Electromagnetic Resonator With Electronic Tuning

Abstract

In a waveguide resonator, either coaxial, or non-coaxial, there are inserted spaced irises which establish capacitances between the conductors of the waveguide structure at the location of each iris. These capacitances tune the resonance of the waveguide cavity, which would generally be used in a bandpass filter. Switch means are provided for each iris, for discretely altering the value of the capacitance established, whereby the resonator or filter may be tuned to a large number of different center frequencies.

Description

BACKGROUND OF THE INVENTION

This invention relates to electronically tuned filters, and more particularly, to improvements therein.

OBJECTS AND SUMMARY OF THE INVENTION

An object of this invention is the provision of a novel construction for affording electronic tuning of a high power VHF, UHF or microwave resonator used in a bandpass or other filter.

Another object of this invention is the provision of a novel and simple tuning arrangement for a waveguide-type of resonator.

Yet another object of this invention is the provision of a VHF/UHF/microwave resonator which is capable of being tuned rapidly and reliably over a long period of time.

The foregoing and other objects of the invention may be achieved in a waveguide resonator which is distributively loaded with several identical, but independent capacitive irises. Switches are connected in these irises which are operable to enable the effective capacitance of an iris to be altered from one discrete value to another to thereby tune the filter resonator to be responsive to different frequencies. Additionally, the distribution pattern for the irises may be determined such that the tuning increment resulting from the switching of each member of the series of irises are related very closely in a binary manner.

The novel features of the invention are set forth with particularity in the appended claims. The invention will best be understood from the following description when read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a bandpass filter resonator in accordance with this invention, which is operated in a half-wave mode.

FIG. 2 is a schematic diagram of an embodiment of this invention, which is operated in a quarter-wave mode.

FIG. 3 is a view in longitudinal section of a bandpass filter in accordance with this invention.

FIG. 4 is a cross-sectional view along the lines 4--4 of FIG. 3. FIG. 5 is a schematic representation of a suitable switching system for tuning each iris of the filter resonator.

FIG. 6 is a longitudinal sectional view showing additional details of the invention.

FIG. 7 is a cross-sectional view illustrating an alternative embodiment for switching each iris in the invention represented by FIG. 6.

FIG. 8 illustrates in cross-section, the installation details of each of the miniature high speed mechanical switches used in the iris embodiment of the invention shown in FIG. 7.

FIG. 9 is a longitudinal section and FIG. 10 is a cross-sectional of an embodiment of the invention that uses a waveguide without a central conductor.

FIG. 11 is an end view, in cross-section, of an embodiment of the invention, that uses pairs of irises in a waveguide without a central conductor.

FIG. 12 is an end view in cross-section illustrating the appearance of the invention in a circular coaxial waveguide.

FIG. 13 is an end view in cross-section of the invention using a circular waveguide without a central inner conductor.

FIG. 14 is a cross-sectional view of a bistable capacitor which may be used with this invention instead of a switch that goes from conducting to non-conducting.

FIGS. 15 and 16 supplement FIG. 14 in showing the moving element of FIG. 14 in its two possible stable positions.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates schematically, the equivalent circuit of a transmission line (generally coaxial) resonator distributively loaded with independent switch-controlled capacitive irises in accordance with this invention. As will be seen in later drawings, each iris is insulatedly supported between a central conductor and the wall of the waveguide (in the case of a TEM-line), or between the waveguide walls, and thereby establishes a capacitance, designated as C.sub.1 between the iris and the central conductor, or one wall, and a capacitance designated as C.sub.2 between the iris and the other wall of the waveguide. The capacitors C.sub.1 and C.sub.2 effectively are connected in series and all of the series connected capacitances C.sub.1, C.sub.2 effectively are distributed along the length of the transmission line and separated by distances that are significant fractions of a wavelength.

