GB1596079A - Composite piezo-electric vibrator - Google Patents

Description

(54) COMPOSITE PIEZO-ELECTRIC VIBRATOR (71) We, KABUSHIKI KAISHA DAINI SEIKOSHA, a Japanese body corporate of 6-31-1, Kameido, Koto-ku, Tokyo, Japan, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention concerns a composite piezo-electric vibrator and, although the invention is not so restricted, it is more particularly concerned with a composite piezo-electric vibrator for use in a small instrument such as an electronic wrist watch.

According to the present invention there is provided a composite piezo-electric vibrator comprising a plurality of individual piezo-electric vibrators which have different frequency-temperature characteristics and which are connected in parallel, each individual piezo-electric vibrator having a said characteristic which is constituted by a quadratic curve which has a maximum.

Preferably there are two individual piezoelectric vibrators whose frequency temperature characteristics have their maxima at relatively low and relatively high temperatures ToL and ToH respectively. In this case, the arrangement is preferably such that ToLATl+(AT/10), and ToH > T2-(AT/10), where AT=ToH-ToL, and the desired working temperature range is from Tl to T2.

ToL is preferably in the range -20"C to 5"C, and ToH is preferably in the range 35"C to 60"C.

The invention also comprises an electronic timepiece provided with a composite piezo-electric vibrator as set forth above.

The invention is illustrated, merely by way of example, in the accompanying drawings, in which: Figure 1 shows the frequencytemperature characteristic of a single conventional quartz crystal vibrator, Figure 2 shows the frequencytemperature characteristics of a conventional AT cut and a conventional GT cut quartz crystal vibrator, Figure 3 shows the frequencytemperature characteristics of two individual piezo-electric vibrators which may form part of a composite piezo-electric vibrator according to the present invention, and also shows two possible frequencytemperature characteristics of a composite piezo-electric vibrator formed by connecting the two said individual vibrators in parallel, Figure 4 is a graph illustrating the frequency-temperature characteristics of two individual piezo-electric vibrators which cannot be used in a composite vibrator according to the present invention, and also shows the frequency-temperature characteristic of the composite vibrator which is produced by connecting the lastmentioned individual vibrators in parallel, Figure 5 is a graph illustrating the impedance-frequency characteristic of two parallel-connected vibrators.

Figure 6 is a graph illustrating certain features of a composite piezo-electric vibrator according to the present invention, and Figure 7 is a circuit diagram illustrating a composite piezo-electric vibrator according to the present invention.

In Figure 1 there is shown the frequencytemperature characteristic 1 of a known quartz crystal vibrator which is used in a conventional electronic wrist watch. The vibrator whose characteristic 1 is shown in Figure 1 may be either of the flexural vibrating mode or of the longitudinal vibrating mode and is the one mainly used in an electronic wrist watch because of its size, frequency, and the possibility of mounting it, properly. However, as clearly shown in Figure 1, such a conventional quartz crystal vibrator, whose frequency temperature characteristic has its maximum at temperature To, does not have a constant frequency over a wide temperature range and, in order to overcome this disadvantage, it has been necessary to use a temperature compensating element such as a temperature compensating condenser.

However, if such a temperature compensating element is incorporated into an oscillating circuit of an electronic wrist watch, the design of the oscillating circuit is thereby made difficult and its power consumption is increased. This increase in the power consumption is especially inconvenient having regard to the fact that it is not practicable in an electronic wrist watch to use a large battery and it is therefore very important that the power consumption should be restricted.

Moroever, one cannot rely on the temperature compensating element remaining fully effective for a long period of time, and consequently there can be some loss of reliability in a watch in which such an element is incorporated.

In contrast, AT cut and GT cut quartz crystal vibrators usually have excellent frequency-temperature characteristics. This is illustrated in Figure 2, where the curve 2 is the characteristic of an AT cut quartz crystal vibrator, while the curve 3 is the characteristic of a GT cut quartz crystal vibrator.

However, it is hard to use the AT cut quartz crystal vibrator in an electronic wrist watch both because of the size of the vibrator and also because it involves an increase in the current consumption with high frequency.

It is also hard to use the GT cut quartz crystal vibrator since it is difficult to massproduce such vibrators with the same characteristics. Thus GT cut quartz crystal vibrators having good characteristics cannot be manufactured unless their external dimensions are very closely controlled indeed.

The present invention is based upon the discovery that the above problems can be overcome by connecting in parallel a plurality of individual piezo-electric vibrators which have different frequencytemperature characteristics, each individual piezo-electric vibrator having a frequencytemperature characteristic which is constituted by a quadratic curve which has a maximum. Each of these individual piezoelectric vibrators may vibrate in the flexural vibration mode, the longitudinal vibration mode, or the contour shear mode. Such a composite piezo-electric vibrator may be provided with an excellent frequencytemperature characteristic which cannot be obtained by the use of one only of the individual piezo-electric vibrators.

