Abstract:
To provide a highly stable crystal oscillator having increased thermal efficiency. The highly stable crystal oscillator comprises; a thermostat mainframe which maintains the temperature of a crystal resonator including a resonator container for sealing a crystal piece constant, an oscillating element which constitutes an oscillation circuit together with said crystal resonator, a temperature control element which controls the temperature inside of said thermostat mainframe, and a circuit board mounted with said thermostat mainframe, said oscillating element, and said temperature control element. The construction is such that a heat generating chip resistor and a highly heat sensitive element having a higher temperature dependency, among said oscillating element and said temperature control element, are arranged on one principal plane of said circuit board, and said heat generating chip resistor, said highly heat sensitive element, and said thermostat mainframe are directly heat bonded by a thermo-conductive material.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     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.  
         [0003]     2. Description of the Related Art  
         [0004]     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.  
         [0005]      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.  
         [0006]     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  1   a  and a second circuit board  1   b . The first circuit board  1   a  is supported by metallic pins  7   a  serving as external terminals which are insulated from and pierce a metallic base  6 . The second circuit board  1   b  is supported by metallic pins  7   b  placed on the first circuit board  1   a . In addition, thermostat  3  is supported by a leg  7   c  with the second circuit board  1   b.    
         [0007]     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  1   b  and adhered to the second circuit board  1   b  by an adhesive  12 .  
         [0008]     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.  
         [0009]     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  1   b . The temperature control elements  5  also contain at least a thermistor  5   a  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  5   a  are arranged on the outside surface of the first circuit board  1   a . The temperature control circuit detects the temperature inside the thermostat  3  by joining the thermistor  5   a  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  1   a  and  1   b , the crystal resonator  2  and the like are covered with a metallic cover  17 .  
         [0010]     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  1   b  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.  
         [0011]     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.  
         [0012]     Moreover, in the conventional crystal oscillator as shown in  FIG. 7A , the second circuit board  1   b  is mounted with the oscillating elements  4  and electrically connected to the first circuit board  1   a  by the metallic pins  7   b , and the metallic pins  7   a  of the first circuit board  1   a  are led out as the external terminals. Therefore, the metallic pins  7   b  of the second circuit board  1   b  are not led out directly to the outside, enabling prevention of heat release to the outside.  
         [0013]     However, in the conventional highly stable oscillator having the above construction, the second circuit board  1   b  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  1   b  is impaired. The general adhesive  12  is poor in pyroconductivity.  
         [0014]     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  1   b  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  1   b . Particularly, together with the thermo-sensor (thermistor  5   a ), 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.  
       SUMMARY OF THE INVENTION  
       [0015]     An object of the present invention is to provide a highly stable oscillator having increased thermal efficiency.  
         [0016]     The present invention, in a crystal oscillator for high stability using a thermostat, comprising; a thermostat mainframe which maintains the temperature of a crystal resonator including a resonator container for sealing a crystal piece constant, an oscillating element which constitutes an oscillation circuit together with said crystal resonator, a temperature control element which controls the temperature inside of said thermostat mainframe, and a circuit board mounted with said thermostat mainframe, said oscillating element, and said temperature control element, has a construction such that a heat generating chip resistor and a highly heat sensitive element having a higher temperature dependency, among said oscillating element and said temperature control element, are arranged on one principal plane of said circuit board, and said heat generating chip resistor, said highly heat sensitive element, and said thermostat mainframe are directly heat bonded by a thermo-conductive material.  
         [0017]     According to such construction, a thermostat where the chip resistor arranged on the circuit board is directly heat bonded to the thermostat mainframe by the thermo-conductive flexible material, is formed. Moreover, since the chip resistor directly supplies heat to the circuit board, the thermal efficiency is improved and the heat distribution of the circuit board is made uniform. Particularly, since the highly heat sensitive element is directly heat bonded to the chip resistor, the highly heat sensitive element has high dependency with respect to the exothermic temperature of the chip resistor. Furthermore, since the chip resistor is used as the heat source, then for example, the coiling operation for the heating coil becomes unnecessary, thus decreasing manufacturing costs.  
         [0018]     Moreover, in the highly stable oscillator of the present invention, said thermo-conductive material is a flexible material, and said flexible material is provided between said heat generating chip resistor, said highly heat sensitive element and said thermostat mainframe and adhered thereto. Accordingly, the heat generating chip resistor, the highly heat sensitive element, and the thermostat mainframe are adhered by the flexible material, so that heat bonding is performed directly.  
         [0019]     Furthermore, in the highly stable oscillator of the present invention, said flexible material is in sheet form and is provided between said heat generating chip resistor, said highly heat sensitive element and said thermostat mainframe. Accordingly, for example if the thickness of the flexible material in sheet form is made larger and is laid between the chip resistor, the highly heat sensitive element and the thermostat mainframe on the circuit board, it can be bit into by the surroundings of the chip resistor and the highly heat sensitive element to give close contact, so that operation of the close contact with the flexible material is improved.  