Switches respectively 10, 12, 14, 16 and 18 are shown for selectively short-circuiting, or otherwise altering the value of, each one of the C.sub.2 capacitances. In the case where the switches are PIN diodes, a biasing adjunct, such as a choke, (not shown) is needed for each switch. The incidental influence of a choke on the circuit is schematically represented by a reactance 20 and a resistance 22. For simplicity, a showing of these in all places has been omitted, except near switch 12. The resistance 24 represents a "switch open" equivalent resistance (R.sub.p) and resistance 26 represents the "switch closed" equivalent resistance (R.sub.s). For clarity, a showing of these in all places has been omitted, except near switch 16.

If the cavity resonator is to be used as a filter e.g., band stop or band pass, coupling to external circuits is required. This is done with one or more coupling probes or loops which can be located anywhere convenient in the cavity resonator. FIG. 1 shows schematic symbols 28, 30 for the case of two coupling loops at the opposite short-circuited ends of the transmission-line resonator.

In the absence of the irises, the fundamental resonant frequency, f.sub.0, of the transmission-line resonator is c/4l where c is the phase velocity of the waveguide, and l is the cavity half length, and the (transverse) electric field in the resonator would have a half-sine-wave distribution with a maximum in the mid-plane. The irises that are added are identified according to their distance from the nearest short circuiting plane; it is convenient to specify the distances .theta..sub.i as "electrical angles" relative to .theta..sub.1 (distance from short to mid-plane) being defined as 90.degree.. That is, as shown on the drawing, the mid-plane iris is a distance .theta..sub.1 or 90.degree. from the end of the waveguide. An iris is considered to be more or less "significant" to the resonator as .theta. is larger or smaller.

The principal capacitance provided by the iris is C.sub.1, but since the switches exhibit capacitance when "open," and are supported between conductors providing additional capacitance, it is necessary to include the capacitance C.sub.2 as shown in the drawing. The iris capacitance is thus two valued

C.sub.max = C.sub.1 with switch closed.

C.sub.min = C.sub.1 .sup.. C.sub.2 /C.sub.1 + C.sub.2 with switch opened.

Since the usefulness of an iris depends on having C.sub.max /C.sub.min large, it is seen as desirable to have C.sub.2 < C.sub.1, and preferably C.sub.2 << C.sub.1.

Having all the irises identical, has many advantages, including keeping C.sub.max /C.sub.min optimal. The significance of an iris for tuning purposes is then determined solely by its location relative to the E field distribution. If iris loading is assumed not to alter this half-sin-wave distribution, and irises are assumed not to interact with one another, the capacitive effect of an iris should be weighted as sin .theta..sub.i, i = 1, 2, 3, . . . , N.

Further, assuming that it is a law of nature for a "frequency tuning effect" to vary as the (-1/2) power of a "capacitive effect," a binary tuning program would be achieved if sin.sup.2 .theta..sub.i = 1, 1/2, 1/4, 1/8 . . . 1/2.sup.N.sup.-1, where N is the total number of irises. The following table is derived from the last equation.

TABLE 1 --------------------------------------------------------------------------- IRIS PLACEMENT FOR A BINARY TUNING

PROGRAM Iris Nominal No. "Frequency- Distance (i=) Tuning Effect" sin.sup.2 .theta..sub.i .theta..sub.i from Short __________________________________________________________________________ 1 1/2 of the tuning range 1 90.degree. l 2 1/4 of the tuning range 1/2 45.degree. 0.500 l 3 1/8 of the tuning range 1/4 30.degree. 0.333 l 4 1/16 of the tuning range 1/8 20.7.degree. 0.230 l 5 1/32 of the tuning range 1/16 14.5.degree. 0.161 l . . . . . . . . . . . . . . . N 1/2.sup.N of the tuning range 1/2.sup.N.sup.-1 sin.sup.-.sup.1 [2.sup.-.sup.(N.sup.-1)/2 ] 2.theta ..sub.i __________________________________________________________________________ l/.pi.

