Abstract:
A crystal unit includes: a crystal piece; an excitation electrode configured to excite the crystal piece; a case configured to accommodate the crystal piece; an external electrode formed in the case and configured to be electrically connected to the excitation electrode; and an antenna formed in the case and configured to be electrically connected to the external electrode.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2014-138722 filed on Jul. 4, 2014, the entire contents of which are incorporated herein by reference. 
       FIELD 
       [0002]    The embodiments discussed herein are related to a crystal unit and a method of measuring the characteristics of the crystal unit. 
       BACKGROUND 
       [0003]    There has been known a technique in which, in a centrifugal acceleration test system of an oscillation unit using a crystal unit, an electromagnetic wave is broadcast at an oscillation frequency of the oscillation unit from a transmitting antenna formed in the oscillation unit and is received to measure the oscillation frequency. In the centrifugal acceleration test system, the oscillation unit includes an oscillation stage (oscillation circuit) having a crystal unit, an interference amplification stage having a buffer amplifier, a transmission amplification stage having a power amplifier, and a transmitting antenna. The transmitting antenna is formed outside the oscillation stage having the crystal unit. 
         [0004]    However, in the above-described configuration, since the transmission antenna is formed outside the crystal unit, the oscillation unit needs to include components other than the crystal unit, which may result in an increase in the size of the oscillation unit. 
         [0005]    The following is a reference document.
   [Document 1] Japanese Laid-open Patent Publication No. 2009-092544.   
 
       SUMMARY 
       [0007]    According to an aspect of the invention, a crystal unit includes: a crystal piece; an excitation electrode configured to excite the crystal piece; a case configured to accommodate the crystal piece; an external electrode formed in the case and configured to be electrically connected to the excitation electrode; and an antenna formed in the case and configured to be electrically connected to the external electrode. 
         [0008]    The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
         [0009]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0010]      FIGS. 1A and 1B  are schematic views illustrating a crystal unit  100  according to one example (Embodiment 1); 
           [0011]      FIG. 2  is a schematic view illustrating a reception state of electromagnetic wave by a receiving antenna  70  from an antenna  50 ; 
           [0012]      FIG. 3  is a schematic view illustrating one example of a circuit configuration incorporating the crystal unit  100 ; 
           [0013]      FIG. 4  is a view illustrating one example of a mounted state of the crystal unit  100 ; 
           [0014]      FIGS. 5A and 5B  are schematic views illustrating a crystal unit  102  according to another example (Embodiment 2); 
           [0015]      FIGS. 6A and 6B  are schematic views illustrating a crystal unit  103  according to another example (Embodiment 3); and 
           [0016]      FIG. 7  is an equivalent circuit diagram of an oscillation circuit incorporating the crystal unit  103 . 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0017]    Hereinafter, embodiments will be described in detail with reference to the accompanying drawings. 
         [0018]      FIGS. 1A and 1B  are schematic views illustrating a crystal unit  100  according to one example (Embodiment 1),  FIG. 1A  being a top view and  FIG. 1B  being a sectional view taken along the line B-B in  FIG. 1A . In  FIG. 1A , a cover of a case  30  is not illustrated to allow the interior of the crystal unit  100  to be viewed. In the following description, for the convenience of description, it is assumed that a thickness direction of a crystal piece  10  (a vertical direction in  FIG. 1B ) is a vertical direction and a side in which the cover of the case  300  is present is an “upper side.” However, a direction of a mounted state of the crystal unit  100  is optional. In addition, as used herein, the term an “outer surface” refers to a surface exposed to the outside of the case  30  and the term an “inner surface” refers to a surface exposed to the inner space of the case  30 . 
         [0019]    The crystal unit  100  includes a crystal piece  10 , an excitation electrode  20 , a case  30 , external electrodes  41  to  44  and an antenna  50 . The crystal unit  100  is of a surface mounting type as illustrated in  FIGS. 1A and 1B . 
         [0020]    The crystal piece  10  may be, for example, an AT cut synthetic crystal substrate. The crystal piece  10  may be supported in a cantilever structure to the case  30 . In the example illustrated in  FIGS. 1A and 1B , the crystal piece  10  is supported in the cantilever structure on a dam portion  31  of the case  30 . 