         [0020]     Furthermore, in the highly stable oscillator of the present invention, said thermostat mainframe has a space section in a cross sectional C shape with a pair of legs extending from the opposite ends of a housing section, and said crystal resonator is heat bonded to said thermostat mainframe to be arranged in said space section, and said heat generating chip resistor and said highly heat sensitive element are arranged in said space section. Accordingly, the heat capacity of the thermostat is increased compared to the case of using only the resonator container, and is insensitive with respect to temperature variations, and the momentary fluctuations in the oscillating frequency of the crystal resonator are prevented.  
         [0021]     In the highly stable oscillator of the present invention, said thermostat mainframe has a space section in a cross sectional C shape with a pair of legs extending from the opposite ends of a slab part, and said crystal resonator is heat bonded to said thermostat mainframe to be arranged in the upper part of said space section, and said heat generating chip resistor and said highly heat sensitive element are arranged at the lower part of said space section. Accordingly, similarly, the heat capacity of the thermostat is increased, and the space section in the cross sectional C shape may be formed by folding a plate material in the C shape. Hence manufacturing costs can be decreased.  
         [0022]     Moreover, in the highly stable oscillator of the present invention, said thermostat mainframe serving as said resonator container, has said heat generating chip resistor and said highly heat sensitive element arranged on the lower face of said resonator container. Accordingly, the thermostat mainframe can be made unnecessary, so that manufacturing costs can be further decreased. In this case, since the temperature response becomes sensitive, the starting characteristics of the crystal resonator become quick. However, stability of the oscillating frequency of the crystal resonator with respect to rapid temperature variations is decreased.  
         [0023]     Furthermore, in the high stability oscillator of the present invention, said highly heat sensitive element is a thermo-sensor among the temperature control elements, or a voltage variable capacitative element among the oscillating elements. Accordingly, for example if the highly heat sensitive element is the thermo-sensor, the temperature of the heat generating chip resistor and the thermostat mainframe can be detected in real time, so that the response of the temperature control can be improved. Moreover, if the highly heat sensitive element is the voltage variable capacitative element, the temperature of the thermostat mainframe which depends on the exothermic temperature of the chip resistor becomes constant, so that the decrease in characteristics due to temperature variation is prevented.  
         [0024]     In the highly stable oscillator of the present invention, the oscillating element is arranged on the other principal plane of said circuit board. Accordingly, the temperature of the oscillating element is kept constant, and the frequency stability of the crystal resonator is improved. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]      FIG. 1  is diagrams for explaining a first embodiment of a highly stable oscillator of the present invention,  FIG. 1A  being an assembly exploded view of the main point, and  FIG. 1B  being a vertical sectional view along the line A-A of  FIG. 1A .  
         [0026]      FIG. 2  is a vertical sectional view in the longitudinal direction showing a thermostat mainframe of the highly stable oscillator of the present invention, with one leg not illustrated herein, and showing the thermostat and the like shown in  FIG. 1B  inverted and arranged in the normal position.  
         [0027]      FIG. 3  is a perspective view showing a thermostat mainframe for explaining another example where a plurality of cavities are provided in the thermostat mainframe in the first embodiment of the highly stable oscillator of the present invention.  
         [0028]      FIG. 4  is a vertical sectional view of the main points of the thermostat in the width direction, for explaining a second embodiment of the highly stable oscillator of the present invention.  
         [0029]      FIG. 5  is a vertical sectional view of the main points of the thermostat in the width direction for explaining another example in the second embodiment of the highly stable oscillator of the present invention.  
         [0030]      FIG. 6  is a vertical sectional view of the main points of the thermostat in the width direction, for explaining a third embodiment of the highly stable oscillator of the present invention.  
         [0031]      FIG. 7  is diagrams for explaining a conventional highly stable oscillator,  FIG. 7A  being a fragmentary vertical sectional view of the highly stable oscillator, and  FIG. 7B  being a conceptual diagram showing a procedure for inserting a crystal resonator into a thermostat. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0000]     [First Embodiment] 
         [0032]      FIG. 1  and  FIG. 2  are diagrams for explaining a highly stable oscillator of a first embodiment of the present invention,  FIG. 1A  being an assembly exploded view of the main points, and  FIG. 1B  being a sectional view of the main points in the width direction.  FIG. 2  is a sectional view in the longitudinal direction showing a thermostat mainframe, with one leg omitted.  
