Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2001-07-18
2003-03-25
Dougherty, Thomas M. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S348000, C310S366000
Reexamination Certificate
active
06538361
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to piezoelectric oscillators in which piezoelectric resonators and capacitors are combined. More particularly, the present invention relates to piezoelectric oscillators having piezoelectric resonators connected to load capacitances and parallel capacitances.
2. Description of the Related Art
Conventionally, piezoelectric oscillators have been widely used to obtain oscillation frequencies for clock circuits or other suitable devices. In such piezoelectric oscillators, outstanding frequency accuracy and reliability are required.
Japanese Unexamined Utility Model Application Publication No. 5-18120 provides a piezoelectric oscillator.
FIG. 7
shows a sectional view of the piezoelectric oscillator. In this figure, a piezoelectric oscillator
101
includes a piezoelectric resonator
103
mounted on a capacitor substrate
102
. The piezoelectric resonator
103
is enclosed by a cap
104
. External electrodes
102
a
to
102
c
are provided on outer surfaces of the capacitor substrate
102
. The external electrodes
102
a
and
102
b
are provided on each end surface of the capacitor substrate
102
. The portions of the external electrodes
102
a
and
102
b
that extend to the upper surface of the capacitor substrate
102
are electrically connected to vibration electrodes
103
a
and
103
b
of the piezoelectric resonator
103
.
Internal electrodes
102
d
and
102
e
are provided inside the capacitor substrate
102
. The internal electrodes
102
d
and
102
e
are arranged to overlap each other via a dielectric layer. The internal electrode
102
d
is connected to the external electrode
102
b
and the internal electrode
102
e
is connected to the external electrode
102
a
. The external electrode
102
c
extends from the upper surface of the capacitor substrate
102
to the lower surface through a pair of side surfaces of the substrate
102
. In addition, the external electrode
102
overlaps with the external electrode
102
d
and
102
e
via a dielectric layer.
With the above arrangement, a parallel capacitance C
p
is provided between the external electrodes
102
a
and
102
b
based on the dielectric layer between the internal electrodes
102
d
and
102
e
. In addition, a load capacitance C
11
is provided between the internal electrode
102
d
and the external electrode
102
c
and a load capacitance C
12
is provided between the internal electrode
102
e
and the external electrode
102
c
.
FIG. 8
shows the circuit structure of the piezoelectric oscillator
101
.
In the piezoelectric oscillator
101
, the parallel capacitance C
p
is connected in parallel to the piezoelectric resonator
103
. Thus, by adjusting the magnitude of the parallel capacitance C
p
, a spurious response occurring between a resonance frequency and an anti-resonance frequency can be shifted. As a result, spurious responses occurring on the high frequency side can be suppressed.
Furthermore, Japanese Unexamined Patent Application Publication No. 4-192709 provides a piezoelectric oscillator, which is shown in FIG.
9
. In a piezoelectric oscillator
111
, a piezoelectric resonator
113
is mounted on a capacitor substrate
112
. The piezoelectric resonator
113
is enclosed by a cap
114
. A plurality of internal electrodes
112
a
to
112
e
are provided inside the capacitor substrate
112
. The internal electrodes
112
a
and
112
b
oppose each other via a predetermined distance at the same height position. The internal electrode
112
a
is connected to a via-hole electrode
112
f
and the internal electrode
112
b
is connected to a via-hole electrode
112
g
. Similarly, the internal electrodes
112
d
and
112
e
oppose each other at the same height position and the internal electrode
112
d
is connected to the via-hole electrode
112
f
and the internal electrode
112
e
is connected to the via-hole electrode
112
g
.
The internal electrode
112
c
is arranged to overlap the internal electrodes
112
a
,
112
b
,
112
d
, and
112
e
via dielectric layers. The internal electrode
112
c
is connected to a via-hole electrode
112
h
. The via-hole electrodes
112
f
to
112
h
extend to the lower surface of the capacitor substrate
112
. External electrodes
112
i
to
112
k
are provided on the lower surface of the capacitor substrate
112
to be electrically connected to the outside. The external electrodes
112
i
to
112
k
are connected to the via-hole electrodes
112
f
to
112
h.
In the piezoelectric oscillator
111
, the internal electrodes
112
a
to
112
e
are provided inside the capacitor substrate
112
, and a load capacitance is thereby connected to the piezoelectric resonator
113
. In this oscillator
111
, since the external electrodes
112
i
to
112
k
are provided on the lower surface of the capacitor substrate
112
, the lower-surface side of the capacitor substrate
112
can be mounted on a printed circuit board.
Recently, in such a piezoelectric oscillator, as in other kinds of electronic devices, miniaturization, and particularly, height reduction has been strongly demanded. When a piezoelectric oscillator is miniaturized, the size of a piezoelectric resonator used in the oscillator is also reduced. As a result, a vibration-electrode facing area in the piezoelectric resonator is reduced. Consequently, a capacitance C
f
between terminals in the piezoelectric resonator is reduced. For example, when the piezoelectric resonator is made of a ceramic material including Pb having a high Q factor, since the relative bandwidth is broadened, the frequency accuracy varies due to variations in the magnitudes of external capacitances with respect to the piezoelectric resonator.
As the piezoelectric ceramic defining the piezoelectric resonator becomes finer and stronger, the high frequency characteristics are greatly improved. However, with the high frequency characteristics improved, high-order mode spurious responses such as a third harmonic and a fifth harmonic causing abnormal oscillation are substantially increased. Thus, such high order mode spurious responses must be suppressed.
In addition, as the height of the oscillator is reduced, the thickness of a gap between the piezoelectric resonator and the capacitor substrate on which the piezoelectric resonator is mounted is reduced to a few &mgr;m. As a result, for example, in the oscillator as described in Japanese Unexamined Utility Model Application Publication No. 5-18120, when the piezoelectric resonator
103
resonates, the vibration electrodes which extend on the lower surface of the piezoelectric resonator
103
come in contact with the external electrode
102
b
connected to a ground potential on the capacitor substrate and are eventually short-circuited.
In the piezoelectric oscillator
111
described in Japanese Unexamined Patent Application Publication No. 4-192709, no external electrode is provided on the upper surface of the capacitor substrate
112
. Thus, when the height of the oscillator is reduced, the above-mentioned short circuit does not occur. However, in the piezoelectric oscillator
111
, as the size of the oscillator is reduced, as described above, the influences caused by the variations in external electrodes are greater. Thus, it is very difficult to improve the frequency accuracy. In addition, the influences of high order mode spurious responses including a third harmonic and a fifth harmonic are increased.
SUMMARY OF THE INVENTION
To overcome the above-described problems with the prior art, preferred embodiments of the present invention provide an improved piezoelectric oscillator. In this piezoelectric oscillator, since the deterioration of the frequency accuracy caused by variations in external capacitances is minimized, excellent frequency characteristics are obtained. Moreover, even when the height of the oscillator is reduced, undesired short circuits do not occur. Thus, the piezoelectric oscillator of preferred embodiments of the present invention has outstanding accuracy and reliability.
In addition to this, in
Dougherty Thomas M.
Keating & Bennett LLP
Murata Manufacturing Co. Ltd.
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