Variable capacitance element

Details Variable capacitance element Variable capacitance element

2003-03-05

2004-12-21

Thomas, Eric (Department: 2831)

Electricity: electrical systems and devices

Electrostatic capacitors

Variable

C361S278000, C361S290000

Reexamination Certificate

active

06833985

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a variable capacitance element included in a high-frequency circuit. In particular, the present invention relates to a variable capacitance element, for use as, for example, a variable capacitance switch or a variable capacitor. In this case, the variable capacitance switch performs switching operations on high-frequency signals by changing electrostatic capacitance.
2. Description of the Related Art
Generally, a variable capacitor is known. A variable capacitor displaces, for example, a movable electrode with respect to a fixed electrode by using electrostatic gravity so as to change the spacing between these electrodes. Thus, the electrostatic capacitance is variably selected.
This kind of conventional variable capacitor is substantially similar to an electrostatically-driven switch as disclosed in Japanese Unexamined Patent Application Publication No. 2000-188050. The variable capacitor includes a movable electrode on the right side of the substrate including a flexible supporting bar. The flexible supporting bar bends toward the front side of the substrate. The movable electrode is spaced from and faces toward the fixed electrode provided on the substrate. Driving electrodes are provided on the substrate side and the movable electrode side. Voltage is applied between the driving electrodes externally such that electrostatic gravity is produced.
When voltage is not applied between the driving electrodes, the supporting bar freely supports the movable electrode. Thus, a predetermined space (electrostatic capacitance) is set between the fixed electrode and the movable electrode. When voltage is applied between the driving electrodes, the supporting bar is bent and is deformed due to the electrostatic gravity such that the movable electrode is displaced toward the fixed electrode. Thus, the electrostatic capacitance between these electrodes increases.
FIG. 4A
schematically shows an example of a shunt switch element, which is a variable capacitance element. A shunt switch element
130
includes a substrate
131
containing a dielectric. A coplanar line
132
is provided on the substrate
131
. High-frequency signals are conducted through the coplanar line
132
. Three lines
133
g
1
,
133
s
and
133
g
2
are aligned at a desired interval on the substrate
131
. The middle line
133
s
is a signal line. The lines
133
g
1
and
133
g
2
on both sides of the signal line
133
s
are ground lines.
On the coplanar line
132
, both ends of an electrode bridge
134
are joined with the ground lines
133
g
1
and
133
g
2
. The electrode bridge
134
crosses over the signal line
133
s
.
FIG. 4B
is a top view of
FIG. 4A
schematically showing the coplanar line
132
and the electrode bridge
134
.
When direct-current voltage is applied between the signal line
133
s
and the electrode bridge
134
included in the shunt switch element
130
, electrostatic gravity is produced between the signal line
133
s
and the electrode bridge
134
. As a result, the electrode bridge
134
is pulled toward the signal line
133
s
due to the electrostatic gravity. Therefore, the electrostatic capacitance changes between the electrode bridge
134
and the signal line
133
s
of the coplanar line
132
.
It is important to note that equivalent circuits of the coplanar line
132
and the electrode bridge
134
can be expressed as shown in FIG.
4
C. In
FIG. 4C
, the reference letter C indicates an electrostatic capacitance between the signal line
133
s
and the electrode bridge
134
. The reference letter L indicates an inductance component of the electrode bridge
134
. The reference letter R indicates a resistance component of the electrode bridge
134
.
When the space between the signal line
133
s
and the electrode bridge
134
is reduced, and when the electrostatic capacitance C between the signal line
133
s
and the electrode bridge
134
is increased, the self-oscillating frequency of the LC series circuit in
FIG. 4C
decreases. At the self-oscillating frequency of the LC series circuit, the impedance of the LC series circuits is minimized. Thus, when viewing the ground lines
133
g
1
and
133
g
2
from the signal line
133
s
through the electrode bridge
134
, a short circuit occurs at high frequencies of the self-oscillating frequencies of the LC series circuit. As a result, the conducting of high frequency signals through the coplanar line
132
(signal line
133
s
) is turned OFF.
On the other hand, when the space between the signal line
133
s
and the electrode bridge
134
is increased and when the electrostatic capacitance C between the signal line
133
s
and the electrode bridge
134
is decreased, the self-oscillating frequency of the LC series circuit in
FIG. 4C
increases. As a result, when viewing from the signal line
133
s
through the electrode bridge
134
, the ground lines
133
g
1
and
133
g
2
are open to high frequencies. Therefore, the conducting of high-frequency signals through the coplanar line
132
is turned ON.
As described above, the shunt switch element
130
controls ON and OFF of the conducting of high frequency signals through the coplanar line
132
. In this case, the electrode bridge
134
is displaced and the electrostatic capacitance C is variable between the electrode bridge
134
and the signal line
133
s.
It is important to note that, in the above-described conventional technology, the movable electrode is held at a predetermined location by its own force (spring force) when voltage is not applied between the driving electrodes. However, when a variable capacitance element is used, an external force such as vibration and impact may be applied to the substrate. Thus, when external forces act thereon in a direction perpendicular to the substrate, the supporting bar is bent and is deformed by the external force. As a result, the movable electrode is displaced with respect to the fixed electrode.
Therefore, while the variable capacitor is operating, the electrostatic capacitance of the capacitor may be changed due to vibration and/or impact even without voltage being applied to the driving electrode. Therefore, the vibration resistance and reliability are reduced. This is a disadvantage.
Furthermore, in the conventional variable capacitance element, when two driving electrodes are too close to each other and contact each other, they remain fixed together even after the voltage is terminated. In this case, returning the variable capacitance element to the normal operating state is difficult using only the stability of the supporting bar. This is another disadvantage.
In the construction of the shunt switch element
130
, the electrode bridge
134
functions as both a driving electrode and an electrode for electrostatic capacitance. In this case, the driving electrode is paired with a fixed driving electrode to produce electrostatic gravity. The electrode for electrostatic capacitance is paired with the signal line
133
s
to determine the self-oscillating frequency of the LC series circuit shown in FIG.
4
C.
However, when high frequency signals are conducted through the coplanar line
132
, such as extremely high frequency (EHF) signals, the amount of electrode surface area of the electrode bridge
134
must be reduced. Thus, the conducting of high frequency signals through the coplanar line
132
can be turned ON and OFF precisely by using changes in the self-oscillating frequency of the LC series circuit using the change in electrostatic capacitance as described above. On the other hand, when the electrode bridge
134
is small, large direct-current voltages must be applied between the electrode bridge
134
and the fixed driving electrode in order to produce electrostatic gravity for displacing the electrode bridge
134
. However, the electrode bridge
134
is preferably displaced by using low direct-current voltage. Therefore, from the viewpoint of the displacement driving, the size of the electrode bridge
134
is preferably increased.
As described abov

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