The inductive reasoning used in deriving the table is far from rigorous. Nevertheless, it has in practice proved workable; computations and measurements made with the resonator loaded with five irises positioned as indicated have yielded an essentially binary tuning logic as evidenced by a substantially uniform spacing of 32 resonances over a 21 percent tuning range obtained by operating the switches of the five irises in all of their combinations.

The locations of the irises may be determined as those which satisfy the following relationship:

sin.sup.2 .pi./2 .sup.. L.sub.1 /L = 2 sin.sup.2 .pi./2 .sup.. L.sub.2 /L = 4 sin.sup.2 .pi./2 .sup.. L.sub.3 /L = 8 sin.sup.2 .pi./2 .sup.. L.sub.4 /L = . . . = 2.sup.N.sup.-1 sin.sup.2 .pi./2 .sup.. L.sub.N /L,

where L.sub.1, L.sub.2, L.sub.3, L.sub.4 . . . L.sub.N are each locations of an iris measured from the short-circuited end of the waveguide, and L is the distance from the short-circuited end to the mid-plane, where the waveguide is operated at half wavelength resonance.

FIG. 2 is the equivalent circuit of a transmission-line resonator which is shorter than in FIG. 1 because it is quater-wave resonant, having one end open-circuited and the other end short-circuited. The switches 31 through 34, respectively, are employed for selectively short circuiting (or otherwise modifying) the capacitors C.sub.2. As previously, the resonator is coupled to external circuits through one or more coupling probes or loops. The illustration shows symbols for 2 coupling loops, 35, (one for input, and one for output in the case of a bandpass filter) both located at the short-circuited end of the resonator. Other arragnements would work as well. A choice as to which the 2 embodiments is to be used depends upon the particular application to be served and the frequencies of interest. However, the resonator arrangement shown in FIG. 2 is less advantageous at higher frequencies than the one shown in FIG. 1, due to the mechanical inconvenience of having irises closer together in the embodiment of FIG. 2 and the greater likelihood of unwanted interactions between the irises.

Here, the locations of the irises may be determined as those which satisfy the equation

sin.sup.2 .pi./2 .sup.. L.sub.1 /L = 2 sin.sup.2 .pi./2 .sup.. L.sub.2 /L = 4 sin.sup.2 .pi./2 .sup.. L.sub.3 /L = 8 sin.sup.2 .pi./2 .sup.. L.sub.4 /L - . . . = 2.sup.N.sup.-1 sin.sup.2 .pi./2 .sup.. L.sub.N /L,

where L.sub.1, L.sub.2, L.sub.3, L.sub.4 . . . L.sub.N are each locations of an iris measured from the short-circuited end of the waveguide, and L is the distance from the short-circuited end to the open-circuited end of the waveguide, where the waveguide is operated at quarter wavelength resonance.

FIG. 3 is a cross-sectional and plan view of an embodiment of the invention, and FIG. 4 is a slightly enlarged view along the lines 4--4 of FIG. 3. The TEM waveguide 36 has a central conductor 37. By way of example, five pairs of irises such as pair 38, 39, are insulatingly supported in the same plane on opposite sides of the central conductor 37. For binary tuning, these pairs of irises are placed at locations corresponding to those shown in FIG. 1. As will become more clear later herein, a pair of irises is treated as equivalent to a single iris. The irises 38, 39 are the central irises. The irises are supported in position, within the waveguide, by pairs of dielectric spacers, such as spacers, 40, 41, 42 and 44.

In one embodiment of the invention, diodes such as "PIN" diodes, were used as switches for shorting the capacitance C.sub.2, which is established between the iris and the wall of the waveguide. C.sub.1 is the capacitance established between the iris and the inner conductor of the waveguide. The diodes respectively, 44, 46, 48 and 50, may be seen in FIG. 4, supported from the waveguide wall and connected between the wall and the respective irises 38, 39 of the pair. It should be clear to those familiar with the art that interchanging the positions of fixed capacitance, C.sub.1, and the switch-associated capacitance, C.sub.2, with respect to being near either the inner or the outer conductor of the coaxial line, does not constitute a deviation from the intent of this invention, and therefore is within the scope of this invention and the claims directed thereto.

