Abstract:
A method of evaluating quality of a crystal unit, capable of performing quantitative measurement of an actual operation of a crystal unit which is to be oscillated in an actual oscillator to ensure an accurate quality evaluation, is provided which comprises the steps of increasing a DC input voltage of a crystal oscillator, said crystal oscillator having at least one AGC amplifier whose amplification rate varies depending on the DC input voltage and having a crystal unit connected thereto; measuring a maximum value of the DC input voltage at a start of oscillation of the crystal oscillator; and evaluating quality of the crystal unit by the measured maximum value.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a method of evaluating quality of a crystal unit, more specifically, to a novel method capable of an accurate quality evaluation of a crystal unit.  
           [0003]    2. Description of the Related Art  
           [0004]    Conventionally, for quality evaluation of a crystal unit, there is used a method using a network analyzer (hereinafter referred to as a network analyzer method). In this network analyzer method, a crystal unit is considered as a passive element constituted of inductance L, capacitance C, resistance R, and a serial resonance frequency and a serial resonance resistance CI (crystal impedance) of the crystal unit is obtained by giving an AC signal thereto, and then the quality of the crystal unit is evaluated from the resultant magnitude of the CI.  
           [0005]    However, in the network analyzer method, parameters such as CI are measured under a condition in which the crystal unit is oscillated in a stable state by the externally given signal without evaluating any transient state at the start of the oscillation in an actual oscillator. This causes a problem that a crystal unit evaluated as having good quality by the network analyzer method does not oscillate often in an actual oscillator in which the oscillation is not always in a stable state.  
         SUMMARY OF THE INVENTION  
         [0006]    In view of the foregoing, it is an object of the present invention to provide a novel method of evaluating quality of a crystal unit, capable of performing quantitative measurement of an actual operation of a crystal unit which is to be oscillated in an actual oscillator to ensure an accurate quality evaluation of the crystal unit.  
           [0007]    Here, “the quality evaluation of crystal unit” means to evaluate whether the crystal unit has good or poor quality, in other words, whether the crystal unit is acceptable or unacceptable for practical use. “The crystal unit has good quality (or is acceptable)” means that in an actual oscillator, an oscillation starts normally and its frequency variation is little. Hereinafter, this crystal unit is referred to simply as “an acceptable crystal unit”. “The crystal unit has poor quality (or is unacceptable)” means that in an actual oscillator, an oscillation does not start at all, or the value of CI or frequency variation is abnormally large due to variation in driving power. Hereinafter, this crystal unit is referred to simply as “an unacceptable crystal unit”. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    The foregoing objects, features and advantages of the present invention will be better understood from the following detailed description, taken in conjunction with the accompanying drawings, in which:  
         [0009]    [0009]FIG. 1 is a diagram showing an example of a crystal oscillator used in a method of evaluating quality of a crystal unit according to the present invention;  
         [0010]    [0010]FIG. 2 is a diagram showing an example of an equivalent circuit of a crystal unit;  
         [0011]    [0011]FIG. 3 is a graph for explaining an example of measurement and evaluation relating to oscillation states of the crystal oscillator in FIG. 1;  
         [0012]    [0012]FIG. 4 is a graph for explaining an example of measurement and evaluation after a stable oscillation state of the crystal oscillator in FIG. 1; and  
         [0013]    [0013]FIG. 5 is a diagram showing another example of a crystal oscillator used in the method of evaluating quality of a crystal unit according to the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0014]    Embodiment 1  
         [0015]    [0015]FIG. 1 shows an example of a crystal oscillator used in the method of evaluating quality of a crystal unit according to the present invention.  
         [0016]    In the crystal oscillator shown in FIG. 1, XTAL 1  is a crystal unit to be evaluated;  
         [0017]    GV 1  is an AGC (automatic gain control) amplifier whose amplification rate changes depending on a DC input voltage VAGC;  
         [0018]    C 1  is an input capacitor of the crystal oscillator;  
         [0019]    R 1 , C 2  and AV 1  denote, respectively, a resistance, capacitor and high gain amplifier, together comprising an integrating circuit, wherein the AGC amplifier GV 1  is connected to its input side;  
         [0020]    AV 2  is an error amplifier for driving the crystal unit with a stabilized power (or a stabilized current, or a stabilized voltage);  
         [0021]    VAV 2  is an output voltage from the error amplifier;  
         [0022]    Vp (Vi, Vv) is a DC voltage in proportion to a driving power (or a driving current, or a driving voltage) of the crystal unit;  
         [0023]    VP (VI, VV) is a DC voltage for setting a driving power (or a driving current, or a driving voltage) of the crystal unit;  
         [0024]    D 1  and E 1  are a diode and a voltage source, both for setting the maximum voltage of the DC input voltage VAGC; and  
         [0025]    R 2  is a voltage dividing resistor.  