         [0021]    The excitation electrode  20  excites the crystal piece  10 . The excitation electrode  10  includes an upper excitation electrode formed on the upper surface of the crystal piece  10  and a lower excitation electrode  22  formed on the lower surface of the crystal piece  10 . The excitation electrode  20  excites the crystal piece  10  using a potential difference between the upper excitation electrode  21  and the lower excitation electrode  22 . The excitation electrode  20  may be made of gold, silver, or aluminum. 
         [0022]    The case  30  accommodates the crystal piece  10 . The case  30  is made of, for example, ceramic material. The case  30  includes a cover  34  (see, e.g.,  FIG. 2 ) and air-tightly seals the crystal piece  10  in its internal space. For example, the internal space of the case  30  is vacuous or filled with dry nitrogen and is sealed with the cover  34 . The cover  34  may be a metal plate or a ceramic plate. 
         [0023]    The external electrodes  41  to  44  are formed in the case  30 . In the example illustrated in  FIGS. 1A and 1B , the external electrodes  41  to  44  are formed on the outer surface of the bottom of the case  30 . The external electrodes  41  and  43  are electrically connected to the upper excitation electrode  21  and the lower excitation electrode  22 , respectively. In the example illustrated in  FIGS. 1A and 1B , the external electrode  41  is electrically connected to the upper excitation electrode  21  via a conductor pattern  45  formed on an inner layer of the case  30  and a conductor pattern  47  formed on the upper surface of the crystal piece  10 . The conductor pattern  45  has both ends exposed from the inner layer to the surface of the case  30 , with one end electrically connected to the external electrode  41  and the other end electrically connected to the conductor pattern  47  by a conductive adhesive  49 . 
         [0024]    Similarly, the external electrode  43  is electrically connected to the lower excitation electrode  22  via a conductor pattern  46  formed on the inner layer of the case  30  and a conductor pattern  48  formed on the lower surface of the crystal piece  10 . The conductor pattern  46  has both ends exposed from the inner layer to the surface of the case  30 , with one end electrically connected to the external electrode  43  and the other end electrically connected to the conductor pattern  48  by the conductive adhesive  49 . The conductive adhesive  49  is formed at an edge of the crystal piece  10  (an edge of a cantilever-supported side). In the example illustrated in  FIGS. 1A and 1B , the external electrodes  42  and  44  may be omitted. 
         [0025]    The antenna  50  is formed in the case  30 . In the example illustrated in  FIGS. 1A and 1B , the antenna  50  is formed on the inner layer of the case  30 , as in the conductor patterns  45  and  46 . For example, the antenna  50  is formed by firing a conductor on a ceramic material forming the case  30 . The shape of the antenna  50  is optional. In the example illustrated in  FIGS. 1A and 1B , the antenna  50  extends linearly. As illustrated in  FIG. 1A , the antenna  50  may have a bent portion such that its entire length becomes a predetermined length. The predetermined length may be determined depending on the oscillation frequency (designed value) of the crystal piece  10 . In the example illustrated in  FIGS. 1A and 1B , the antenna  50  has one end electrically connected to the conductor pattern  46  on the dam  31  and the other end which is a free end. The antenna  50  may extend in the same plane over the entire length or may extend in a partial section in a vertical direction or in an oblique and vertical direction. 
         [0026]    In operation of the crystal unit  100 , when the crystal piece  10  is oscillated at a certain frequency, an electric field (standing wave) is generated in the antenna  50  at that frequency. Accordingly, as schematically indicated by R in  FIG. 2 , an electromagnetic wave having a frequency corresponding to the oscillation frequency of the crystal piece  10  is radiated from the antenna  50 . Accordingly, as schematically illustrated in  FIG. 2 , by forming a receiving antenna  70  outside the crystal unit  100  and receiving the electromagnetic wave in the receiving antenna  70 , it becomes possible to measure the oscillation frequency of the crystal unit  100 . 
         [0027]    With the crystal unit  100  illustrated in  FIGS. 1A and 1B , since the antenna  50  is installed in the crystal unit  100 , it becomes possible to measure the oscillation frequency of the crystal unit  100  externally. Thus, for example, for the crystal unit  100  in the mounted state, it is possible to measure the oscillation frequency. As the oscillation frequency may be measured, it becomes possible to make comparison of relative characteristics with non-defective products. In addition, since the antenna  50  is installed in the case  30 , it becomes possible to make the crystal unit  100  compact as compared to a case where the antenna is externally attached to the outside of the case  30 . 
