Abstract:
This is a crystal oscillator comprising a heater whose heater line is multiplied and a control unit for controlling the heater.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a crystal oscillator for guaranteeing high frequency precision against temperature fluctuations.  
         [0003]     2. Description of the Related Art  
         [0004]     As a crystal oscillator with the high stability of an oscillating frequency against the fluctuations of ambient temperature, a temperature compensated crystal oscillator (TCXO) and an oven controlled crystal oscillator (OCXO) are known.  
         [0005]     The TCXO comprises a temperature-compensated circuit for correcting an oscillating frequency according to the fluctuations of ambient temperature. In the OCXO, a crystal oscillator element or an oscillation circuit is disposed in a constant-temperature oven whose internal temperature is kept constant to reduce its influence on ambient temperature.  
         [0006]     Although the TCXO is suitable for low power, it is technically difficult to guarantee high frequency precision of  10   −7  or less against temperature fluctuations. However, although the OCXO has an advantage over the TCXO in achieving high frequency precision, it stands at a disadvantage in low power.  
         [0007]     In order to solve the problem, for example, Patent reference 1 (the specification (FIG. 1) of U.S. Pat. No. 5,917,272) discloses an OCXO which comprises a heater on a heat conductive substrate in order to efficiently heat by heat conduction and radiation and to save power. Since in this configuration, a crystal element cannot be disposed in such a way as to enclose the heater, the influence of ambient temperature increases.  
         [0008]     One factor of the high consumption power of the OCXO is a complex temperature control circuit for keeping the temperature of the constant-temperature oven.  
         [0009]     For temperature control, there are an analog method in which it is difficult to miniaturize/integrate circuits and a pulse width modulation (PWM) method in which circuits can be easily miniaturized and integrated.  
         [0010]     Since a heater drive circuit can be fairly miniaturized, control by PWM is used to control the temperature of a laser diode. However, a pulse residue is superimposed on a temperature control driving signal. If such a driving signal is applied to a heater, an electromagnetic field generated from the heater is superimposed on the oscillating signal of a crystal oscillator disposed adjacently to it. Therefore, it is unsuitable for temperature control.  
         [0011]     When temperature control is attempted to realize by control by PWM, there is no conventional method for effectively eliminating noise due to control by PWM. Therefore, a signal obtained by increasing/decreasing DC voltage without noise must be used. In this case, since for a heater drive transistor, one with a large collector loss must be used, the setting of a circuit constant becomes complex and also a large device must be used. Therefore, a control circuit becomes complex and large.  
       SUMMARY OF THE INVENTION  
       [0012]     It is an object of the present invention to provide a small low-powered crystal oscillator and a temperature-keeping method thereof.  
         [0013]     In order to solve the above-described problem, the crystal oscillator according to the present invention comprises a heater and a control unit.  
         [0014]     The heater has multiplied heater lines.  
         [0015]     The control unit controls the heater.  
         [0016]     In this configuration, an object whose temperature is kept constant is heated by the multiplied heater.  
         [0017]     Since each heater line of the heater is duplicated, the control unit can also flow two pieces of driving current each with an opposite phase to each pair of duplicated heater lines.  
         [0018]     Since in this configuration, by the pair of heater lines through each of which current with an opposite phase, respective noise can be mutually killed by the heater lines, the object to be heated by the heater is not affected by the noise on the heater lines.  
         [0019]     Furthermore, the heater can also be configured in such a way that the object whose temperature is kept is constant by the heater may be enclosed with the heater lines.  
         [0020]     Thus, the object whose temperature is kept constant can be actually kept at a preset temperature without being affected by ambient temperature.  
         [0021]     The control unit controls the heater by pulse width modulation (PWM). Thus, the miniaturization and power saving of an oscillator can be realized.  
         [0022]     The present invention covers not only a crystal oscillator but also the temperature-keeping method of an object whose temperature is kept constant in a crystal oscillator.  
         [0023]     According to the present invention, since an object whose temperature is kept constant can be actually kept at a preset temperature without being affected by ambient temperature, highly precise oscillation which is stable against temperature fluctuations can be realized.  
         [0024]     A control method in which noise is superimposed on heater lines, such as PWM can be adopted for the control of a heater. Furthermore, by adopting PWM control, the miniaturization and power saving of a temperature control circuit can be realized.  
