Vacuum variable capacitor

Details Vacuum variable capacitor Vacuum variable capacitor

2000-10-16

2002-10-29

Nguyen, Chau N. (Department: 2831)

Electricity: electrical systems and devices

Electrostatic capacitors

Variable

C361S278000, C361S279000, C361S283300, C361S296000, C361S292000

Reexamination Certificate

active

06473289

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to vacuum variable capacitors.
2. Related Art
Vacuum variable capacitors are used in a variety of applications in a variety of industries. For example, vacuum variable capacitors are used in RF matching devices to vary capacitance as part of tuning the RF impedance.
FIG. 1
is a cross-sectional view of a previous vacuum variable capacitor
100
. Each of two opposed capacitor plate structures
102
and
103
include a mounting plate
102
b
or
103
b,
respectively, having formed thereon a multiplicity of concentric cylindrical capacitor plates
102
a
or
103
a,
respectively. (In the following description, the reference numerals
102
a
and
103
a
can refer to one or multiple capacitor plates.) The capacitor plates
102
a
and
103
a
are formed on the mounting plates
102
b
and
103
b,
respectively, at locations and with a spacing such that, as shown in
FIG. 1
, the capacitor plate structures
102
and
103
can be positioned with respect to each other to cause capacitor plates
102
a
of the capacitor plate structure
102
to fit between adjacent capacitor plates
103
a
of the capacitor plate structure
103
(and vice versa) so that a desired spacing (“gap distance” ) between adjacent capacitor plates
102
a
and
103
a
results. The capacitor plate structures
102
and
103
are electrically connected to a voltage source so that the adjacent capacitor plates
102
a
and
103
a
act as a capacitor.
The capacitor plate structures
102
and
103
are positioned in a housing
101
. The capacitor plate structure
103
is attached to the housing
101
so that the position of the capacitor plate structure
103
remains fixed with respect to the housing
101
. As explained further below, the capacitor plate structure
102
is attached to the housing
101
so that the position of the capacitor plate structure
102
can move with respect to the housing
101
.
An end of a hollow shaft
104
is attached to the, capacitor plate structure
102
. A female threaded member
109
is attached to an end of the shaft
104
opposite the end attached to the capacitor plate structure
102
. A male threaded member
105
is screwed into the threaded member
109
. The threaded member
105
is attached to an adjustment, head
108
which is, in turn, attached to the housing
101
so that the adjustment head
108
and threaded member
105
are held in place with respect to the housing
101
along a longitudinal axis
110
of the vacuum variable capacitor
100
. Rotating the adjustment head
108
causes the threaded member
105
to move into or out of the threaded member
109
, causing corresponding motion of the threaded member
109
, shaft
104
and capacitor plate structure
102
(including associated capacitor plates
102
a
) with respect to the housing
101
along the longitudinal axis
110
. Since the position of the capacitor plate structure
103
with respect to the housing
101
is fixed, the adjustment head
108
can therefore be used to change the relative positions of the capacitor plates
102
a
and
103
a,
thereby adjusting the capacitance between the capacitor plates
102
a
and
103
a,
as known to those skilled in the art.
A bellows
106
surrounds the shaft
104
. A bearing
107
enables the shaft
104
to rotate relative to the bellows
106
and the housing
101
. The housing
101
, bearing
107
, bellows
106
and mounting plate
102
b
form a sealed enclosure, held at a vacuum pressure, within which the capacitor plates
102
a
and
103
a
are positioned. The bellows
106
expands and contracts as necessary to allow movement of the threaded member
109
, bearing
107
, shaft
104
and capacitor plate structure
102
along the longitudinal axis
110
. The bellows
106
also provides an electrical connection from the capacitor plate structure
102
to complete the electrical circuit including the capacitor formed by the capacitor plates
102
a
and
103
a.
The above-described vacuum variable capacitor
100
is illustrative of previous vacuum variable capacitors. (Variations exist: for example, in another type of previous vacuum variable capacitor, the capacitor plates are formed as spirals, rather than as concentric cylinders as in the vacuum variable capacitor
100
.) The vacuum variable capacitor
100
has a number of characteristics which can be undesirable.
The structure for effecting movement of the capacitor plate structure
102
is subject to mechanical friction which can cause adjustment of the capacitance to take an undesirably long time and/or require an undesirably large amount of power. Additionally, movement of the capacitor plate structure
102
may be opposed by a force due to the differential pressure between the vacuum pressure within the sealed enclosure and the atmospheric pressure outside of the sealed enclosure. The necessity of overcoming this force can further cause adjustment of the capacitance to take an undesirably long time and/or require an undesirably large amount of power. Illustratively; in the vacuum variable capacitor
100
, capacitance can be adjusted at a rate of about
200
picofarads per second.
As indicated above, the bellows
106
provides an electrical connection from the capacitor plate structure
102
(i.e., from the capacitor represented by the capacitor plates
102
a
and
103
a
) to complete the electrical circuit of which the capacitor is part. However, the bellows
106
has associated therewith a parasitic inductance and resistance which degrades the performance of the capacitor.
The capacitor plate structures
102
and
103
are typically made by brazing the capacitor plates
102
a
or
103
a
on to the corresponding mounting plate
102
b
or
103
b.
The brazing process softens the material (typically copper) of which the capacitor plates
102
a
and
103
a
are made. This can make the capacitor plates
102
a
and
103
a
undesirably susceptible to deformation that degrades the performance of the vacuum variable capacitor
100
or renders the vacuum variable capacitor
100
unusable.
The size of the vacuum variable capacitor
100
along the longitudinal axis
110
may be larger than desired for some applications. (For convenience, the overall size of the vacuum variable capacitor
100
in this dimension is referred to herein as the “length” of the vacuum variable capacitor
100
.) For example, the presence of the bellows
106
adds to the length of the vacuum variable capacitor
100
. The shape of the capacitor plates
102
a
and
103
a
can also affect the length of the vacuum variable capacitor
100
, as explained in more detail below with respect to the description of the invention.
The housing
101
of the vacuum variable capacitor
100
is cylindrical. In previous vacuum variable capacitors, the housing has been made cylindrical to avoid stress concentrations that may otherwise occur at corners of a rectangular housing as a result of the differential pressure between the vacuum pressure within the housing and the atmospheric pressure outside of the housing. However, while, for the reason given above, the use of a cylindrical housing may be desirable if only the construction of the vacuum variable capacitor is considered, the use of a cylindrical housing may not be desirable from the standpoint of a system with which the vacuum variable capacitor is to be used, since a cylindrical shape may not be as space-efficient as a rectangular shape when the vacuum variable capacitor is integrated with other components of the system.
SUMMARY OF THE INVENTION
A vacuum variable capacitor according to the invention can include one or more characteristics that provide advantages over previous vacuum variable capacitors. In particular, a vacuum variable capacitor according to the invention can be constructed so as to enable the capacitance to be adjusted more easily, reduce parasitic electrical characteristics that degrade the performance of the vacuum variable capacitor, increase the strength of the capacitor plates of the vacuum variable capacitor, reduce the siz

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