         [0026]    Vp(Vi, Vv) means that the DC voltage may be any one of DC voltages Vp, Vi and Vv proportional to a driving power, driving current and driving voltage of the crystal unit, respectively. VP(VI, VV) means that the DC voltage may be any one of DC voltages VP, VI and VV for setting a driving power, driving current and driving voltage of the crystal unit, respectively.  
         [0027]    [0027]FIG. 2 shows an example of an equivalent circuit of the crystal unit. In FIG. 2, Lx, Cx and Rx denote, respectively, an equivalent serial resonance inductance, equivalent capacitance and equivalent resistance of the crystal unit, connected in series to each other.  
         [0028]    As apparent from FIGS. 1 and 2, the condition of continuous oscillation of the crystal oscillator having at least one AGC amplifier whose amplification rate changes depending on the DC input voltage VAGC, can be denoted by following equation with the equivalent resistance Rx of the crystal unit taken as CI.  
       CI   =     GV1       ω   2     ·   C1   ·   C2   ·   R1             where         ω   =       1       Lx   ·   Cx              (     1   +     1   2     +     Cx   C1       )                             
 
         [0029]    In this equation, GV 1  is the amplification rate of the AGC amplifier GV 1  (hereinafter referred to simply as “the amplification rate GV 1 ”), and CI is proportional to the amplification rate GV 1 . When the DC input voltage VAGC to the AGC amplifier is monotonously increasing in relation to the amplification rate GV 1 , the VAGC is not necessarily required to be proportional to the amplification rate GV 1 . Conversely, with the DC input voltage VAGC proportional to the amplification rate GV 1 , CI of the crystal unit is to be proportional to the DC input voltage VAGC when the oscillation in the crystal oscillator becomes in a stable state. In other word, with the AGC amplifier having the amplification rate GV 1  being proportional to the DC input voltage VAGC, the DC input voltage VAGC can be measured as a value proportional to the CI of the crystal unit. This is the same for measurements of the DC input voltages VAGC in later described embodiments 2 to 4.  
         [0030]    In the method of the present invention, using the crystal oscillator as shown in FIG. 1, the maximum value of the DC input voltage at a start of oscillation in the crystal oscillator is measured with the DC input voltage VAGC made increased. From the measured maximum value, quality of a crystal unit can be evaluated.  
         [0031]    [0031]FIG. 3 is a graph explaining the measurement and evaluation of quality of a crystal unit in the above case. In FIG. 3, the horizontal axis represents the voltage of the voltage source E 1  (hereinafter, this voltage is referred to as E 1 ), and the vertical axis represents the DC input voltage VAGC provided by E 1  to the AGC amplifier.  
         [0032]    First, when E 1 =0, no oscillation is started, so that the DC voltage Vp (Vi, Vv) proportional to the driving power (driving current, driving voltage) of the crystal unit is equal to  0 , which is inputted to one of the input terminals of the error amplifier AV 2  as a differential amplifier. To the other input terminal, there is inputted the DC voltage VP (VI, VV) set at a specified value, which allows the error amplifier AV 2  to output the output voltage VAV 2  as a differential output that is equal to VP (VI, VV). Since VAV 2  is larger than E 1  (=0), a forward voltage is applied to the diode D 1 , which is taken as being zero. This makes the DC input voltage VAGC supplied from a connection of the resistor R 2  and the diode D 1  equal to E 1 , so that VAGC is made zero.  
         [0033]    As E 1  is increased from zero, VAGC increases with an increase in E 1  with the value thereof equal to E 1  until the crystal unit starts to oscillate because the DC voltage Vp (Vi, Vv) is remained as being zero 0 and the output voltage VAV 2  is kept to be VP (VI, VV) which is larger than E 1 .  
