Variable capacitor

A variable capacitor includes a first electrode (2), a second electrode (3) and, located between the first and second electrodes, an array of gas discharge tubes (7). The capacitor includes means for producing an electrical discharge in the tubes, thereby affecting the capacitance of the capacitor.

BACKGROUND OF THE INVENTION 
This invention relates to a variable capacitor and to a method of varying 
the capacitance of a capacitor. The invention further relates to a 
digital-to-analog, converter and to an electrostatic loudspeaker. 
Variable capacitors are widely used in electronics but have restrictions 
where high voltages or large capacitance ranges are required. Moreover, 
the value of the capacitor is adjusted mechanically, usually manually or 
by means of a servo motor. This limits the usefulness of the capacitors 
where automatic control is required or where very rapid changes in 
capacitance are necessary, with precision. 
It is an object of the present invention to mitigate at least some of the 
afore-mentioned problems. 
SUMMARY OF THE INVENTION 
According to the present invention there is provided a variable capacitor 
including a first electrode, a second electrode separated therefrom and, 
located between the first and second electrodes, gas discharge means 
including an ionizable gas and means for producing an electrical discharge 
in the ionizable gas, thereby affecting the capacitance of the capacitor. 
The present invention provides a capacitor whose capacitance may be 
adjusted electrically, according to analog or digital input signals, 
without the need for a mechanical interface. This allows the capacitance 
to be precisely adjusted automatically and with great rapidity. As the 
capacitor has no moving parts, it is also very reliable and needs minimal 
maintenance. 
The capacitor is also capable of operating at high voltages, can provide a 
relatively large capacitance and has a large range of variation. 
The gas discharge means may include a plurality of gas discharge tubes, 
said means for producing an electrical discharge in the ionizable gas 
comprising the electrodes of the gas discharge tubes. 
The gas discharge tubes may be connected so as to permit the number of 
tubes activated to be controlled. 
Advantageously, each of said first and second electrodes comprises a 
plurality of plate elements, the plate elements of the first electrode 
being interleaved with those of the second electrode. 
Gas discharge means may be located between each adjacent pair of 
interleaved plate elements. 
Voltage supply means may be connected to said gas discharge means and 
arranged to supply a voltage sufficient to activate said gas discharge 
means. Said voltage supply means may include control means for selectively 
activating a predetermined number of said gas discharge tubes. 
Alternatively or in addition, the voltage supply means may include control 
means for controlling the voltage applied to the gas discharge tubes. 
Alternatively, the voltage supply means may include means for modulating 
the voltage applied to the gas discharge tubes. 
The present invention further provides a digital-to-analog analog converter 
including a variable capacitor including a first electrode, a second 
electrode separated therefrom and, located between the first and second 
electrodes, a plurality of gas discharge tubes, the gas discharge tubes 
being connected so as to permit the number of tubes activated to be 
controlled, and voltage supply means connected to the gas discharge tubes, 
the voltage supply means including control means for selectively 
activating a number of the gas discharge tubes in response to a digital 
input signal received by the control means. 
Since the capacitance may be relatively large, analog power may be drawn 
from the capacitor. It may thus be employed as a digital-to-analog power 
transducer, without the need for a power amplifier. 
According to the present invention there is further provided an 
electrostatic loudspeaker including a diaphragm, at least one 
electrostatic drive element. arranged to drive the diaphragm, the 
electrostatic drive element including a first electrode, a second 
electrode separated therefrom and, located between the first and second 
electrodes, a plurality of gas discharge tubes, and voltage supply means 
connected to the electrostatic drive element, the voltage supply means 
including control means for selectively activating a predetermined number 
of the gas discharge tubes in response to a digital input signal received 
by the control means. 
The present invention further provides a method of varying the capacitance 
of a capacitor comprising a first electrode, a second electrode separated 
therefrom and, located between the first and second electrodes, gas 
discharge means including an ionizable gas and means for producing an 
electrical discharge in the ionizable gas, the method comprising 
controlling the electrical discharge produced in the ionizable gas. 
Advantageously, the gas discharge means includes a plurality of gas 
discharge tubes and the electrical discharge is controlled by selectively 
activating a predetermined predetermined number of the gas discharge 
tubes. Alternatively, the electrical discharge may be controlled by 
adjusting the voltage applied to the gas discharge means, or by modulating 
the voltage applied to the gas discharge tubes. 
