example, the heating resistors are removed from the bottom face of the crystal unit 1, and a plurality of heating resistors are disposed so as to surround the outer circumference of the crystal unit. It is considered that the oscillator is reduced in height size by the configuration. Incidentally, the heating resistor 4th and the like are omitted in FIG. 4 for the sake of convenience.
However, in this case, since the flange 14 of the crystal unit 1 touches the circuit substrate 5 to be inclined, the projecting part of the flange becomes taller. Then, a load is applied to the flexural areas of the pair of lead wires, and, for example, cracks are generated in the glass at the airtight terminals or the like, which results in the problem that an installation work to the circuit substrate is made difficult. Moreover, the thickness of the heat conducting resin at the metal base side is increased, which results in the problem that heat conduction to the circuit substrate from the principal surface of the metal cover is made nonuniform.