         [0034]    When the increased E 1 , hence VAGC, reaches a certain voltage VM 1  or VM 2 , transient oscillation is started. Once the oscillation is started, the DC voltage Vp (Vi, Vv) is generated which is proportional to a driving power (driving current, driving voltage) of the crystal unit. This reduces VAV 2  as the differential output voltage of the error amplifier AV 2  to further reduce the DC input voltage VAGC from VM 1  or VM 2  to a certain value of VS 1  or VS 2  because a reverse voltage is applied to the diode D 1  at this time to cut the connection from the E 1 . With this, the oscillation is brought from the transient state to a stable state. Here, VS 1  is the DC input voltage VAGC after the start of oscillation of an acceptable crystal unit, and VS 2  is that of an unacceptable crystal unit.  
         [0035]    As explained above, the value of the driving power (driving current, driving voltage) of the crystal unit at a transition from a transient state to a stable state becomes proportional to the DC voltage VP (VI, VV) for setting the driving power, or to the detected value of the voltage Vp (Vi, Vv) of actual operation. In addition, a voltage VM 2 , at which an unacceptable crystal unit starts oscillation, is larger than a voltage VM 1  at which an acceptable crystal unit starts oscillation. The level of VS 2  sometimes becomes approximately equal to that of VS 1 . Both of the levels of VS 1  and VS 2  are determined by the conditions of the crystal oscillator and crystal unit, namely, the levels are characteristic of the crystal oscillator and crystal unit.  
         [0036]    Therefore, it is possible to accurately evaluate quality of a crystal unit by increasing the DC input voltage VAGC with the DC voltage VP (VI, VV) being fixed, measuring the maximum value of VAGC at the start of oscillation in the crystal oscillator shown in FIG. 1, and determining whether the measured maximum value is VM 1 , the oscillation starting voltage characteristic of the acceptable crystal unit, or VM 2 , the oscillation starting voltage characteristic of the unacceptable oscillator.  
         [0037]    In addition, the oscillation starting voltages VM 1  and VM 2  become larger as an increasing speed of the voltage E 1  becomes faster. This is due to inertia relating to the equivalent inductance and equivalent capacitance of the crystal unit. However, the voltage levels VS 1  and VS 2  are not changed regardless of the increasing speed of E 1 . Thus, it is desirable to keep the increasing speed of E 1 , that is, the increasing speed of the DC input voltage VAGC, constant.  
         [0038]    Moreover, it is not necessary for the DC input voltage VAGC (or E 1 ) to be increased from the initial value zero, but may be from an arbitrary value less than VM 1  insofar as the measurement of the maximum value thereof is accurately made.  
         [0039]    Embodiment 2  
         [0040]    In the Embodiment 1, the DC input voltage VAGC (or E 1 ) is made continuously increased. However, as shown in FIG. 3, the DC input voltage VAGC (or E 1 ) can be set at an arbitrary voltage as an evaluation voltage VMC between the voltages VM 1  and VM 2 . In this case, a crystal unit starting oscillation with the above set VAGC is proved to be acceptable one, which is to be brought in a stable state at the oscillation starting voltage VM 1 . While, a crystal unit starting no oscillation with the above set VAGC is proved to be unacceptable one, which is to be brought in a stable state at the oscillation starting voltage VM 2 . This allows the quality evaluation to be carried out more simply.  
         [0041]    Embodiment 3  
         [0042]    [0042]FIG. 4 is a graph showing an example of a relationship between the DC input voltage VAGC and the DC voltage VP (VI, VV) for setting the driving power (driving current, driving voltage) of the crystal unit after the start of oscillation. The insert is a simplified FIG. 3 as a reference.  
         [0043]    As is apparent from FIG. 4, an acceptable crystal unit {circle over ( 1 )} exhibits little change in the DC input voltage VAGC to the change in the setting DC voltage VP (VI, VV). This expresses that there is little change in CI of the crystal unit. Contrary to this, unacceptable crystal units {circle over ( 2 )} and {circle over ( 3 )} exhibit various changes in the DC input voltage VAGC to the change in the setting DC voltage VP (VI, VV). In general, the values of the DC input voltages VAGCs in this case become larger than that for the acceptable crystal unit.  
