Abstract:
A constant-temperature type crystal oscillator includes: a crystal unit that is installed on one principal surface of a circuit substrate, and chip resistors, which function as heating elements, and which are installed on the other principal surface of the circuit substrate so as to face a principal surface of the crystal unit, the chip resistors heating up the crystal unit to keep an operational temperature of the crystal unit constant. A heating metal film facing the principal surface of the crystal unit is provided on the one principal surface of the circuit substrate. A heat conducting material is interposed between the principal surface of the crystal unit and the heating metal film to perform thermal coupling therebetween. The heating metal film is thermally coupled to electrode terminals of the chip resistors via a plurality of electrode through holes.

Description:
[0001]    This application claims priority from Japanese Patent Application No. 2008-224560 filed on Sep. 2, 2008, the entire subject matter of which is incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to a technical field of a constant-temperature type crystal oscillator (hereinafter called constant-temperature type oscillator) using chip resistors as heating elements, and in particular, to a constant-temperature type oscillator in which the heat conduction property to the crystal unit is enhanced. 
         [0004]    2. Description of the Related Art 
         [0005]    Constant-temperature type oscillators keep the operational temperatures of their crystal units constant. Therefore, the constant-temperature type oscillators are especially adopted for communication devices for a base station with high frequency stability of, for example, 0.1 ppm or more. JP-A-2005-341191 discloses one example of the constant-temperature type oscillator, for in which downsizing has been attempted by use of chip resistors as heating elements. 
         [0006]      FIGS. 4A to 4C  are diagrams for explanation of a related art constant-temperature type oscillator.  FIG. 4A  is a cross-sectional view of the related art constant-temperature type oscillator,  FIG. 4B  is a view of an oscillator output circuit thereof, and  FIG. 4C  is a view of a temperature control circuit thereof. 
         [0007]    The constant-temperature type oscillator shown in  FIGS. 4A to 4C  includes an oscillator output circuit  2  including a crystal unit  1  and a temperature control circuit  3 . In the constant-temperature type oscillator, respective circuit elements  4  are installed on a first circuit substrate  5   a  and a second circuit substrate  5   b  along with the crystal unit  1 . Then, the constant-temperature type oscillator is configured to house those in an oscillator case  6 . The crystal unit  1  has a crystal element  1   a  which is formed as, for example, an AT-cut crystal element or an SC-cut crystal element. The crystal element  1   a  is electrically and mechanically connected to a pair of lead wires  1   c  of a metal base  1   b , and is hermetically encapsulated with a metal cover  1   d.    
         [0008]    The crystal unit  1  has a frequency-temperature characteristic that approximately 80° C. at the higher temperature side higher than or equal to 25° C. as room temperature is an extreme value, and the oscillating frequency varies according to a temperature in any cases of both of the AT-cut and SC-cut crystal elements. For example, in an AT-cut crystal element, the frequency-temperature characteristic shows a cubic curve (curve A in  FIG. 5 ), and in an SC-cut crystal element, the frequency-temperature characteristic shows a quadratic curve (curve B in  FIG. 5 ). 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. 
         [0009]    The oscillator output circuit  2  is composed of an oscillating stage  2   a  serving as an oscillator circuit and a buffering stage  2   b  having a buffer amplifier or the like. The oscillating stage  2   a  is formed as a Colpitts type circuit having a voltage dividing capacitor (not shown) and transistor for oscillation, that form a resonance circuit along with the crystal unit  1 . Here, the oscillating stage  2   a  is formed as a voltage control type circuit having a voltage-controlled capacitative element  4 Cv in an oscillatory loop, for example. In the drawing, Vcc is a power source, Vout is an output, and Vc is a control voltage. 
         [0010]    In the temperature control circuit  3 , a temperature sensing voltage Vt by a temperature sensing element (for example, thermistor)  4   th  and a resistor  4   r   1  is applied to one input terminal of an operational amplifier  40 A, and a reference voltage Vr by resistors  4   r   2  and  4   r   3  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  4 Tr, and electric power from the power source Vcc is supplied to the chip resistors (hereinafter called heating resistors)  4   h  serving as heating elements. In this example, the number of heating resistors  4   h  ( 4   h   1 ,  4   h   2 ) is two. Thereby, the supply current to the heating resistors  4   h  is controlled with a temperature-dependent resistance value of the temperature sensing element  4   th , and a quantity of heat from the power transistor  4 Tr as well is added thereto to keep the operational temperature of the crystal unit  1  constant. 