         [0025]     Furthermore, since an oscillator can be miniaturized, stable oscillation output can be realized in a short time after the oscillator is activated. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]      FIG. 1A and 1B  show the configurations of the heater of the conventional oscillator and that of the oscillator in this preferred embodiment of the present invention, respectively.  
         [0027]      FIG. 2  shows one disposition of the heater of the crystal oscillator and an object whose temperature is kept constant in this preferred embodiment.  
         [0028]      FIG. 3  shows another disposition of the heater of the crystal oscillator and an object whose temperature is kept constant in this preferred embodiment.  
         [0029]      FIG. 4  shows an example of the circuit configuration of the crystal oscillator in this preferred embodiment.  
         [0030]      FIG. 5  is the section view showing one disposition of components constituting the oscillation circuit in this preferred embodiment.  
         [0031]      FIGS. 6A, 6B  and  6 C show examples of the configurations of the differential-driven heater (DDH). 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0032]     One preferred embodiment of the oscillator according to the present invention is described below with reference to the drawings.  
         [0033]     In the oscillator of this preferred embodiment, a plurality of heater lines of a heater for keeping components which affect the oscillating frequency of the oscillator by the fluctuations of their temperature, such as a crystal oscillator element and the like, in a constant temperature as objects whose temperature should be kept constant are disposed adjacently to each other. The crystal oscillator is disposed in an area with a weak electromagnetic field, which is enclosed with a heater and in which AC noise superimposed on a driving signal is mutually killed by flowing two pieces of driving current each with an opposite phase to each pair of heater lines. Thus, even if a control method in which there is a possibility that noise is superimposed on a driving signal, such as control by PWD for the temperature control of the heater or the like, is used, respective noise can be mutually killed by the respective electromagnetic fields of each pair of heater lines.  
         [0034]      FIGS. 1A and 1B  show the configurations of the heater of the conventional oscillator and that of the oscillator in this preferred embodiment of the present invention, respectively.  
         [0035]     As shown in  FIG. 1A , in the conventional heater  11   a  of a crystal oscillator, a loop-shaped heater line  12  is provided on a substrate in such a way as to enclose an object whose temperature is kept constant, such as a crystal element or a circuit device constituting an oscillation circuit, disposed at the center of the substrate.  
         [0036]     However, in the heater  11   b  of the crystal oscillator in this preferred embodiment, each heater line is duplicated as shown in  FIG. 1B , and a heater line  14  is disposed inside a heater line  13 . The two heater lines  13  and  14  are connected to a temperature control circuit for driving the heater crosswise, and current is applied to each of the heater lines  13  and  14  in an opposite direction. Thus, noise superimposed on the heater line  14  and one superimposed on the heater line  13  are mutually killed and an object whose temperature is kept constant is protected from noise. Therefore, even if control by pulse width modulation (PWM) is used for this heater control, noise superimposed on a signal for driving the heater does not affect the output of the crystal oscillation circuit, thereby realizing an oscillator capable of outputting a highly precise oscillating signal.  
         [0037]     In the following description, a general heater shown in  1 A and the heater of the oscillator in this preferred embodiment shown in  FIG. 1B  are called “single-driven heater (SDH)” and “differential-driven heater (DDH)”, respectively.  
         [0038]      FIGS. 2 and 3  shows disposition examples of a heater and an object whose temperature is kept constant.  
         [0039]     In  FIG. 2 , a crystal oscillation circuit is disposed inside the heater as an object whose temperature is kept constant by the heater.  
         [0040]     In  FIG. 2 , a DDH  22  obtained by forming a thick film-baked heater resistor is disposed on a ceramic substrate  21 . An IC chip  23  obtained by integrating a crystal oscillator element and circuit components constituting a VCXO and packaging them into a ceramic or the like, a temperature sensor  24  for sensing the temperature inside the DDH  22 , such as a thermistor or the like and a discrete component  25 , such as a large-capacity capacitor which cannot be accommodated in the IC chip  23  and the like are disposed at the center of the enclosure of the DDH  22  in such a way as to be enclosed with the DDH  22  as objects whose temperature should be kept constant.  
         [0041]     In  FIG. 3 , a temperature control circuit  36  for controlling DDH 32  as well as the IC chip  33 , a temperature sensor  34  and discrete components  35  shown in  FIG. 2  are disposed inside the DDH  32  as objects whose temperature should be kept constant as an example.  