         [0044]    Thus, in a stable state after the start of oscillation, with the setting DC voltage VP (VI, VV) being set at one or several points in evaluation voltage value, that is, with the driving power (or may be driving current or driving voltage) of a crystal unit being set at one or several points in evaluation power value (or may be current value or voltage value), evaluation of quality of the crystal unit can be accurately carried out by measuring, at each of the above set values, either one of the DC input voltage VAGC or oscillation frequency or both of them.  
         [0045]    In the case of the evaluation power value being set at one point, when the measured value of the DC input voltage VAGC or frequency is the same as the value intrinsic to an acceptable crystal unit {circle over ( 1 )} in stable oscillation, the crystal unit can be evaluated as being acceptable. While, when the measured value is the same as the value intrinsic to an unacceptable crystal unit {circle over ( 2 )} or {circle over ( 3 )} in stable oscillation, the crystal unit can be evaluated as being unacceptable.  
         [0046]    In the case of the evaluation power value being set at a plurality of points, when the change in the measured values of the DC input voltages VAGCs or frequencies for respective evaluation power values is the same as the change intrinsic to an acceptable crystal unit {circle over ( 1 )} in stable oscillation, that is, the values are constant, the crystal unit can be evaluated as being acceptable. While, when the change in the measured values is the same as the change intrinsic to an unacceptable crystal unit {circle over ( 2 )} or {circle over ( 3 )} in stable oscillation, the crystal unit can be evaluated as being unacceptable.  
         [0047]    Embodiment 4  
         [0048]    In the above Embodiment 3, with the driving power (or may be driving current or driving voltage) of the crystal unit being set at one or several points of evaluation power value, the DC input voltage VAGC or oscillation frequency is measured at each of the set points. On the other hand, a continuous measurement of one or both of the DC input voltage VAGC or oscillation frequency to the continuously changed DC voltage VP (VI, VV), that is, to the continuously changed driving power (may be driving current or driving voltage) of a crystal unit, may be performed in the method of the present invention, thereby to allow the quality of the crystal unit to be more efficiently evaluated. Namely, with a continuously measured value found to be constant, the crystal unit can be evaluated as being acceptable. While, with a continuously measured value being changed, the crystal unit can be evaluated as being unacceptable.  
         [0049]    Embodiment 5  
         [0050]    [0050]FIG. 5 is a diagram showing another example of a crystal oscillator used in the method of the present invention. The oscillator is configured so that one more AGC amplifier GV 2  is inserted in the previously described crystal oscillator shown in FIG. 1.  
         [0051]    In FIG. 5,  
         [0052]    f/V is an F-V (frequency to voltage) converter;  
         [0053]    V/V 2  is a square multiplication circuit;  
         [0054]    f is an oscillation frequency of the crystal oscillator;  
         [0055]    Vf is a DC output voltage proportional to the oscillation frequency f; and  
         [0056]    Vf 2  is a DC output proportional to the square of Vf.  
         [0057]    At the F-V converter f/V, the inputted oscillation frequency f is converted to the DC output voltage Vf proportional to the oscillation frequency f. The DC output voltage Vf is then inputted to the square multiplication circuit V/V 2 , from which the DC output Vf 2  is outputted with a voltage proportional to the square of Vf and then inputted to the AGC amplifier GV 2 .  
         [0058]    With the additional AGC amplifier GV 2  as in FIG. 5, an influence of the oscillation frequency of the value of CI in the aforementioned equation is effectively compensated so as to allow the DC input voltage VAGC to be taken as the CI of the crystal unit driven with various frequencies.  
         [0059]    It is to be understood that the present invention is not limited to the embodiments as explained above, but various changes and modifications in detail are possible without departing from the spirit and scope of the invention. For example, the crystal oscillator, although shown in FIGS. 1 and 5 as being provided with one and two AGC amplifiers, respectively, may have more AGC amplifiers. This, like in the above embodiments, makes it possible to measure various values necessary for evaluating quality of a crystal unit for realization of accurate quality evaluation. It is also to be understood that circuit elements other than the crystal units and AGC amplifiers are not limited to those shown in FIGS. 1 and 5. They can be changed and modified insofar as the above values for can be measured for realization of quality evaluation.  
         [0060]    As is explained above in detail, according to the present invention, a novel method of evaluating quality of a crystal unit can be provided. The method can quantitatively measure an actual operation of a crystal unit which is to be oscillated in an actual oscillator to permit sure and accurate evaluation of the quality of the crystal unit.