         [0033]     As shown in  FIG. 1  and  FIG. 2 , the highly stable oscillator of the first embodiment of the present invention comprises; a crystal resonator  2  with lead wires  8  led out therefrom, a thermostat  3 , oscillating elements  4  and temperature control elements  5 , and a circuit board  1 , with all arranged thereon. Here, the thermostat  3  comprises a thermostat mainframe (metallic cylinder)  13  and heat generating chip resistors  14 . The thermostat mainframe  13  supported by a leg  7   c  with the circuit board  1  has a housing section  15   a  for the crystal resonator  2 , and a cavity  15   b  as a space on one principal plane having legs  13   b  extending from widthwise opposite sides of a slab part  13   a . The housing section  15   a  and the cavity  15   b  are open on the longitudinally opposite sides. The housing section  15   a  is filled with a resin  19   a  composed of a thermo-conductive flexible material and thermally bonded to the top region of the crystal resonator  2  (resonator container).  
         [0034]     Three of the chip resistors  14 , for example, are arranged in parallel in the central region of the one principal plane of the circuit board  1 , and electrically connected and adhered to a metallic pattern  16  with holes  18  by soldering. Between the respective chip resistors  14  are arranged, the thermistor  5   a  as the temperature control element, and the voltage variable capacitative element  4   a  as the oscillating element. On the chip resistors  14 , the thermistor  5   a , and the voltage variable capacitative element  4   a , is provided, for example, a member  19   a  composed of a thermo-conductive flexible material combined with an insulating function.  
         [0035]     In this first embodiment, for the thermo-conductive flexible material, for example λGEL made by GELTECH Co. Ltd., (registered trademark) in sheet form is laid as a resin  19   b . In this case, as shown in  FIG. 2 , the thickness of the resin  19   b  is made larger than a gap g between the chip resistors  14  and the bottom of the thermostat mainframe  13 . Moreover, the resin  19   b  in sheet form is interposed between the chip resistors  14 , and the resin  19   b  is pressed by the thermostat mainframe  13 . Accordingly, the resin  19   b  in sheet form can be bitten into by the surroundings of the chip resistor  14 , the thermistor  5   a  and the voltage variable capacitative element  4   a , to contact tightly therewith.  
         [0036]     Moreover, as shown in  FIG. 1B , the thermostat mainframe  13  has the legs  13   b  in contact with the metallic pattern  16  provided on the circuit board  1  and is fixed to the circuit board  1  by screws inserted through holes  18 . The thermistor  5   a  detects the temperature in the cavity  15   b  of the thermostat mainframe  13 . Moreover, the voltage variable capacitative element  4   a  makes the oscillation circuit have a variable oscillating frequency of the crystal oscillator, due to the control voltage circuit being a voltage-controlled type. As shown in  FIG. 1A  , the oscillating elements except for the voltage variable capacitative element  4   a  are arranged in the central region of the other principal plane of the circuit board  1 . The temperature control elements  5  except for the thermistor  5   a  are arranged on the peripheral region of the other principal plane.  
         [0037]     According to such a construction, the chip resistors  14  arranged on the circuit board  1  are directly heat bonded to the thermostat mainframe  13  by the resin  19   b  in sheet form being the thermo-conductive flexible material, so that the thermostat  3  is formed. In this case, the thermo-conductive flexible material (resin  19   b ) increases the heat capacity using three chip resistors  14  as one heat source. Moreover, the chip resistors  14  directly supply the heat to the circuit board  1  so that the thermal efficiency is improved and the heat distribution of the circuit board  1  is made uniform. In this embodiment, as shown in  FIG. 1B , the legs  13   b  of the thermostat mainframe  13  are joined to the metallic pattern  16  to which the chip resistors  14  are connected so that the thermal efficiency is further improved. Therefore, frequency fluctuations of the oscillating elements  4  can be prevented by the temperature characteristic being maintained constant.  
         [0038]     In particular, since the thermistor  5   a  and the voltage variable capacitative element  4   a  being the highly heat sensitive elements are directly heat bonded to the chip resistors  14 , the dependency with respect to the exothermic temperature of the chip resistors  14  is high. Therefore, if the thermistor  5   a  is used, the temperature of the thermostat mainframe  13  can be detected directly (in real time), so that response (followingness) is improved. Moreover, if the voltage variable capacitative element  4   a  is used, the temperature dependent voltage/capacity characteristic is kept from fluctuating, and the oscillating frequency of the crystal oscillator is stabilized. In particular, since the voltage variable capacitive element  4   a  is covered by the cavity  15   b , there is no effect of convection, and the temperature can be maintained constant compared to the case where they are arranged on the other principal plane of the circuit board  1 .  
         [0039]     Moreover, since the thermostat  3  uses the chip resistors  14  as the heat source, for example, the coiling operation for the heating coil becomes unnecessary so that manufacturing cost can be reduced. In this first embodiment, the circuit board  1  is composed of one layer, and the oscillating elements  4  and the temperature control element  5  are mounted thereon. Therefore the manufacturing cost can be further decreased. Furthermore, since the circuit board  1  is composed of one layer having an efficient construction, heat radiation is minimal. Moreover since the heat capacity is increased by the thermostat mainframe  13 , changes in the oscillating frequency with respect to rapid temperature variations can be prevented. However, in this case, the heat capacity becomes the sum of the thermostat mainframe  13  and the thermo-conductive flexible material (resin  19   a  and  19   b ).  