         [0011]    The first and second circuit substrates  5   a  and  5   b  are both made of epoxy materials, and are formed into a multistage (two-stage) structure in which the outer circumferential portion of the first circuit substrate  5   a  is supported on the second circuit substrate  5   b  with metal pins  7 , and their board faces face each other. The metal pins  7  electrically connect circuit patterns (not shown) on the first and second circuit substrates  5   a  and  5   b . The heating resistors  4   h   1  and  4   h   2 , the power transistor  4 Tr, and the temperature sensing element  4   th  of the temperature control circuit  3  are installed on (firmly fixed to) the central area which is the top face of the first circuit substrate. 
         [0012]    Then, the principal surface of the crystal unit  1  (the principal surface of the metal cover  1   d ) is installed on these elements  4 ( h   1 ,  h   2 , Tr, and  th ) so as to interpose heat conducting resin  8  therebetween. In this case, the power transistor  4 Tr which is the highest is disposed on the principal surface at the leading end side of the crystal unit  1  (the metal base  1   b ), and the temperature sensing element  4   th  is to be between the heating resistors  4   h   1  and  4   h   2 . The pair of lead wires  1   c  led out from the metal base  1   b  of the crystal unit  1  penetrates through to be connected to the first circuit substrate  5   a.    
         [0013]    Then, a circuit element  4  for adjustment such as, for example, a trimming capacitor (not shown) is installed on the top face of the first circuit substrate  5   a , and the circuit elements  4  of the oscillating stage  2   a  including the voltage-controlled capacitative element  4 Cv having an affect on an oscillating frequency are installed on the top and bottom faces. The second circuit substrate  5   b  is, for example, a laminated plate having a shielding metal film  9  on its lamination plane, and has mounting terminals  10  on its outer bottom face. Then, the other circuit elements  4  of the buffering stage  2   b  and the temperature control circuit  3  are installed on the inner bottom face of the second circuit substrate  5   b . The oscillator case  6  is formed such that a metal cover  11  is bonded to the second circuit substrate  5   b  serving as the base as well. 
         [0014]    Incidentally, JP-A-2008-28620 also discloses a related art constant-temperature type oscillator. 
         [0015]    However, in the constant-temperature type oscillator having the above-described configuration, because not only the heating resistors  4   h , but also the power transistor  4 Tr as well serves as a heating source, the energy efficiency of the power source is enhanced, meanwhile, the power transistor  4 Tr is greater in height than the heating resistors  4   h   1  and  4   h   2 , which generates a step between those. Accordingly, the thickness of the heat conducting resin  8  interposed between the crystal unit  1  and the heating resistors  4   h   1  and  4   h   2  as well is increased, and heat loss through the heat conducting resin  8  is generated, which results in the problem of a lack of efficiency of heat conduction. 
         [0016]    Further, with respect to the crystal unit  1 , the lead wires  1   c  led out from the metal base  1   b  are bent to be connected to the first circuit substrate  5   a . Then, the principal surface of the crystal unit  1  is installed (laminated) so as to face the heating resistors  4   h  so as to include the power transistor  4 Tr, which results in the problem that the oscillator is basically increased in height size. 
         [0017]    For this reason, as shown in JP-A-2005-341191, for example, a circuit substrate  14  is made of ceramic, and a crystal unit  2  formed for surface mounting is installed on one principal surface of the circuit substrate  14 , and heating resistors  15   a  and  15   b  facing the crystal unit  2  are installed on the other principal surface. Thereby, the crystal unit  2  and the heating resistors  15   a  and  15   b  are installed on both principal surfaces of the circuit substrate  14  so as not to be laminated, which reduces the oscillator in height size. Then, because heat from the heating resistors  4   h  is conducted to the principal surface (the bottom face) of the crystal unit  2  by utilizing the heat conducting property of the ceramic (the first circuit substrate  5   a ), it is possible to heat up the crystal unit  2 . 
         [0018]    However, in this case, the ceramic is applied as a heat conducting material, meanwhile, the heat conductivity of ceramic is approximately less than or equal to 1/10 of that of metal, which results in the problem of a lack of efficiency of heat conduction. Then, because the power transistor is not used as a direct heat source, there has been the problem of a decrease in energy efficiency as well. 