Diodes of the PIN type, which were found suitable for this type of switching operation includes, for example, the UN7000C series diodes, which are manufactured by Unitrode Corporation, Watertown, Massachusetts. The diodes were preferably connected in parallel between the iris and the biasing sources supplied, and have a forward current applied to simulate a switch-closed position and a reverse bias voltage applied when they simulate a switch-open position. The number of diodes to be used is determined by considerations of resonator Q, as influenced by diode losses when "closed" (forward-bias) and when "open" (reverse-bias). In the former case, having more diodes in parallel reduces the loss, but in the latter case, it increases the loss. A compromise is therefore made so that the two situations will yield about the same loss. This is necessary because, in general, some of the irises in the filter will have forward-biased diodes at the same time as other irises have reverse-biased diodes. It may also be noted that if too few diodes are used in parallel, the resonator tuning plan, and the resonator Q, may be adversely affected by switch inductances and RF current crowding problems. In the UHF resonator being discussed as an illustrative embodiment of the invention and using the diodes mentioned, two diodes per iris half gave the substantially correct balance.

The rectangular-cross-section construction shown in the drawings is very convenient, although the invention should not be considered as limited thereto. A circularly symmetrical cross-section might lead to smaller dimensions for a given resonator, but the iris sub-assembly might be more difficult to place.

A fine tuning iris, 52, is shown in FIG. 4, supported on dielectric spacers 56, 58, from the side of the waveguide, and it has a single diode switch 60. The fine tuning iris is so called because the tuning increment that its switching in or out provides is less than that of any of the broad double-sided "coarse-tuning" irises distributed throughout the resonator. The entire set of fine-tuning irises form a second set of binary tuning increments. For example, if the tuning increments provided by the coarse-tuning set are 1/2, 1/4, 1/8, 1/16 and 1/32 of the tuning range, then the tuning increments of the fine-tuning set are 1/64, 1/128, 1/256, 1/512 and 1/1024 of the tuning range. Each fine-tuning iris is located in the same place as a coarse-tuning iris pair because the locations of the fine-tuning set are also derived from the table given above. Creating a second type or style of iris is done when there is mechanically too little room to insert irises of the original type having locations .theta..sub.i between 0.degree. and 14.5.degree..

A highly asymmetrical rectangular cross-section was considered for the coaxial waveguide with the RF fields concentrated on one side, so that a single iris plate (instead of two) would suffice, located on that same side. It was found however, that for the same impedance and resonator Q, an excessively large volume would be needed on the unused side. That is, when the resonator volume and, hence, cross-sectional area are limited, one obtains the greatest unloaded Q for a given impedance when using the available area for a bilaterally symmetrical cross-section. An asymmetrical cross-section nevertheless, should not be excluded from the intent of this invention.

In actual application, it has been found that after five or six irises have been installed in a UHF filter, the spacing between irises, and between an iris and an end wall, start to become inconveniently small. Hence, if the tuning range is required to be divided into more than 32 or 64 parts, more irises must be added, though not very many, since the number of tuning channels doubles for each additional iris. Although there are several solutions to this problem, the one chosen here was to shape the 5 original (coarse tuning) irises such that they would not fill up the entire resonator cross-section. Room is left for the small, interpolating, or "fine tuning" iris, such as 52, which is in the same transverse plane as the coarse tuning iris. If the tuning effectiveness of the fine tuning iris is 1/32 of that of the coarse tuning iris, and if there is negligible interaction between them, the progressive halvings of tuning increments by each iris in turn would be continued from the five coarse to the five fine tuning irises, whereby a total of 1024 resonances can be made available.