The present invention yet further provides a method of converting a signal 
from digital to analog form by means of a variable capacitor including a 
first electrode, a second electrode separated therefrom and, located 
between the first and second electrodes, a plurality of gas discharge 
means, the method comprising applying a first voltage to the first and 
second electrodes, receiving a digital input signal, applying a second 
voltage to a predetermined number of the gas discharge means to activate 
the predetermined number of the gas discharge means, and controlling the 
number of activated gas discharge means in response to the digital input 
signal, thereby generating an analog variation in the charge carried by 
the capacitor.

DETAILED DESCRIPTION OF THE INVENTION 
As shown schematically in FIG. 1, the capacitor 1 includes two electrodes 
2,3, each of which comprises a set of parallel conductive plates. The 
plates 2a,2b,2c of the first electrode 2 are connected to a first 
conducting strip 4 and are interleaved with the plates 3a,3b of the second 
electrode 3, which are connected to a second conducting strip 5. In 
practice, each electrode 2,3 will typically include many more plates than 
are illustrated in FIG. 1 (for example, 10 or more plates), only a few of 
those plates being shown for clarity. 
Located between each pair of adjacent plates, for example between plates 2a 
and 3a, there is provided a row 6 of miniature gas discharge tubes 7. One 
terminal of each tube 7 is connected via a 4.7 k.OMEGA., 0.25 W load 
resistor 8 to a first connecting rail 9 and the other terminal of each 
tube is connected to a ground connection 10. The first connecting rail 9 
is connected to a regulated DC power supply (not shown). When a suitable 
potential of approximately 80-100 V DC is applied to the rail 9, all the 
tubes 7 in that row are activated. 
In the embodiment shown in FIG. 1, each row 6 of gas discharge tubes 7 is 
separate from the other rows and may be switched on or off independently 
of the other rows. It is possible, however, to connect all the rows 
together in parallel so that all the tubes may be switched on or off 
together. Alternatively, each tube 7 may be connected separately to the 
power supply, so that they may be activated individually. 
A practical embodiment of the invention is shown in FIGS. 2 and 3. In this 
embodiment, each electrode 2,3 comprises eleven plates 2a-2k, 3a-3k that 
are manufactured from strips of copper and mounted on a printed circuit 
board 11 so that the plates 2a-2k of the first electrode 2 are interleaved 
with the plates 3a-3k of the second electrode 3. Each electrode 2,3 is 
connected to a respective connecting device CC'. The plates 2a-2k, 3a-3k 
have an undulating or zigzag form for reasons that are explained below. 
A row 6 gas discharge tubes 7 is located between each adjacent pair of 
plates (for example, plates 2a,3a). In this embodiment, alternate rows 
have nine and ten tubes each, with the tubes of each row being located 
opposite the gaps between the tubes of the adjacent-rows 6. This, and the 
zigzag form of the plates 2a-2k, 3a-3k, allows the rows 6 to be located as 
close to one another as possible, so reducing the spacing between the 
plates and increasing the capacitance of the capacitor. 
As shown in FIG. 3, a first terminal of each tube 7 is connected via a load 
resistor 8 to a connection rail 9 and the second terminal of each resistor 
is connected via the ground plane 10 of the printed circuit board 11 to an 
earthing pin 12. The tubes of each row 6 are thus connected in parallel 
and may be activated simultaneously by applying a suitable potential to 
that row. Each row may be activated independently of the other rows. 
Alternatively, as mentioned above, each tube 7 may have separate power 
supply connections to allow the tubes to be activated independently. The 
rows 6 of tubes 7 are hereinafter collectively referred to as the array 
13. 
The performance of the capacitor was tested using the circuit arrangements 
shown in FIGS. 4 to 7, where: 
PSU is a regulated power supply unit 15 delivering up to 100 V DC 
(adjustable), 
AM is an a stable multivibrator 16 with adjustable frequency, 
OSC is an oscilloscope 17, 
Tl is a mains isolating transformer 18, 
TS is a transistor switch 19, and 
Cx is the capacitor 1 under test. 