Referring now to Figure 3, which illustrates the present invention, curves 4, 5 show the frequency-temperature characteristics of two individual piezoelectric vibrators, each characteristic being constituted by a quadratic curve which has a maximum. The frequency-temperature characteristics of the two individual piezoelectric vibrators have their maxima at relatively low and relatively high temperatures ToL, and ToH respectively.

Thus the two individual piezo-electric vibrators have different frequencytemperature characteristics.

If the individual piezo-electric vibrators whose characteristics are shown by the curves 4, 5 are connected in parallel, then two of the possible frequency-temperature characteristics of the resultant composite piezo-electric vibrator are shown by the curves 6, 6'. As will be seen, the curves 6, 6' each have a relatively large substantially flat portion Tdl, Td2 respectively. The curve 6 relates to the case where the individual piezo-electric vibrators are connected in such a way that the temperature range of the flat portion Tdl, of the characteristic is at its greatest, while the curve 6' relates to the case in which the individual piezoelectric vibrators are connected so that the frequency variation of the substantially flat portion Td2 is less than a certain value.

If the load required to produce the characteristic constituted by the curve 6 is CLI, and the load required to produce the characteristic having the curve 6' is CL2, then CL1 < CL2.

In contrast to the present invention the individual piezo-electric vibrators whose frequency-temperature characteristics are shown in Figure 4 by the curves 7, 8 are constituted by quadratic curves which have a minimum. If vibrators having the characteristics 7, 8 are connected in parallel, then the frequency-temperature characteristic of the resultant composite piezo-electric vibrator is as shown by the sharp quadratic curve 9. Thus if two vibrators having similar characteristics 7, 8 are connected in parallel, the frequency of the composite vibrator so produced is a little higher than the higher frequency of the two single vibrators, as will be clear from the impedance-frequency characteristic of two such parallel-connected vibrators as shown in Figure 5. This impedancefrequency characteristic has been prepared by regarding two parallel-connected vibrators as being equivalent to the electrical circuit shown in Figure 5. The electrical circuit shown therein has a complex impedance Z-Re+JXe, and if the real and imaginary terms of this impedance Z are evaluated and plotted against frequency, a characteristic as shown in Figure 5 results.

It is therefore important that the two vibrators which are connected in parallel should have frequency-temperature characteristics constituted by quadratic curves which have a maximum, as shown for example in Figure 3.

Figure 6 shows the relationship between AT, Tdl and Td2 where AT is the difference between ToH and ToL shown in Figure 3; Tdl is the temperature range of the flat portion of the curve 6 of Figure 3, Tdl having a maximum frequency variation of n=2.P.P.M., and Td2 is the temperature range of the relatively flat portion of the curve 6', Td2 having a maximum frequency variation of n=4 P.P.M.

As shown in Figure 6, the pair of lines 10, 10' shown for each of the cases n=2 P.P.M., and n=4 P.P.M., diverge with increasing AT.

Taking the pair of lines 10, 10' shown for n=2 P.P.M., the distance between lines 10, 10' of this pair in a direction parallel to the Tdl/Td2 axis for any AT gives the value of Tdl for that AT. The same applies to the pair of lines 10, 10' shown for n=4 P.P.M.

Since Td2 is a wider temperature range than Tdl, if the required temperature range is from Tl to T2, then ToLATl+ez and ToH > T2, where a is about AT/10.

Accordingly, if it is desired to obtain a good frequency temperature characteristic within the temperature range Tl to T2, then it should be arranged that TolATl+(AT/10), and To llT2-(AT' 10) As will be appreciated, the load capacitance CL of the circuit which includes the individual vibrators must be varied in accordance with the value of AT in order to obtain the optimum composite frequency-temperature characteristic.

Although the numerical values shown in Figure 3 are those of an XY flexural vibrator, an NT flexural vibrator, and an cut long side vibrator respectively, it will be appreciated that the values of a DT-cut vibrator, or a CT cut vibrator will be very similar.

Figure 7 shows an embodiment of a composite piezo-electric vibrator according to the present invention in which use it made of a Colpitts circuit using C-MOS as an active element. In Figure 7, the individual piezo-electric vibrators are shown at 11, 12. However, other active elements and circuit systems can also be used.

As will be deduced from Figure 3, To can be easily shifted by suitably selecting the dimensions, cut-angle, and vibrating mode of the vibrator.

As described above, reference has been made to only two individual piezo-electric vibrators, but it should be understood that the composite vibrator of the present invention may consist of three or more vibrators if desired.

Since the desired working temperature range of an electronic wrist watch is from 0 C to 40"C, it follows that ToL should be in the range -200C to 50C, while ToH should be in the range 350 to 600 C, having regard to the desirability of using easy production techniques. Piezo-electric vibration with To in these ranges are easily obtained as mentioned above.

As will be appreciated from the above, the composite piezo-electric vibrator of the present invention can be provided with an excellent frequency-temperature characteristic by using conventional circuit techniques and the reliability of the composite piezo-electric vibrator does not decline with age since an element of low reliability, such as a temperature compensating element, is not used. Further, by selecting piezo-electric vibrators with To at the optimum temperatures, a highly accurate electronic timepiece can be obtained.