         [0028]    In the example illustrated in  FIGS. 1A and 1B , the antenna  50  is formed in the aspect of electrical connection to the lower excitation electrode  22 . Instead, the antenna  50  may be formed in the aspect of electrical connection to the upper excitation electrode  21 . Alternatively, in addition to the antenna  50 , a second antenna (not illustrated) may be formed in the aspect of electrical connection to the upper excitation electrode  21 . 
         [0029]      FIG. 3  is a schematic view illustrating one example of a circuit configuration incorporating the crystal unit  100 . 
         [0030]    In the example illustrated in  FIG. 3 , the crystal unit  100  is connected to an IC (Integrated Circuit)  200 . That is, the external electrodes  41  and  43  of the crystal unit  100  are respectively connected to an input terminal  202  and an output terminal  204  of the IC  200 . The crystal unit  100  generates a clock used in the IC  200 . The IC  200  includes an inverting amplifier  206  and an output buffer  208 . A signal input to the input terminal  202  is inverted and amplified by the inverting amplifier  206 . The inverted and amplified signal is input to the output buffer  208  and is supplied to the upper excitation electrode  21  via the external electrode  43 . 
         [0031]    Matching capacitors  300  are connected to the crystal unit  100 . More specifically, a first capacitor  302  is connected between the external electrode  41  of the crystal unit  100  and a ground, and a second capacitor  304  is connected between the external electrode  43  of the crystal unit  100  and the ground. With regard to the IC  200 , for example, terminal internal capacitance, stray capacitance of wiring patterns of a mounting substrate, and resistance limiting a current flown into the crystal unit  100  are not illustrated in  FIG. 3 . The matching capacitors  300  are formed to adjust the oscillation frequency of the crystal unit  100  such that the oscillation frequency becomes a desired value (designed value) when the total capacitance (load capacitance) including all circuit capacitance ranging from the crystal unit  100  to the IC  200  is assumed as a load. In  FIG. 3 , a range enclosed by a dotted line forms an oscillation circuit. 
         [0032]    The IC  200  may include terminals  220  and  222  for monitoring the oscillation circuit. However, these terminals  220  and  222  may be omitted. This is because the oscillation frequency of the crystal unit  100  may be measured (monitored) as the crystal unit  100  includes the antenna  50 , as described above. Accordingly, the crystal unit  100  illustrated in  FIGS. 1A and 1B  eliminates a need to form the terminals  220  and  222 , thereby achieving a simplification of the IC  200 . 
         [0033]    In addition, in the input side and output side of the crystal unit  100 , since the signal is amplified by the inverting amplifier  206 , the output side of the crystal unit  100  has the larger amplitude of the signal than the input side thereof. Accordingly, the antenna  50  may be connected to the output side of the crystal unit  100 , as illustrated in  FIG. 3 . In the example illustrated in  FIG. 3 , the upper excitation electrode  21  and the lower excitation electrode  22  may be reversed. 
         [0034]      FIG. 4  is a view illustrating one example of a mounted state of the crystal unit  100 . 
         [0035]    As illustrated in  FIG. 4 , the crystal unit  100  may be mounted on a substrate  90 . In the example illustrated in  FIG. 4 , a peripheral component  92  is mounted near the crystal unit  100 . 
         [0036]    In the meantime, in recent years, compactness and high density mounting of parts and modules have been progressed to meet the demands for a device downsizing. Compactness (for example, 3.2×2.5 mm, 2.5×2.0 mm and 2.0×1.6 mm) of crystal units serving as clock sources has been also unexceptionally progressed. Under such circumstances, when the functional failure of a device is deemed to have occurred due to the abnormality of a crystal unit, it is useful to be able to measure electrical characteristics of the crystal unit with it mounted in the device. This is because taking out only the crystal unit mounted with high density for measurement is accompanied by a risk of destroying peripheral components when removing the crystal unit. 
