Article comprising a multi-port variable capacitor

Details Article comprising a multi-port variable capacitor Article comprising a multi-port variable capacitor

1998-09-12

2001-06-05

Pascal, Robert (Department: 2817)

Oscillators

With frequency adjusting means

With voltage sensitive capacitor

C361S271000, C361S277000

Reexamination Certificate

active

06242989

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to a micro-machined variable capacitor.
BACKGROUND OF THE INVENTION
Monolithic implementations of many desirable and important circuits have been hitherto unrealizable, or at least commercially impractical, due to the difficulty in fabricating low loss, expensive, linear passive RF components using conventional fabrication methods. That problem is being addressed, with some success, using micro-electromechanical systems (MEMS) technology. Using MEMS technology, devices having the functionality of inductors, variable inductors and variable capacitors can be realized by various silicon IC-compatible, micron-sized electromechanical structures. The latter component, variable capacitors, are important elements of a variety of electrical circuits including variable-frequency oscillators, tuned amplifiers, parametric amplifiers, phase shifters, equalizers, and impedance-matching circuits, to name just a few.
Variable capacitors are devices in which a change in a control voltage charge or current causes a change in capacitance. One well known implementation of the variable capacitor is the varactor, typically realized as a p-n junction diode. In such a varactor diode, changes in the control voltage can yield up to about a factor of 10 change in capacitance. Diode varactors typically have two ports; an input port and an output port. As a two-port device, diode varactors have limited functionality. In particular, the two signals that are fed to the varactor—a DC bias and an AC signal—are received at the input port. The DC bias sets the capacitance of the varactor diode, while the AC signal is the signal being processed in the circuit that includes the varactor. If both signals are AC, mixing non-linearity disadvantageously occurs such that the response of the varactor to the control signal is non-linear. Moreover, such a two port arrangement disadvantageously introduces DC into the AC signal path.
The aforementioned limitation (i.e., only two-ports) of diode-based varactors has been carried over to most MEMS-based variable capacitors that have been proposed to date.
FIG. 1
depicts a simplified schematic of a first MEMS-based variable capacitor
102
in the prior art. Such a variable capacitor typically consists of two parallel plates,
104
and
106
. One of the two plates is non-movable. In conventional MEMS-based variable capacitor
102
, the non-movable plate is lower plate
106
, which is disposed on support or substrate
100
. The other of the two plates, upper plate
104
in the present example, is movable. Upper plate
104
is typically suspended over non-movable lower plate
106
, such as by beams or suitably arranged hinged plates (not shown).
The two plates are electrically connected to a bias supply (not shown) operable to apply a typically DC bias voltage, V
1
, to variable capacitor
102
. The two plates are also electrically connected to signal line
110
for supplying a signal, S, typically AC, to variable capacitor
102
. As bias V
1
is applied across upper and lower plates
104
and
106
, upper plate
104
moves towards fixed lower plate
106
. The capacitance of variable capacitor
102
thereby increases. See,Young et al., “A Micromachined Variable Capacitor for Monolithic Low-Noise VCOs,” Tech. Digest, pp. 86-89, 1996 Solid State Sensor and Actuator Workshop, Hilton Head Island, S.C., Jun. 3-6, 1996.
FIG. 2
depicts a simplified schematic of a second MEMS-based variable capacitor
202
in the prior art. Variable capacitor
202
has three parallel plates, including non-movable upper plate
206
, non-movable lower plate
208
and movable plate
204
. Movable plate
204
is sandwiched between the non-movable plates.
The plates are electrically connected to two bias sources (not shown), operable to apply bias voltages V
1
and V
2
to variable capacitor
202
as depicted in FIG.
2
. The two plates are also electrically connected to signal line
210
for supplying a signal, typically AC, to variable capacitor
202
. When bias V
2
is set to 0 volts and non-zero bias V
1
is applied, movable plate
204
moves upwardly towards non-movable upper plate
206
, increasing the capacitance of variable capacitor
202
. When bias V
1
is set to 0 volts and non-zero bias V
2
is applied, movable plate
204
moves downwardly towards non-movable lower plate
208
, decreasing the capacitance of variable capacitor
202
. The three-plate MEMS-based variable capacitor
202
is described, by its inventors, to provide an increased tuning range over a two plate MEMS-based variable capacitor, such as variable capacitor
102
. See, A. Dec et al. in “Micromachined Varactor with Wide Tuning Range,” Elec. Lett. Online No. 19970628 (Apr. 7, 1997).
In both of the conventional MEMS-based variable capacitors
102
and
202
, the bias (V
1
and V
1
/V
2
) and signal (
110
and
210
) are not electrically isolated (i.e., they are applied to the same port). Being two-port devices, MEMS-based variable capacitors
102
and
202
disadvantageously share some of the limitations, such as those described above, common to conventional diode varactors.
The art would thus benefit from a MEMS-based variable capacitor having more than two ports. Such a device would provide a hitherto unachieved degree of flexibility and utility in comparison with conventional diode- or MEMS-based variable capacitors.
SUMMARY OF THE INVENTION
An article comprising a multi-port variable capacitor is disclosed. In one embodiment, the present article comprises a movable plate that is suspended above first and second coplanar fixed electrodes. A bias supply is electrically connected to the first electrode and the movable plate so that a bias V
1
can be applied to the multi-port variable capacitor. An AC signal-canying line is electrically connected to the second electrode and the movable plate. As bias V
1
is applied, an electrostatic attraction is developed therebetween. Such an attraction causes the movable plate to move downwardly towards the first electrode. As it does so, the separation distance between the movable plate and the fixed electrodes decreases, so that the capacitance of the variable capacitor increases.
Unlike conventional variable capacitors, in some embodiments of the present multi-port variable capacitors, the bias (delivered via the first electrode) and the signal (delivered via the second electrode) are electrically isolated from one another. Since DC and AC are not mixed, a true AC-circuit design can be developed. In other words, the circuit can be designed without regard to the presence of DC, since none will be present. Moreover, the present multi-port variable capacitor can be made to operate in a substantially more linear fashion than conventional variable capacitors. More particularly, since the control signal and the RF signal are on separate ports, electrodes can be suitably designed such that the RF signal will not contribute to a change in the capacitance of the device. Also, mechanical advantage can be used to induce considerable capacitance change for relatively moderate applied voltage at the control terminals.
In some embodiments, the present variable capacitor is structured in such a manner that the movable plate is capable of “tilting.” The ability to tilt increases the utility and functionality of the present variable capacitors. In particular, such “tilting” multi-port variable capacitors are advantageously capable of performing “signal processing” (i.e., logic) functions, such as addition, subtraction, multiplication and comparison functions.
In additional embodiments, an article in accordance with the present teachings comprises a monolithically-integrable, tunable LC circuit. In such LC circuits, the multi-port variable capacitor described herein is used to tune the resonant frequency of the circuit. Such LC circuits are used, in some embodiments, to create a variety of tunable filters. In a further embodiment, an article in accordance with the present teachings comprises a variable-frequency oscillator that incorporates a

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