In the first test, made using the circuit arrangement shown in FIG. 4, the 
variable capacitor 1 was substituted for the timing capacitor of the 
multivibrator 16. The array 13 of tubes was connected to the power supply 
unit 15 and supplied with power at potentials of 85 V, 90 V and 95 V DC. 
The number of tubes activated was varied and the capacitance of the 
capacitor determined from the frequency of the multivibrator output, which 
was observed using the oscilloscope 17. 
The results of the first test are shown in FIG. 8. As may be seen, the 
capacitor 1 has a basic capacitance of about 90 pF in its inactive state 
(with no tubes activated) and this increases to about 140 pF with 130 
tubes activated. 
Increasing the voltage supplied to the tubes also increased the capacitance 
somewhat. 
In the second test, using the circuit arrangement shown in FIG. 5, the 
array 13 of tubes was supplied with power at a potential of 90 V and the 
number of tubes activated was varied. The output of the multivibrator 16 
was connected to the electrodes 2,3 of the capacitor 1 via a 100 .OMEGA. 
series resistor 20. The reactance of the capacitor 1 at a frequency of 30 
kHz was measured by means of an oscilloscope 17 connected across the 
series resistor 20. 
The results of the second test are shown in FIG. 9. As may be seen, the 
capacitor 1 exhibited decreasing reactance as the number of activated 
tubes increased. 
In the third test, using the circuit shown in FIG. 6, the output from the 
power supply unit 15 was delivered to the array 13 of tubes through a 
transistor switch 19, which was modulated by the output of the 
multivibrator 16. This allowed pulsed power to be delivered to the tubes, 
at a frequency of 1 kHz. The electrodes 2,3 of the capacitor 1 were 
charged to 350 V, DC and the change in reactance measured across a 22 
k.OMEGA. series resistor 21. Once again the reactance decreased with the 
number of tubes activated, as shown in FIG. 10. Concurrent with the rise 
in current through the series resistor 21, there was an increase in 
current flow to the array 13 of tubes. 
In the fourth test, the uncharged and isolated electrodes of the capacitor 
were connected to an oscilloscope and a 100 V, 10 kHz AC switching voltage 
was applied to the tubes, It was discovered that an almost exact replica 
of the switching waveform was generated at the terminals of the capacitor, 
even though the capacitor was not itself connected to a voltage supply. 
The voltage generated at the terminals of the capacitor was in phase with 
the switching waveform and about 38 V in magnitude, although the magnitude 
varied slightly with the number of tubes activated, as shown in FIG. 11. 
It is speculated that the initial rise in potential with increasing numbers 
of active tubes probably represents a saturation process, preceding 
behavior as a proper capacitor, although a full explanation of the 
phenomenon cannot yet be provided. The energy contained in the secondary 
voltage is considered to represent the transfer function from the gas 
discharge to the plates. It is anticipated that the subsequent decline in 
voltage increasing numbers of active tubes would have continued had more 
tubes been available. 
In order to estimate the current supplied under these conditions the 
capacitor was connected to a variable resistor. This resistance was 
reduced progressively until the output voltage from the capacitor was 
halved, i.e. 19 V. This corresponded to a resistance of 3.7 k.OMEGA., 
which equated to approximately 5 mA and therefore to a power output of 100 
mW. Considering that less than 10% of the available surface area of the 
capacitor was modified by the gas discharge, it is clear that the device 
will be capable of substantial power output if constructed for higher 
efficiencies. 
In the fifth test, using the circuit arrangement shown in FIG. 7, the 
electrodes 2,3 of the capacitor were connected via a-voltage divider 22 to 
a rectifier 24 and charged with 200 V DC. The voltage across the capacitor 
1 was measured using an oscilloscope 17, which was connected to the. 
capacitor through a reverse biassed diode 26, to preserve charge. The 
array of tubes 13 was connected via a transistor switch 19 to a 100 V DC 
power supply unit 15. The transistor switch 19 was driven at a frequency 
of 10 kHz by an a stable multivibrator 16. 
The results of the fifth test are shown in FIG. 12. The results are 
somewhat similar to those shown in FIG. 11, except that the potential 
difference is much higher. The voltage excursion can therefore clearly be 
increased by applying a polarizing voltage to the plates of the capacitor. 