         [0037]    In this regard, in the mounted state of the crystal unit, it may be possible to make probe measurement of high impedance. However, with recent trend of downsizing, there may be a case where the IC  200  does not have a terminal which may verify an oscillation state (see, e.g., the terminals  220  and  222  in  FIG. 3 ) and terminals are hidden in the back side of an IC package by BGA (Ball Grid Array). In addition, there may be a case where no probing point is present, such as, for example, the matching capacitors  300  being incorporated in the IC  200 , and provision of terminals in the backside of the crystal unit  100 . In addition, with the progress of a high density mounting, there may be a case where there is no site that the probe  78  contacts physically, as schematically illustrated in  FIG. 4 . In addition, even when a probing point is present, if the margin of the design of an oscillation circuit is insufficient, there may be a case where an oscillation state is changed (from oscillation to non-oscillation and vice versa) by only a few pF capacitance applied by the probe  78 , thereby making a correct measurement impossible. 
         [0038]    In this regard, with the crystal unit  100  illustrated in  FIGS. 1A and 1B , since the crystal unit  100  includes the antenna as described above, the oscillation frequency of the crystal unit  100  may be measured with high precision even when probe measurement is impossible or difficult. 
         [0039]      FIGS. 5A and 5B  are schematic sectional views illustrating a crystal unit  102  according to one example (Embodiment 2),  FIG. 5A  being a sectional view and  FIG. 5B  being a side view when viewed in the right side of  FIG. 5A . 
         [0040]    The crystal unit  102  according to Embodiment 2 is different from the crystal unit  100  according to Embodiment 1 in that an antenna  52  is used for the antenna  50 . Other configurations of Embodiment 2 may be substantially the same as the configurations of Embodiment 1. 
         [0041]    The antenna  52  is installed on the outer surface of the case  30 . The antenna  52  may be formed on one side of the case  30  or may be formed over a plurality of sides of the case  30 . For example, the antenna  52  may be formed by firing a conductor on a ceramic material that forms the case  30 . In the example illustrated in  FIGS. 5A and 5B , the antenna  52  is formed on one side of the case  30 . The shape of the antenna  52  is optional. In the example illustrated in  FIGS. 5A and 5B , the antenna  52  extends linearly. As illustrated in  FIG. 5B , the antenna  52  may have a bent portion such that its entire length becomes a predetermined length. The predetermined length may be determined depending on the oscillation frequency (designed value) of the crystal piece  10 . In the example illustrated in  FIGS. 5A and 5B , the antenna  52  has one end electrically connected to the conductor pattern  46  (or the lower excitation electrode  22 ) and the other end which is a free end. 
         [0042]    The crystal unit  102  illustrated in  FIGS. 5A and 5B  illustrates the same effects as the crystal unit  100  illustrated in  FIGS. 1A and 1B . 
         [0043]    In the example illustrated in  FIGS. 5A and 5B , the antenna  52  is installed on the outer surface of the side of the case  30 . However, the antenna  52  may be installed on the outer surface of the bottom of the case  30  or may be installed on the outer surface of the cover  34 . In addition, the antenna  52  needs not be installed on the outer surface of the case  30  but may be installed on the inner layer or the inner surface of the case, as in the antenna  50 . In this way, the antenna  52  may be installed at any place. 
         [0044]    In the example illustrated in  FIGS. 5A and 5B , the antenna  52  is formed in the aspect of electrical connection to the lower excitation electrode  22 . Instead, the antenna  52  may be formed in the aspect of electrical connection to the upper excitation electrode  21 . Alternatively, in addition to the antenna  52 , a second antenna (not illustrated) may be formed in the aspect of electrical connection to the upper excitation electrode  21 . 
         [0045]      FIGS. 6A and 6B  are schematic views illustrating a crystal unit  103  according to another example (Embodiment 3),  FIG. 6A  being a top view and  FIG. 6B  being a sectional view taken along line C-C in  FIG. 6A . In  FIGS. 6A and 6B , a sorter  80  and a variable capacitor  82  are schematically illustrated. In  FIG. 6A , only the sorter  80  is schematically indicated by a broken line.  FIG. 7  is an equivalent circuit diagram of an oscillation circuit incorporating the crystal unit  103 . 
         [0046]    The crystal unit  103  according to Embodiment 3 is different from the crystal unit  100  according to Embodiment 1 in that an antenna  53  is used for the antenna  50 , and the sorter  80  and the variable capacitor  82  are formed. Other configurations of Embodiment 3 may be substantially the same as the configurations of Embodiment 1. In  FIGS. 6A and 6B , substantially the same elements as those in  FIGS. 1A and 1B  are denoted by the same reference numerals and explanation of which will not be repeated. 