       SUMMARY OF THE INVENTION 
       [0019]    An object of the invention is to provide a constant-temperature type oscillator, which can reduce a height size thereof, and which can improve efficiency of heat conduction. 
         [0020]    According to a first aspect of the invention, there is provided a constant-temperature type crystal oscillator comprising: a crystal unit that is installed on one principal surface of a circuit substrate; and chip resistors, which function as heating elements, and which are installed on the other principal surface of the circuit substrate so as to face a principal surface of the crystal unit, the chip resistors heating up the crystal unit to keep an operational temperature of the crystal unit constant, wherein a heating metal film facing the principal surface of the crystal unit is provided on the one principal surface of the circuit substrate, wherein a heat conducting material is interposed between the principal surface of the crystal unit and the heating metal film to perform thermal coupling therebetween, and wherein the heating metal film is thermally coupled to electrode terminals of the chip resistors via a plurality of electrode through holes. 
         [0021]    According to a second aspect of the invention, in the constant-temperature type crystal oscillator, wherein a power transistor for supplying electric power to the chip resistors and a conducting metal film, which is extended in width, and which is conducted to the heating metal film, are disposed on the one principal surface of the circuit substrate, and wherein a collector of the power transistor is connected to the heating metal film through the conducting metal film extended in width. 
         [0022]    According to a third aspect of the invention, in the constant-temperature type crystal oscillator, wherein the chip resistors include electrodes on both sides along their long sides, and wherein the chip resistors are composed of two resistors disposed such that the long sides face each other, and wherein a temperature sensing element is disposed between the chip resistors. 
         [0023]    According to the aspects of the invention, since the crystal unit and the heating resistors are disposed on the surfaces opposite to one another of the circuit substrate, the oscillator can be kept low in height size. Then, heat from the heating resistors installed on the other principal surface is directly conducted to the metal film on the one principal surface of the circuit substrate via electrode through holes. Then, the heat is conducted from the metal film to the principal surface of the crystal unit facing the metal film via the heat conducting material. Accordingly, since metal with heat conductivity ten times or more as high as that of ceramic, is applied as a heat conducting material, it is possible to enhance the efficiency of heat conduction from the heating resistors to the crystal unit via the metal film. 
         [0024]    Further, according to the second aspect of the invention, heat from the power transistor is also added to the heat source. Therefore, heat energy can be effectively used in the same way as in the related art. 
         [0025]    Still further, according to the third aspect of the invention, heat from the chip resistors is conducted via the electrodes along their long sides. Therefore, the heat conduction can be more efficient than a case in which the electrodes are provided along their short sides. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIGS. 1A to 1C  are diagrams for explanation of a constant-temperature type crystal oscillator according to one embodiment of the invention, in which  FIG. 1A  is a cross-sectional view of the constant-temperature type oscillator,  FIG. 1B  is a plan view (top view) of a first circuit substrate of the constant-temperature type oscillator, and  FIG. 1C  is a plan view of a heating resistor of the constant-temperature type oscillator; 
           [0027]      FIG. 2  is a partial schematic view showing wire connections between the first circuit substrate and a temperature control circuit of the constant-temperature type oscillator; 
           [0028]      FIG. 3  is a partial schematic view of the temperature control circuit for explanation of another application example according to the invention; 
           [0029]      FIGS. 4A to 4C  are diagrams for explanation of a related art constant-temperature type oscillator, in which  FIG. 4A  is a cross-sectional view of the related art constant-temperature type oscillator,  FIG. 4B  is a view of an oscillator output circuit of the related art constant-temperature type oscillator, and  FIG. 4C  is a view of a temperature control circuit of the related art constant-temperature type oscillator; and 
           [0030]      FIG. 5  is a frequency-temperature characteristic diagram of a crystal unit according to the related art constant-temperature type crystal oscillator. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0031]      FIGS. 1A to 1C  are diagrams for explanation of a constant-temperature type crystal oscillator according to one embodiment of the invention.  FIG. 1A  is a cross-sectional view of the constant-temperature type oscillator,  FIG. 1B  is a plan view (top view) of a first circuit substrate thereof, and  FIG. 1C  is a plan view of a heating resistor thereof. Incidentally, portions which are the same as those in the related example are denoted by the same symbols, and descriptions thereof will be simplified or omitted. 