WHAT WE CLAIM IS: 1. A composite piezo-electric vibrator comprising a plurality of individual piezoelectric vibrators which have different frequency-temperature characteristics and which are connected in parallel, each individual piezo-electric vibrator having a said characteristic which is constituted by a quadratic curve which has a maximum.

2. A composite piezo-electric vibrator as claimed in claim 1 in which there are two individual piezo-electric vibrators whose frequency-temperature characteristics have their maxima at relatively low and relatively high temperatures ToL and ToH respectively.

3. A composite piezo-electric vibrator as claimed in claim 2 in which ToLATl+(AT/10), and ToH > T2-(AT/l0), where AT=ToHToL, and the desired working temperature range is from Tl to T2.

4. A composite piezo-electric vibrator as claimed in claim 2 or 3 in which ToL is in the range -200C to 5"C, and ToH is in the range 350C to 600C.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. frequency, a characteristic as shown in Figure 5 results. It is therefore important that the two vibrators which are connected in parallel should have frequency-temperature characteristics constituted by quadratic curves which have a maximum, as shown for example in Figure 3. Figure 6 shows the relationship between AT, Tdl and Td2 where AT is the difference between ToH and ToL shown in Figure 3; Tdl is the temperature range of the flat portion of the curve 6 of Figure 3, Tdl having a maximum frequency variation of n=2.P.P.M., and Td2 is the temperature range of the relatively flat portion of the curve 6', Td2 having a maximum frequency variation of n=4 P.P.M. As shown in Figure 6, the pair of lines 10, 10' shown for each of the cases n=2 P.P.M., and n=4 P.P.M., diverge with increasing AT. Taking the pair of lines 10, 10' shown for n=2 P.P.M., the distance between lines 10, 10' of this pair in a direction parallel to the Tdl/Td2 axis for any AT gives the value of Tdl for that AT. The same applies to the pair of lines 10, 10' shown for n=4 P.P.M. Since Td2 is a wider temperature range than Tdl, if the required temperature range is from Tl to T2, then ToLATl+ez and ToH > T2, where a is about AT/10. Accordingly, if it is desired to obtain a good frequency temperature characteristic within the temperature range Tl to T2, then it should be arranged that TolATl+(AT/10), and To llT2-(AT' 10) As will be appreciated, the load capacitance CL of the circuit which includes the individual vibrators must be varied in accordance with the value of AT in order to obtain the optimum composite frequency-temperature characteristic. Although the numerical values shown in Figure 3 are those of an XY flexural vibrator, an NT flexural vibrator, and an cut long side vibrator respectively, it will be appreciated that the values of a DT-cut vibrator, or a CT cut vibrator will be very similar. Figure 7 shows an embodiment of a composite piezo-electric vibrator according to the present invention in which use it made of a Colpitts circuit using C-MOS as an active element. In Figure 7, the individual piezo-electric vibrators are shown at 11, 12. However, other active elements and circuit systems can also be used. As will be deduced from Figure 3, To can be easily shifted by suitably selecting the dimensions, cut-angle, and vibrating mode of the vibrator. As described above, reference has been made to only two individual piezo-electric vibrators, but it should be understood that the composite vibrator of the present invention may consist of three or more vibrators if desired. Since the desired working temperature range of an electronic wrist watch is from 0 C to 40"C, it follows that ToL should be in the range -200C to 50C, while ToH should be in the range 350 to 600 C, having regard to the desirability of using easy production techniques. Piezo-electric vibration with To in these ranges are easily obtained as mentioned above. As will be appreciated from the above, the composite piezo-electric vibrator of the present invention can be provided with an excellent frequency-temperature characteristic by using conventional circuit techniques and the reliability of the composite piezo-electric vibrator does not decline with age since an element of low reliability, such as a temperature compensating element, is not used. Further, by selecting piezo-electric vibrators with To at the optimum temperatures, a highly accurate electronic timepiece can be obtained. WHAT WE CLAIM IS:

1. A composite piezo-electric vibrator comprising a plurality of individual piezoelectric vibrators which have different frequency-temperature characteristics and which are connected in parallel, each individual piezo-electric vibrator having a said characteristic which is constituted by a quadratic curve which has a maximum.

2. A composite piezo-electric vibrator as claimed in claim 1 in which there are two individual piezo-electric vibrators whose frequency-temperature characteristics have their maxima at relatively low and relatively high temperatures ToL and ToH respectively.

3. A composite piezo-electric vibrator as claimed in claim 2 in which ToLATl+(AT/10), and ToH > T2-(AT/l0), where AT=ToHToL, and the desired working temperature range is from Tl to T2.

4. A composite piezo-electric vibrator as claimed in claim 2 or 3 in which ToL is in the range -200C to 5"C, and ToH is in the range 350C to 600C.

5. A composite piezo-electric vibrator

substantially as hereinbefore described with reference to Figures 3, 6 and 7 of the accompanying drawings.

6. An electronic timepiece provided with a composite piezo-electric vibrator as claimed in any preceding claim.