With the use of the PIN diodes in the filter structure, it becomes necessary to bring the bias leads for the diodes into the RF region. This should be done without degrading the RF performance. After giving consideration to a series connection of pairs of diodes with opposite polarities (for d-c) that would insert bypass capacitors rather than bias leads into the RF region, it was decided instead, to use a resonant bias choke, such as the resonant bias chokes 62, 64, 66, which are shown in FIG. 4, which are provided for each iris plate. The bias chokes for the central iris were tuned to resonance (by adjusting the number of turns) at the center of the upper half of the tuning range because the diodes short this choke over the entire lower half of the tuning range.

The remaining bias chokes were tuned to approximately the center of the tuning range. Each choke, as may be seen in FIG. 4, has one end which is connected to an iris, and the other end is connected to the respective choke shield, 68, 70, 72, which surround the chokes. A "DC break" is provided between the choke shield and the wall of the waveguide. This break is nothing more than a spacing which has very little effect on the circuit, since these breaks are located as close as possible to the high impedance end of the choke. However, to minimize any possible RF leakage due to the break, the gap is filled with a thin Mylar tape forming a by-pass capacitance of at least 65 pf. Input and output coupling loops, 74, 76, are shown in FIG. 3. While these loops are shown at one end of the resonator, it will be appreciated that these may be used at both ends if required.

FIG. 5 is a circuit diagram of an arrangement for biasing the diodes in a forward or reverse direction (switch closed or switch open). While the circuit is shown for a single iris, and its associated diodes, it will be appreciated that a switching arrangement is required for each iris. Pairs of plates for the double irises are switched simultaneously. Two diodes, respectively 82, 84, are connected in parallel between the iris 80 and the waveguide walls 81, by way of illustration. The bias choke 86 has one end connected to the iris 80, and the other end connected to one terminal of a selector switch 88. When in one position, (the upper) the selector switch allows a direct current to flow from anodes to cathodes of both diodes. When the switch 88 is in its other position, a large negative voltage is applied to the anodes of the two diodes.

Operation of the switch is performed by the switch control 90.

It will be appreciated that the switch 88 and its control 90 are representative of any electronic switching arrangement, many of which are well known in the art, which can operate at the speed desired to effectuate switching. It should also be appreciated that the total number of switches 88 and controls 90 equals the number of pairs of coarse-tuning iris plates plus the number of fine-tuning irises, or 10 in the case being illustrated. This is consistent with the total number of resonant frequencies for the illustrated resonator being 2.sup.10 or 1024.

FIG. 6 is a schematic view in longitudinal section of the interior of the waveguide 36, shown to illustrate, by way of example, and not by way of a limitation, some typical dimensions for a UHF bandpass filter, made in accordance with this invention, and in FIG. 4, the cross-section, which is also shown to demonstrate the weights, or effects, of the coarse and fine tuning irises on the tuning of the filter. Only four fine tuning irises are shown in FIG. 6, and these are given the numbers six, seven, eight and nine, which indicate their weights, as 1/2.sup.N where N is the weighting number. The numbers one, two, three, four and five indicate the weights of the coarse tuning irises. This illustration thus gives a filter with 2.sup.9 or 512 resonant frequencies rather than the 2.sup.10 or 1024 discussed previously. The depth of the interior of the waveguide (top to bottom dimensions of the view shown in FIG. 4) is 3.24 in.

While each of the five coarse tuning irises is comprised of two halves, that could be switched separately if so desired, (although the 1024 resultant resonances with the fine tuning irises omitted would definitely not cover the tuning range uniformly), generally, asymmetrical iris switching results in RF current crowding that is very detrimental to the unloaded Q. When the switches are low loss, such asymmetries must be avoided; however, in the present invention, the switch losses predominate, and so mask these asymmetry-crowding losses. Nevertheless, this invention is compatible with low loss switches, and therefore, the halves of the coarse tuning irises are switched in unison.

Table 2 reproduced below, represents a small excerpt from the results which were obtained by measuring the performance of the filter using an automatic network analyzer. The column with the heading "Code" designates the "off-on" settings of the iris switches. 0 designates off, or reverse biased diodes, and 1 designates on, or forward biased diodes. The five binary digits commencing with the left side of the code number represent the settings of the five coarse tuning irises, and the remaining four digits represent the settings of the four fine tuning irises.