This has the benefit of increasing the available energy in the capacitor, 
since this is proportional to the square of the voltage (as given by the 
equation W=1/2CV.sup.2). Again, it is not clear at this stage what causes 
this phenomenon, although speculation surrounds the possibility that it 
may be related to a photovoltaic effect, radio frequency transmission or 
induction, or a combination of those factors. 
Many possibilities exist to enhance the performance of the device. Some of 
these are described below: 
The diameter of the tubes 7 may be reduced. This would allow greater 
density of tubes per unit area, thus allowing a larger proportion of the 
plates of the capacitor 1 to be occluded by the region of electrical 
discharge within the tubes. In addition, reducing the diameter of the 
tubes 7 will allow closer spacing of the capacitor plates, resulting in a 
larger capacitance per unit area. 
The area of the cathode in the tubes 7 may be increased. It is believed 
that the enhancing effect of the tubes on the capacitance of the capacitor 
is generated by the plasma envelope around the cathode. If the area of the 
cathode were to be increased in relation to the diameter of the tube, a 
further improvement to the effect could be expected. By coating the inside 
surface of the tube with an electrically conductive layer to constitute 
the cathode, the plasma cloud would occupy practically the entire diameter 
of the tube. By controlling the resistivity of this layer it could be made 
to constitute the load resistor as well. The anode would occupy the 
central axis of the tube and would be of minimum diameter. 
Suitable ionizing substances may be introduced in the tubes 7. It is 
believed that the effect on the capacitance is enhanced by greater numbers 
of ions in the gas. Ionizing substances could be introduced in the gas, or 
as a coating in the tubes 7. 
The materials chosen for the electrodes of the capacitor may be selected to 
enhance the effect. There may be an advantage in using conductors with 
photo-voltaic properties for the plates of the capacitor, to capitalize on 
the light emitted from the tubes. 
One possible design based on the above points is shown in FIGS. 13 and 14. 
In this embodiment, the tubes 7 are elongate and hexagonal in 
cross-section. Each row 6 is made up of a number of such tubes 7, laid 
edge to edge. The cathode 28 is formed on the inner surface of each tube 
and the anode 30 runs along the central axis of the tube, so that when the 
tube is activated, the plasma fills substantially the entire volume of the 
tube. 
The plates 2a,2b,2c and 3a,3b,3c etc. of the electrodes 2,3 are tightly 
interposed between the rows of tubes and have a zigzag or undulating form, 
to provide a high packing density and minimum plate separation. On their 
outer surfaces, the electrodes 2,3 are provided with fins 32, for good 
heat dissipation. 
As explained above, the capacitance of the capacitor depends on the number 
of tubes activated. The capacitor may be used to supply power and, as the 
number of tubes activated may be controlled digitally, this allows the 
capacitor to be used as a digital-to-analog power converter. 
For example, assuming that the tubes have a diameter of approximately 1 mm 
and a length of 20 mm, a total of 65536 tubes required for 16-bit digital 
conversion would occupy an area of 256 mm.times.256 mm. The resting 
capacitance (with no tubes activated) would be approximately 25 nF and if 
a ten-fold change in capacitance (i.e. a change of 225 nF) could be 
effected, this would produce an energy change of approximately 2.8 J at 5 
kV. This could be increased by increasing the length of the tubes, or by 
connecting a number of capacitors in parallel. 
A further application of the digital capacitor relates to its possible use 
as a digital-to-analog converter (DAC) or decoder, for example for use in 
a compact disk player. In the case of a 16-bit decoder, the capacitor 
would be arranged to receive 16-bit digital data along sixteen parallel 
lines. The first of those lines, which carried the least significant data 
bit, would be connected to one gas discharge tube, the second line would 
be connected to two tubes, the third to four tubes, the fourth to eight 
tubes and so on. Altogether, 65536 (2.sup.16) tubes would be required. 
By activating the tubes in accordance with the digital data received on the 
sixteen parallel lines, the capacitance of the capacitor can be varied in 
direct proportion to the value of the digital data. The digital data can 
thus be decoded and converted directly into a voltage to drive an 
amplifier or a speaker directly. 