         [0040]     In the first embodiment, the cavity  15   b  of the thermostat mainframe  13  has the opposite ends opened, however they may be blocked off surfaces rather than open. In this case, the heat radiation from the opened surfaces can be prevented, giving further efficiency. Furthermore, as shown in  FIG. 3 , both ends of the thermostat mainframe  13  may be blocked off surfaces and the cavity  15   b  may be divided into a plurality of cavities  15   b  with the chip resistors  14  and the highly heat sensitive elements arranged into the plurality of cavities  15   b.    
         [0000]     [Second Embodiment] 
         [0041]      FIG. 4  is a vertical sectional view of the main points of a highly stable oscillator, for explaining a second embodiment of the highly stable crystal oscillator of the present invention.  
         [0042]     In the second embodiment of the present invention, the thermostat mainframe  13  comprises a pair of legs  13   b  extending from the opposite ends of a slab part  13   a  and forming a space section S in a cross sectional C shape. For example, the thermostat mainframe  13  is formed by folding a metal plate in the C shape.  
         [0043]     Moreover, a crystal resonator  2  is arranged in the upper part (slab part  13   a  side) of a space section S and the crystal resonator  2  is adhered and heat bonded to the slab part  13   a  for example by a resin  19   a  being a thermo-conductive flexible material. On a circuit board  1  located at the lower part of the space section S, heat generating chip resistors  14  and a thermistor  5   a  and a voltage variable capacitative element  4   a  being highly heat sensitive elements are arranged. A resin  19   b  in sheet form composed of a thermo-conductive flexible material is adhered and interposed between the crystal resonator  2  and the highly heat sensitive element.  
         [0044]     According to such a construction, similarly to the first embodiment, a thermostat  3  which transmits heat from the chip resistors  14  to the crystal resonator  2  by the resin  19   b  composed of the thermo-conductive flexible material, is formed. Therefore, a similar effect to that of the first embodiment of the present invention is demonstrated. Moreover, since the heat capacity of the thermostat mainframe  13  in the C shape is increased, the oscillating frequency can be stably controlled with respect to rapid temperature variations. Furthermore, since the thermostat mainframe  13  may be formed by folding a metal plate in the C shape, the cutting operation becomes unnecessary, and manufacturing cost is reduced.  
         [0045]     In this second embodiment, as shown in  FIG. 5 , when forming the thermostat mainframe  13  by folding the metal plate in the C shape, a pair of projections  21  may be provided so as to reliably hold the crystal resonator  2 .  
         [0000]     [Third Embodiment] 
         [0046]      FIG. 6  is a vertical sectional view of the main points of a third embodiment of a highly stable crystal oscillator of the present invention.  
         [0047]     In the third embodiment, a thermostat mainframe  13  is constructed as a resonator container of a crystal resonator  2 . On a lower face of the resonator container (crystal resonator  2 ), a heat generating chip resistor  16 , a thermistor  5   a  and a voltage variable capacitative element  4   a  serving as highly heat sensitive elements, are arranged. Here, over the chip resistor  14  and the highly heat sensitive element (thermistor  5   a ) mounted on the circuit board  1 , is coated a resin  19   c  composed of a thermo-conductive flexible material, and the crystal resonator  2  is superposed thereon, and the resin then cured.  
         [0048]     According to such construction, the heat from the chip resistors  14  is transmitted to the crystal resonator  2  by the resin  19   c  serving as the thermo-conductive flexible material, so that the thermostat is formed. Therefore, a similar effect to that of the first and second embodiments is demonstrated.  
         [0049]     Moreover, since the aforementioned thermostat mainframe  13  is not used, the cost can be further decreased. In this case, the heat capacity can be controlled according to the amount of the coated thermo-conductive flexible material (resin  19   c ).  
         [0050]     In the first to third embodiments described above, the circuit board is made of one layer to decrease manufacturing cost. However, if importance is attached to the functionality of the highly stable oscillator, then similarly to the conventional crystal oscillator, a double layered circuit board may be used. Even if a double layered circuit board is used, the circuit board  1  mounted with the oscillating elements  4  is arranged on the thermostat mainframe  13 .  
         [0051]     Moreover, the thermistor  5   a  and the voltage variable capacitative element  4   a  are arranged in the cavity  15   b  of the thermostat mainframe  13 . However, for example a transistor or the like of the oscillation circuit may also be arranged. The point is that the circuit elements having the high temperature dependency are arranged in the cavity  15   b . Moreover, one or four or more of the chip resistors may also be arranged as required.