         [0047]    The crystal unit  103  includes a crystal piece  10 , an excitation electrode  20 , a case  30 , a seal portion (one example of an insulating portion)  32 , external electrodes  41  to  44  (not partially illustrated), an antenna  53 , a sorter (one example of a conductor)  80 , and a variable capacitor  82 . 
         [0048]    The seal portion  32  is formed between the antenna  53  and the external electrode  43 , and makes electrical isolation between the antenna  53  and the external electrode  43 . In the example illustrated in  FIGS. 6A and 6B , the seal portion  32  is formed on the circumference of the cover  34  in which the antenna  53  is formed. The seal portion  32  is formed to increase sealability of the cover  34  on the circumference (airtightness of the case  30 ). The seal portion  32  is made of, for example, insulating material such as, for example, glass. In the example illustrated in  FIGS. 6A and 6B , the external electrode  43  is connected to an upper electrode  60  via a conductor pattern  62 . The conductor pattern  62  is formed on the outer surface of the side of the case  30 , and the upper electrode  60  is formed on the outer surface of the top of the case  30 . Accordingly, in the example illustrated in  FIGS. 6A and 6B , as the seal portion  32  is formed between the upper electrode  60  and the antenna  53 , the seal portion  32  makes an electrical isolation between the antenna  53  and the external electrode  43 . 
         [0049]    The antenna  53  is formed on the outer surface of the cover  34 . For example, the antenna  53  is formed by firing a conductor on a ceramic material that forms the cover  34 . The shape of the antenna  53  is optional. In the example illustrated in  FIGS. 6A and 6B , the antenna  53  extends linearly. As illustrated in  FIG. 6A , the antenna  53  may have a bent portion such that its entire length becomes a predetermined length. The predetermined length may be determined depending on the oscillation frequency (designed value) of the crystal piece  10 . In the example illustrated in  FIGS. 6A and 6B , the antenna  53  has one end extending to the vicinity of the upper electrode  60  (but being electrically isolated from the upper electrode  60 ) and the other end which is a free end. 
         [0050]    The sorter  80  may be attached to the case  30 . In the example illustrated in  FIGS. 6A and 6B , the sorter  80  may be attached between the antenna  53  and the upper electrode  60  in the case  30 . An aspect of attachment of the shorter  80  to the case  30  is optional. For example, the sorter  80  may be attached via, for example, a fastener or may be simply mounted. The sorter  80  is formed by a conductor and its resistance may be substantially zero (0). The sorter  80  may have a shape of a jumper element (jumper chip/jumper lead). The sorter  80  electrically connects the antenna  53  and the external electrode  43  under a state where the sorter  80  is attached to the case  30 . In the example illustrated in  FIGS. 6A and 6B , under the state where the sorter  80  is attached to the case  30 , the sorter  80  electrically connects the antenna  53  and the external electrode  43  by making a connection between the antenna  53  and the upper electrode  60 . As illustrated in  FIG. 7 , the sorter  80  acts as a switch as a circuit and the state of attachment of the shorter  80  to the case  30  corresponds to a state where the switch is closed. 
         [0051]    The variable capacitor  82  has one end connected between the shorter  80  and the antenna  53  and the other end connected to a ground G. That is, the variable capacitor  82  is formed in a line  84  connecting the sorter  80  to the ground G. Under the state where the sorter  80  is attached to the case  30 , as illustrated in  FIG. 7 , the variable capacitor  82  is connected in parallel to the second capacitor  304  of the matching capacitors  300  between the antenna  53  and the external electrode  43 . At this time, when the capacitance of the variable capacitor  82  is varied, the frequency of an output signal of the inverting amplifier  206  is varied, and the frequency of an electromagnetic wave transmitted from the antenna  53  is also varied. 
         [0052]    The crystal unit  103  illustrated in  FIGS. 6A and 6B  illustrates the same effects as the crystal unit  103  illustrated in  FIGS. 1A and 1B . In addition, with the crystal unit  103 , since the electrical isolation between the antenna  53  and the external electrode  43  is made by the seal portion  32  under a state where the sorter  80  is not attached to the case  30 , it is possible to reduce an effect of an external noise which may be received via the antenna  53 . That is, the antenna  53  may serve as a receiving antenna, which means that the external noise received from the antenna  53  may have an effect on the operation of the crystal unit  103 . In this regard, with the crystal unit  103 , by attaching the sorter  80  to the case  30  only when measuring the oscillation frequency of the crystal unit  103 , it is possible to eliminate the effect of the antenna  53  on the operation of the crystal unit  103  for other cases. 
