Abstract:
If a piezoelectric crystal is used in circuits containing ICs with low supply voltage, the additional capacitors required in oscillator or filter circuits cannot be implemented as junction capacitances. To eliminate the need for additional discrete capacitors, these capacitances are implemented with additional films on the crystal.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to piezoelectric resonators with electrode films deposited on the crystal and with holding members holding the crystal at vibration nodes. 
     If such resonators are used in oscillator or filter circuits, small load, trimming or coupling capacitances are needed in most cases. As an example, FIG. 1 shows part of the oscillator circuit of a quartz-crystal watch module. 1 is a piezoelectric resonator, 2 is an amplifier, and C L1  and C L2  are load capacitors. The other divider stages present on an integrated-circuit chip used for such purposes, such as the IC 1115, are not shown. Because of the low supply voltage of such an integrated circuit, the load capacitors C L1  and C L2  can be realized therein as junction capacitances only with very great difficulty and with low Qs. Therefore, they had to be inserted into the circuit as discrete components. Similarly, filter circuits with piezoelectric resonators require capacitances which also had to be implemented with discrete components. 
     SUMMARY OF THE INVENTION 
     The present invention therefore has for its object to integrate these capacitances into the piezoelectric resonator, thereby eliminating the need for special components which would take up additional space. This object is attained by the means set forth in the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention will now be described by way of example with reference to the accompanying drawing, in which: 
     FIG. 1a shows the oscillator circuit diagram mentioned above; 
     FIG. 1b shows the circuit diagram of FIG. 1a with the equivalent circuit of the resonator; 
     FIGS. 2a and 2b show a conventional circular resonator in top and sectional views; 
     FIG. 3 shows a circular resonator according to the invention with additional insulating layers and films for two additional capacitances; 
     FIG. 4 shows a circular resonator according to the invention with additional counter films deposited for the pads; 
     FIG. 5 shows the resonator of FIG. 4 in a holder; 
     FIG. 6 shows a rectangular resonator according to the invention with additional integrated capacitances whose corners rest on the rim of a circular opening, and 
     FIG. 7 shows a resonator as illustrated in FIG. 6 which is mounted by means of blocks attached at the four corners. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In FIG. 1b, the piezoelectric resonator 1 in the above-described circuit diagram of FIG. 1a has been replaced by its equivalent circuit. C 1 , L 1 , and R 1  are the equivalent values for the series resonance used, and C o  is the unavoidable shunt capacitance given by the capacitance of the electrode films. If the electrode films are designated 5 and 6, it can be seen that the film 5 is connected to a plate of the load capacitor C L1 , while the film 6 is connected to a plate of the load capacitor C L2 . FIGS. 2a and 2b show a conventional circular resonator in top and cross-sectional views, respectively. The piezoelectric crystal is designated 1; 5 and 6 are the electrode films, and 9 and 10 are the pads of the electrode films. 
     To accommodate capacitances on the piezoelectric plate in addition to the shunt capacitance C o  provided by the electrode films, an electrode film 5 or 6 can be used as a plate for such a capacitance, while the necessary counter (corresponding opposite plate) film 7 or 8 is deposited on a dielectric insulating layer 13 or 14, as shown in FIG. 3. 9 and 10 are the pads f the electrode films 5 and 6, and 11 and 12 are the pads of the counter films 7 and 8. While the dielectric insulating layers 13, 14 and the counter films 7, 8 are really bonded to the electrode films 5, 6 and their pads 9, 10 on the piezoelectric crystal 1 with no space therebetween, spaces are shown in FIG. 3 to permit a graphical representation. 
     FIG. 4 shows another possibility of implementing two capacitances each having one plate connected to one of the electrode films. One of the plates is formed by the pads of the electrode films, while the associated counter film is deposited opposite them on the other side of the crystal plate. 1 is again the crystal plate, 5 and 6 (invisible) are the electrode plates, 9 and 10 are their pads, 7 and 8 are the counter films, and 11 and 12 are the pads of the counter films. 
     FIGS. 5 to 7 show holders for such piezoelectric resonators with integrated capacitances. In the arrangement of FIG. 5, the resonator shown in FIG. 4 is secured by means of three retaining springs mounted on a three-pole plug base 3. The reference numerals correspond to those of FIG. 4. In FIG. 6, the corners of a rectangular resonator 1 rest on the rim of a circular opening at the points 4, and its films 5, 6 and 7, 8 are connected, via pads 9, 10 or directly at these corners, to conductive strips on the substrate 3 by soldering or by means of a conductive adhesive. The resonator may also be supported by and attached to chamfered corners of the rectangular opening. The resonator shown in FIG. 7 has the same film structure and is connected to the conductive strips on the substrate 3 via blocks at the points 4 by soldering or by means of a conductive adhesive. 
     It is obvious that, as in the case of the resonator of FIG. 3, the capacitance counter films of the resonators shown in FIGS. 4 to 7 may also be deposited on a dielectric insulating layer applied to the pads 9 and 10. In the same manner, capacitances may be implemented which use neither of the two electrode films.