         [0032]    As described above, the constant-temperature type oscillator has a circuit configuration having the crystal unit  1 , the oscillator output circuit  2 , and the temperature control circuit  3  (refer to  FIGS. 4A to 4C ). Incidentally, the temperature control circuit  3  functions as a temperature compensation circuit of the crystal unit  1 . The crystal unit  1  and the respective circuit elements  4  are installed on the first and second circuit substrates  5   a  and  5   b  which are both made of epoxy materials. The second circuit substrate  5   b  has the mounting terminals  10 , and serves as a base of the oscillator case  6  as well, and the metal cover  11  is bonded to the second circuit substrate  5   b.    
         [0033]    The crystal unit  1  is installed as a bottom face of the first circuit substrate  5   a  in contrast to the above description, and the two heating resistors  4   h   1  and  4   h   2 , the power transistor  4 Tr, and the temperature sensing element  4   th  are installed on the top face thereof as described above. However, an adjuster element such as a trimming capacitor as well is installed on the top face. Then, the circuit elements  4  of, in particular, the oscillating stage  2   a  having an affect on an oscillating frequency are installed on the top and bottom faces of the first circuit substrate  5   a . Further, the circuit elements  4  of the buffering stage  2   b  and the temperature control circuit  3  having little effect on an oscillating frequency are installed on the second circuit substrate  5   b.    
         [0034]    The metal film  12  is formed on the central area of the bottom face of the first circuit substrate  5   a , and the principal surface of the crystal unit  1  (the metal cover  1   d ) is installed to face the metal film  12 . The protruded outer circumferential portion of the metal base  1   b  of the crystal unit  1  is inserted into a notched part (without numbering) provided in the first circuit substrate  5   a , and the heat conducting resin  8  formed to be sheet-shaped is interposed between the principal surface of the crystal unit  1  and the metal film  12  to firmly adhere thereto. Then, the metal film  12  has a projecting portion  12   a  projecting from the side of the leading end of the crystal unit  1  (the leading end side of the metal cover  1   d ), to gradually narrow. 
         [0035]    The heating resistors  4   h   1  and  4   h   2  are formed into rectangular shapes long in one direction, and both have electrode terminals  15   a  and  15   b  on both sides along the long side directions. Then, the electrode terminals  15   a  and  15   b  of the heating resistors  4   h   1  and  4   h   2  are connected to respective circuit terminals ( 13   a ,  13   b ) and ( 13   c ,  13   d ) formed on the top face of the first circuit substrate  5   a  with a solder (not shown). The respective circuit terminals ( 13   a ,  13   b ) and ( 13   c ,  13   d ) are led out to the bottom face of the first circuit substrate  5   a  via electrode through holes ( 14   a ,  14   b ) and ( 14   c ,  14   d ). Then, the circuit terminals  13   b  and  13   c , which are respectively at the inner sides of the respective circuit terminals ( 13   a ,  13   b ) and ( 13   c ,  13   d ), are connected to the metal film  12  provided on the bottom face. In contrast thereto, the circuit terminals  13   a  and  13   d , which are respectively at the outer sides of the respective circuit terminals ( 13   a ,  13   b ) and ( 13   c ,  13   d ), are independent of the metal film  12 , to be not connected thereto. 
         [0036]    That is, as shown in a partial diagrammatic view as  FIG. 2 , the one terminals of the heating resistors  4   h   1  and  4   h   2 , which are at the collector side of the power transistor  4 Tr, are conducted to the metal film  12  via the electrode through holes  14   b  and  14   c , to have a common connection. That is, the metal film  12  functions as an electrically-conducting path through which the collector of the power transistor  4 Tr and the one terminals of the heating resistors  4   h   1  and  4   h   2  are connected, and heat from the heating resistors  4   h   1  and  4   h   2  is directly conducted to the metal film  12 . In contrast thereto, the other terminals of the heating resistors  4   h   1  and  4   h   2 , which are at the side of the power source (Vcc), are connected to an annular electrode  12   x . However, the other terminals of the heating resistors  4   h   1  and  4   h   2  are not conducted, to be not connected to the metal film  12 . However, the other terminals of the heating resistors  4   h   1  and  4   h   2  are electrically connected to the power source (Vcc) via a line path (not shown) on the top face. 