Frequencies of up to 44 kHz are required for audio purposes and as gas 
discharge tubes have switching rates of up to 80 kHz, these should be more 
than adequate for such purposes. This also allows for the possibility of 
running the device at, say, 60 kHz and interposing values calculated from 
real; data at the revised clock rate. 
It may be possible to increase the switching rate further by using 
different gases, such as helium, in the gas discharge tubes. 
A 16-bit decoder may be constructed as a 256.times.256 matrix, although it 
would preferably be slightly larger than this, for example as a 
300.times.300 matrix, so that there are some spare tubes. Some of these 
tubes may be permanently activated to provide a "bias", offsetting the 
non-linear performance of the capacitor in the pre-saturation part of the 
performance curve, and others may be made available to replace faulty 
tubes. Yet others may be used to provide compensation for any 
non-linearities that may be present in the device as a whole. This 
compensation can be hard-wired during manufacture or provided as a 
software feature. 
A possible construction of a 16-bit digital-to-analog converter (DAC) is 
shown in FIGS. 15 to 19. The basic functional element of the DAC comprises 
a digital capacitor unit 34, which includes a pair of capacitor plates and 
a plurality of individually addressable gas discharge tubes. 
The digital capacitor unit 34 is constructed from a sandwich of four thin 
plates of a non-conductive substrate material, such as glass. These plates 
will be referred to in the following description as the top plate 36, the 
upper inner plate 38, the lower inner plate 40 and the bottom plate 42. It 
will be understood, however, that the device may be operated in any 
orientation and the terms "top", "upper", "lower" and "bottom" are used 
herein purely for ease of reference. 
The vertical dimensions of the plates 36-42 are exaggerated in FIG. 15 for 
the sake of clarity. In practice, all four plates 36-42 will be made as 
thin as possible to reduce the overall dimensions of the device. The total 
thickness of the device 34 may, for example, be approximately 1-3 mm. 
The top and bottom plates 36,42 are each provided on their inner faces with 
a plurality of parallel grooves 44, each of which may be for example 0.5 
mm wide and 20 mm long. A total of 256 grooves 44 may be provided on each 
plate 36,42 (only some of which are shown in FIG. 15). Each groove 44 in 
the top plate 36 lies directly above a corresponding groove 44 in the 
lower plate 42 and each such pair of grooves 44 are connected to one 
another by perforations 46 that extend perpendicularly through the inner 
plates 38,40. Each pair of grooves 44 thus forms a single rectangular 
chamber 48 for ionizable gas. 
The grooves 44 in the top and bottom plates 36,42 are rectangular in 
cross-section, each such groove comprising two side walls 50 and an end 
wall 52. A conductive element 54 is provided on the end wall 52 of each 
groove 44 in the top plate 36, the conductive element covering 
substantially the entire surface of the end wall 52. This conductive 
element 54 serves as the cathode of the gas discharge tube and also as the 
negative terminal of the capacitor unit 34. 
A conductive element 56 is also provided on the end wall 52 of each groove 
4 in the bottom plate 42. This conductive element 56 extends substantially 
the entire length of the groove 44 but is very narrow for minimal 
capacitive load. The conductive element 56 serves as the anode of the gas 
discharge tube and is connected via an electronic switch (not shown) to a 
power supply for activating the gas discharge tubes. 
Sandwiched between the two inner plates 38,40 there is provided a third 
conductive element 58, which serves as the positive terminal of the 
capacitor unit 34. This third conductive element 54 may for example be 
formed by deposition of a conductive layer on one of the inner plates 
38,40. The conductive element 58 does extend to the edges of the 
perforations 46 and it is thus insulated from the ionizable gas in the 
discharge tubes. 
The perforations 46 may serve as load resistors for the gas discharge 
tubes, by mechanically restricting the flow of gas ions between the upper 
and lower halves of each gas discharge tube. For this purpose, the size of 
the perforations must be selected according to criteria such as the 
potential difference, the current, the size of the molecules and the 
pressure of the gas. The perforations 46 may also serve as a series 
inductive load to the current through the tubes, to offset the 
capacitative load. 
Each digital capacitor unit 34 is provided with a frame (not shown) that 
provides the electrical connections for the conductive elements 54,56,58. 
As it has a relatively large surface area, the capacitor unit has plenty 
of room for the electrical connections. 