         [0042]     This temperature control circuit  36  keeps the respective temperature of the IC chip  33 , temperature sensor  34 , discrete components  35  and temperature control circuit  36  which are disposed inside the DDH  32  formed on the ceramic substrate  31  by PWM-controlling the DDH  32 , based on the resistance value of the temperature sensor  34  which changes with temperature fluctuation.  
         [0043]     In the oscillator configured as shown in  FIG. 2  or  3 , since an object whose temperature is kept constant is enveloped and heated in the DDH  22  (or DDH  32 ), the temperature of the object is actually kept at a preset temperature without being affected by ambient temperature.  
         [0044]     By adopting the control by PWM of the temperature control circuit and controlling temperature by changing the pulse width of current for driving the DDH  22  (or DDH  32 ), even if as a result, AC noise superimposed on current flowing through the DDH  22  (or DDH  32 ), an object whose temperature is kept constant, such as the chip of an oscillation circuit disposed inside the DDH  22  (or DDH  32 ) can realize essential oscillation with high frequency precision without being affected by noise superimposed on the heater lines since respective noise can be mutually killed by the respective electromagnetic fields of the two duplicated heater lines of the DDH  22  (DDH  32 ).  
         [0045]     Furthermore, since temperature control by PWM is possible, the miniaturization and low power of the entire device can be realized, and the device can also be adopted for portable equipment or the like. By the miniaturization of equipment, time required to make the temperature of the object whose temperature is kept constant a specified value can be shortened, and time required until stable oscillation output is secured after activation can be shortened.  
         [0046]     Although in  FIGS. 2 and 3 , only one of the temperature sensors  24  and  34  is disposed in the neighborhood of the object whose temperature is kept constant, a plurality of temperature sensors can also be disposed inside the DDHs  22  and  32 . In this case, the plurality of temperature sensors is connected in series, and temperature is controlled based on the total resistance value. Alternatively, the plurality of temperature sensors is connected in parallel, and temperature is controlled by determining the value of each temperature sensor by majority. In this case, the temperature sensors are disposed in appropriate positions, such as in the four corners, at the center of the DDHs  22  and  32  and the like, taking into consideration the temperature distribution of the substrate and the like.  
         [0047]      FIG. 4  shows an example of the circuit configuration of the crystal oscillator in this preferred embodiment.  FIG. 4  shows the case where a DDH is controlled PWM. In  FIG. 4 , mainly a temperature control circuit is described, and descriptions other than a part concerning the control of the DDH are simplified.  
         [0048]     In the crystal oscillator of this preferred embodiment, the oscillation circuit  45 , DDH  46  and temperature sensor  49 , such as a thermistor or the like, which are shown in  FIG. 2  are thermally connected by a substrate made of ceramic or the like, and the heater lines  47  and  48  of the DDH  46  are disposed so as to enclose the oscillation circuit  45  and the temperature sensor  49  disposed in the neighborhood of the oscillation circuit  45  doubly.  
         [0049]     The DDH  46  and temperature sensor  49  is electrically connected to the temperature control circuit composed of an error signal generator  41 , an integrator  42  and a PWM setter  43 . The temperature control circuit PWM-controls the DDH  46 , based on the change by heat of the resistance value of the temperature sensor  49 .  
         [0050]     The error signal generator  41  compares a specified voltage generated by resistors R 1  and R 3 , an operational amplifier A 1  and a variable resistor VR with the output voltage of an amplifier composed of the temperature sensor  49 , resistors R 2  and R 4  and an operational amplifier A 2 , using a differential amplifier composed of a chopper amplifier A 3  and resistors R 5  and R 6 , and inputs the differential value to the integrator  42 . A voltage source E provides the error signal generator  41  and integrator  42  with their reference voltages.  
         [0051]     In the integrator  42 , after unwanted noise is cut from the output of the chopper amplifier A 3 , using a low-pass filter composed by resistors R 7  and R 8  and a capacitor C 1 , an error signal whose timing is synchronous with a temperature time constant is generated by an integrator composed of an amplifier A 4 , capacitors C 2  and C 3  and a resistor R 9  and inputted to the PWM setter  43 .  