         [0053]    In the example illustrated in  FIGS. 6A and 6B , the antenna  53  is formed on the outer surface of the cover  34 . However, the antenna  53  may be formed on the inner layer or inner surface of the cover  34  in such a manner that the antenna  53  is exposed to the outer surface of the cover  34  in an attachment portion of the sorter  80 . In addition, the antenna  53  may be installed on the outer surface of the side of the case  30  or may be install on the outer surface of the bottom of the case  30 . In addition, the antenna  53  may be formed by the cover  34  itself. In addition, in the example illustrated in  FIGS. 6A and 6B , although the conductor pattern  62  is formed on the outer surface of the case, it may be formed on the inner layer of the case  30 . 
         [0054]    In the example illustrated in  FIGS. 6A and 6B , the antenna  53  is formed in the aspect of electrical connection to the lower excitation electrode  22 . Instead, the antenna  53  may be formed in the aspect of electrical connection to the upper excitation electrode  21 . Alternatively, in addition to the antenna  53 , a second antenna (not illustrated) may be formed in the aspect of electrical connection to the upper excitation electrode  21 . 
         [0055]    In the example illustrated in  FIGS. 6A and 6B , the variable capacitor  82  may be formed. However, the variable capacitor  82  may also be omitted as well. The meaning of the variable capacitor  82  will be described in connection with an oscillation frequency measuring method as described below. 
         [0056]    Next, a method of measuring the oscillation frequency of the crystal unit  103  illustrated in  FIGS. 6A and 6B  will be described. 
         [0057]    In measurement of the oscillation frequency, first, the sorter  80  is attached to the case  30  and the antenna  53  and the external electrode  43  are electrically connected. The crystal unit  103  enters an operation state. The receiving antenna  70  ( FIG. 7 ) is installed and an electromagnetic wave is received from the antenna  53 . A signal according to the electromagnetic wave received in the receiving antenna  70  is processed in a signal processing apparatus  76  and the oscillation frequency of the crystal unit  103  is measured. For example, the signal processing apparatus  76  outputs the peak frequency of the received signal by performing a process such as, for example, FFT (Fast Fourier Transform). 
         [0058]    At this time, if a plurality of peak frequencies exists near a designed value of the oscillation frequency of the crystal unit  103  due to a noise effect, the capacitance of the variable capacitor  82  is changed. The capacitance of the variable capacitor  82  may be changed manually or may be changed according to a command from the signal processing apparatus  76  (a command generated by executing a program). As described above, when the capacitance of the variable capacitor  82  is changed, the frequency of the output signal of the inverting amplifier  206  is changed and the frequency of the electromagnetic wave transmitted from the antenna  53  is also changed. On the other hand, with regard to noise, even when the capacitance of the variable capacitor  82  is changed, a frequency is not remarkably changed. 
         [0059]    This is used to select a peak whose frequency is changed, of a plurality of peaks obtained as a result of FFT when the capacitance of the variable capacitor  82  is changed, as an object to be measured. This selection may be achieved either manually (by naked eyes of an examiner) or by the signal processing apparatus  76 . When the peak of the object to be measured is selected, the capacitance of the variable capacitor  82  is changed to zero (0) (or the variable capacitor  82  is removed) and a frequency according to the peak of the object to be measured is determined as the oscillation frequency of the crystal unit  103 . This determination may be achieved by an examiner or the signal processing apparatus  76 . 
         [0060]    In this way, since the crystal unit  103  illustrated in  FIGS. 6A and 6B  includes the variable capacitor  82 , even when a plurality of peak frequencies exists near the designed value of the oscillation frequency of the crystal unit  103 , it is possible to measure the oscillation frequency of the crystal unit  103  with a high precision. This is particularly useful when the electromagnetic wave transmitted from the antenna  53  is weak. In other words, it is possible to measure the oscillation frequency of the crystal unit  103  with high precision without high performance of the antenna  53 . 
         [0061]    All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a illustrating of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.