The digital capacitor unit 34 described above has 256 individually 
addressable gas discharge tubes. A 16-bit decoder would thus consist of a 
stack of 256 such units. Alternatively, the decoder may consist of sixteen 
64.times.64 stacks., which may be arranged as shown in FIGS. 18 and 19. 
Referring to FIG. 18, there is shown a side view of a single 64.times.64 
stack 60, consisting of 64 digital capacitor units 34, each of which has 
64 individually addressable gas discharge tubes 48. At least one tube 48a 
in each unit 34 will preferably be permanently lit to provide an offset 
bias and a source of ions to aid striking. 
The first, least significant, bit is represented by the first gas discharge 
tube of the first plate, the second bit is represented by the next two 
tubes, the third bit by the next four tubes and so on. The first six bits 
are therefore represented by the tubes of the first digital capacitor unit 
34a, the seventh bit by the entire second digital capacitor unit 34b, the 
seventh bit by the third and fourth digital capacitor units 34c,34d and so 
forth. Altogether, the stack 60 of 64 digital capacitor units 34 acts as a 
twelve bit decoder. 
A sixteen bit decoder comprises sixteen such stacks. As shown in FIG. 19, 
the first stack 60a represents the first twelve bits, the second stack 60b 
represents the thirteenth bit, the third and fourth stacks 60c,60d 
represent the fourteenth bit, and so on. 
Various modifications of the invention are possible: for example, instead 
of arranging the digital capacitor unit 34 so that the grooves in the top 
plate are parallel to those in the bottom plate, the top plate may be 
turned through 90.degree. so that the grooves lie perpendicular to one 
another. This will give the device improved mechanical strength and will 
ensure that some ions are present in all channels, to assist reliable 
striking. 
The DAC may be employed for example as the drive for a digital loudspeaker. 
As shown in FIGS. 20 and 21, the loudspeaker may be based on a 
conventional electrostatic loudspeaker and includes an electrically 
conductive diaphragm 62 that is mounted in the gap between two variable 
capacitors 1 and is charged by a high voltage source 64 through its 
built-in resistivity Rd. The inner plates 2 of the capacitors 1 are 
similarly charged through high value resistors R1 and R2. The outer 
electrode 3 of each capacitor is connected to the negative terminal of the 
power supply, which is earthed. 
The array 13 of tubes of each capacitor 1 is connected to a digital control 
device 66 that supplies digitally encoded positive half cycles to one of 
the arrays 13 and digitally encoded negative half cycles to the other 
array. The capacitance of the capacitors, and hence the electrical charge 
carried by the capacitors, is thus digitally controlled. As the charge 
carried by the capacitors varies, the diaphragm 62 is affected by the 
electrostatic field between the capacitors and thus vibrates in accordance 
with the digital signals supplied to the capacitors. 
Since each half cycle is represented by 16 bits, the complete signal will 
have the equivalent of 17 bit resolution. Assuming that a 250 nF capacitor 
could be manufactured according to the techniques described above, and 
that a twenty-fold enhancement is possible, this represents a potential 
capacitance change of 4750 nF. This would produce an energy output of 60 J 
per half cycle at 5000 V. 
The invention thus provides for the possibility of a true digital 
loudspeaker. Power amplification in the conventional sense is avoided. 
Instead, the DAC is powered up to maximum energy and then tapped of only 
enough to move the diaphragm. Tone and volume control can be achieved 
through software. 
The digital capacitor may also be used for driving low impedance loads. 
Suitable circuits are shown in FIGS. 22 and 23, in which R1 and R2 are 
high value resistors, C2 a conventional HV capacitor and T1 a transformer 
(for example a ferrite transformer). Varying the capacitance of the 
digital capacitor C1 causes a current to flow through the primary coil of 
the transformer T1, thereby generating a voltage in the secondary coil. 
The invention may also find a use in any application where digital control 
of power is required. For example, the capacitor may be used for motor 
control or as a video decoder, for example for driving a flat panel 
display. An operating frequency of up to 44 MHz may be required for such 
applications, and Helium is probably the most suitable gas for the 
discharge tubes. 
Further modifications and applications of the invention will be apparent to 
those skilled in the art.