         [0037]    As shown in  FIGS. 1A to 1C , the projecting portion  12   a  of the metal film  12  and two metal films  12   b  and  12   c  facing the projecting portion  12   a  serve as circuit terminals  13 (C),  13 (E) and  13 (B) to which the collector (C), the emitter (E), and the base (B) of the power transistor  4 Tr are respectively connected. Then, the respective electrode terminals of the power transistor  4 Tr formed as a surface mounting type are connected to the circuit terminals  13 (C),  13 (E) and  13 (B). 
         [0038]    According to such a configuration, heat from the heating resistors  4   h   1  and  4   h   2  is directly conducted to the metal film  12  entirely facing the principal surface of the crystal unit  1  via the electrode through holes  14   b  and  14   c  respectively connected to the one electrode terminals thereof. Then, since the principal surface of the crystal unit  1  and the metal film  12  firmly adhere to each other with the heat conducting resin  8 , the heat from the metal film  12  is conducted to the crystal unit  1  without any loss of heat. In this case, since the projecting portion of the metal base  1   b  of the crystal unit  1  is inserted into the notched part of the first circuit substrate  5   a , it is possible to minimize the thickness of the heat conducting resin  8 . Accordingly, it is possible to minimize loss of heat through the heat conducting resin  8  to enhance the effect of heat conduction. 
         [0039]    Further, here, with respect to the other terminals of the heating resistors  4   h   1  and  4   h   2 , heat therefrom is conducted to the bottom face of the first circuit substrate  5   a  via the electrode through holes  14   a  and  14   b . Then, in this example, in particular, the other terminals of the heating resistors  4   h   1  and  4   h   2  are not connected to the metal film  12 , but connected to the annular electrode  12   x . Accordingly, heat from the annular electrode  12   x  as well is conducted to the principal surface of the crystal unit  1  via the heat conducting resin  8  firmly adhering to the annular electrode  12   x , which further enhances the effect of heat conduction. However, the effect of heat conduction is enhanced even without the annular electrode  12   x.    
         [0040]    Then, since the heating resistors  4   h   1  and  4   h   2  have the electrode terminals  15   a  and  15   b  on both sides along their long sides, heat radiation from the heating resistors  4   h   1  and  4   h   2  is made more efficient as compared with a case in which the electrode terminals  15   a  and  15   b  are provided on both sides along their short sides. Moreover, the collector (C) of the power transistor  4 Tr is connected to the projecting portion  12   a  (one example of a conducting metal film) extended in width to directly conduct heat to the metal film  12 . Accordingly, since heat generated from the collector resistance of the power transistor  4 Tr as well is utilized, energy efficiency is enhanced. 
         [0041]    Then, the power transistor  4 Tr, the heating resistors  4   h   1  and  4   h   2 , and the temperature sensing element  4   th  are on the top face of the first circuit substrate  5   a  and the crystal unit  1  is on the bottom face thereof. Accordingly, as compared with the case in which the principal surface of the crystal unit is disposed on the top face of the power transistor  4 Tr, the heating resistors  4   h   1  and  4   h   2 , and the temperature sensing element  4   th  as in the related example, it is possible to reduce the oscillator in height size even in the case in which the crystal unit  1  having the lead wires  1   c  is used. 
       Modified Examples 
       [0042]    In the above-described embodiment, the electrode through holes  14  are provided one by one to each of the respective electrode terminals  15   a  and  15   b  of the heating resistors  4   h   1  and  4   h   2 . However, a plurality of the electrode through holes  14  may be provided to each of those, to enhance the efficiency of heat conduction. Further, the respective chip resistors  4   h   1  and  4   h   2  are provided in parallel with the power transistor  4 Tr. However, as shown in  FIG. 3 , for example, power transistors  4 Tr 1  and  4 Tr 2  may be provided to each of the respective chip resistors  4   h   1  and  4   h   2 . Those are determined according to the specification or the like of the power transistor  4 Tr. 
         [0043]    Further, the heating resistors  4   h  are two. However, the number of heating resistors  4   h  may be randomly determined as needed. Then, the constant-temperature type oscillator is formed as a surface mounting type. However, the constant-temperature type oscillator may be applied to a type in which lead wires are led out from a metal base (not shown). In particular, any constant-temperature type oscillator primarily having the substrate configuration of the first circuit substrate  5   a  is incorporated in the technical scope of the invention.