         [0052]     This error signal notifies the PWM setter  43  that temperature inside the DDH  46  deviates from a set temperature. If the temperature inside the DDH  46  exceeds a temperature set by the variable resistor VR and the resistance value of the temperature sensor  49  increases, an error signal with plus voltage is inputted from the integrator  42  to the PWM setter  43 . If conversely, the temperature drops below the set temperature and the resistance value of the temperature sensor  49  decreases, an error signal with minus voltage is inputted from the integrator  42  to the PWM setter  43 . The PWM setter  43  controls temperature by expanding/contracting the pulse width of current for driving the DDH  46 , according to the voltage value of this error signal. In this case, if necessary, a low-pass filter  44  can also be provided between the PWM setter  43  and DDH  46  and an error signal can also be inputted to the DDH  46  after noise which is superimposed on the error signal outputted from the PWM setter  43  is eliminated by this low-pass filter  44 .  
         [0053]      FIG. 5  is the section view showing one disposition of components constituting the oscillation circuit in this preferred embodiment. In the oscillation circuit of this preferred embodiment, each component is three-dimensionally disposed in a container in order to realize miniaturization.  
         [0054]     In  FIG. 5 , in the oscillation circuit of this preferred embodiment, a chip  53  constituting an oscillator and a temperature sensor  54  for detecting temperature, which are objects whose temperature is kept constant, are disposed inside a DDH  52  formed on a ceramic substrate  51 , using a thick-film resistor and are vacuum-sealed by an insulation material  55 . A glass epoxy substrate  56  on which a capacitor  57  and an inductance  58 , which constitute a low-pass filter, are mounted is connected to the opposite side of the ceramic substrate  51  by couplers  59   a  and  59   b.    
         [0055]     An Integrated circuit  61  obtained by integrating temperature control circuits composed of the error signal generator  41 , integrator  42  and PWM setter  43  which are shown in  FIG. 4 , decoupling capacitors  62  and  63  for power supply and heater current monitor and a resistor  64  for controlled temperature setting and reference voltage adjustment are disposed on the glass epoxy substrate  60 . This substrate  60  is opposed to and coupled with the glass epoxy substrate  56  by couplers  65   a  and  65   b , and are sealed by a metal cover  66 .  
         [0056]     By adopting such a configuration, the area of the ceramic substrate  51 , which is heated by the DDH  52 , can be reduced and also its consumption power can be reduced. Thus, inside temperature vacuum-sealed by the DDH  52  can be adjusted well responsively.  
         [0057]     In this configuration, firstly the DDH  52  is affected by the fluctuations of ambient temperature, and then, the respective temperatures of the temperature sensor  54  and chip  53  are affected. An influence on the temperature sensor  54  by the fluctuations of ambient temperature is extracted as an error signal, and by the temperature control circuit feeds back it to the DDH  52  as heater current, temperature can be controlled. Thus, since heat is difficult to go to the outside in a part sealed inside the DDH  52  in which the chip  53  and the like are disposed, temperature drop inclination in the area can be suppressed to a low level.  
         [0058]      FIGS. 6A, 6B  and  6 C show other configurations of the DDH.  
         [0059]     Although so far duplicated heater lines  72   a  and  73   a  are arrayed and formed on a substrate  75 , as shown in  FIG. 6A , the structure of the DDH in the preferred embodiment is not limited to this. For example, the components of the DDH can also be three-dimensionally formed against the substrate  75 .  
         [0060]      FIGS. 6B and 6C  show such structures of the DDH.  
         [0061]     In  FIG. 6B , one heater line  72   b  constituting the DDH is formed on the same surface as an object whose temperature is kept constant  71  of the substrate  75 , and the other heater line  73   b  is formed on the opposite surface of the substrate  75  as that on which the heater line  72   b  is formed.  
         [0062]     In  FIG. 6C , one heater line  72   c  is formed on the same surface of the substrate  75  as the object whose temperature is kept constant, as in  FIG. 6A . However, as for the other heater line  73   c , an insulation layer  74  is formed on the heater line  72   c , and the other heater line  73   c  is formed on the insulation layer.  
         [0063]     Even if the DDH is formed in any of the forms shown in  FIGS. 6A, 6B  and  6 C, in the oscillator of the preferred embodiment, an object whose temperature is kept constant can be enveloped in and heated to keep its temperature constant by a heater. Even when noise is superimposed on the heater lines, since respective electro-magnetic fields of a pair of heater lines mutually cancel, circuit components disposed at the center of the DDH are not affected by the noise.  
         [0064]     Although in the above-described preferred embodiments, in the DDH, an object whose temperature is kept constant is enveloped doubly in two heater lines, it can also be enveloped in three or more heater lines triply as long as respective noise can be mutually killed by the electromagnetic fields of a plurality of heater lines.