Column 2 which is labeled "DEBC" represents the decimal equivalent of the binary number shown under "Code." The next column indicates the frequency in megahertz.

The following columns list, respectively, frequency (in megahertz) and insertion loss (in decibels) corresponding to phase shifts through the filter of -45.degree., 0.degree., and +45.degree. respectively. The phase shift notation 0 indicates the central resonant frequency of the filter while the phase shifts -45.degree. and +45.degree. locate the edges of its pass band (where the insertion loss is within 3dB of that at band center) at a given switch control combination. Frequency difference from the previous entry is also listed.

Using a computer command system and electronic switching, the center frequency of the illustrated bandpass filter could be rapidly tuned from 359.25 MHz at decimal number 0 (binary 000000000) down to 293.84 MHz at decimal number 511 (binary 111111111). ##SPC1##

FIG. 7 is a waveguide cross-section which illustrates another embodiment of this invention, based on high-speed mechanical switches instead of semiconductor diode switches. The outer wall 92 of the coaxial waveguide encloses the inner conductor 94 and two coarse tuning iris halves 96, 98 are supported perpendicularly thereto on opposite sides and in the same plane by means of insulators 100, 102, 104, and 106. For insuring a low-loss but flexible connection to the switches (112, 113, for example) which are used to connect across the capacitance established between the wall of the waveguide and the iris plates, a plurality of U-shaped copper braids, 108, 110, for example, are used. Each of these copper braids is conductively connected to an iris by the ends of the U, and the center of each U is connected to one terminal of a switch (112, 113, for example) whose other terminal is connected to the waveguide walls 92. By way of example, each of the iris halves 96, 98 has 5 of these copper braids or flexible straps.

The details of a braid connection and a switch mounting are shown in FIG. 8, which is an enlarged and sectional view of these structures, such as braid 114, for example. Some of the qualifications for a switch which is to be used are that it be bistable and have a high open switch resistance (R.sub.p), rapid and positive operation, longevity and a very low closed switch resistance (R.sub.s). By way of illustration, one suitable switch which was incorporated in an embodiment of the invention is known as a Logcell I, which is manufactured by Fifth Dimension Inc., of Princeton, N.J. This switch has mercury contacts, is glass enclosed, and is solenoid operated. This switch is referred to by the reference numeral 116. At one end is pin 118 which is grasped by pin socket 120 that is inserted conductively into the copper braid 114. The socket 120 also has springs 122, 124, which help grasp the switch pin 118. As a result, a low loss connection is made between the switch 116 and the copper braid without applying undue mechanical stress to the thin glass walls of the switch capsule.

The part of the switch below the pin 118 is formed of glass, and thus is an insulator. The switch is held in place, by a clamping shim 126, which is attached to the waveguide cavity wall. The switch central contact 125 is grounded to the wall. A solenoid 128, which is used for the operation of the switch is adjacent the other end thereof, and is placed within the waveguide wall. The solenoid is connected to a switch control circuit 130, which provides the necessary current pulse for energizing the solenoid when it is desired to open or close the switch. The other end of the switch is also a pin 132 which is surrounded by a soft iron sleeve 134. The pin 132 is not connected to anything, but serves to locate the iron sleeve 134 which is part of the magnetic circuit. If desired, however, pin 132 may be connected to an electro-visual indicator that monitors the state of the switch; when the switch 116 is open on the RF side (upper end), the pin 132 is at ground potential, while pin 132 is floating when switch 116 is closed at the RF end. It will be understood that a switch is required for each copper braid connected to an iris. As illustrated in FIG. 7, each iris has 5 parallelled switches. The switches for each pair of coarse tuning iris plates are operated simultaneously in a manner well known to the art.

FIG. 9 is a longitudinal section of an electronically controllable bandpass filter, in accordance with this invention, showing irises 131 A, B, C, D and E, insulatingly supported in a waveguide 133, without a central conductor. FIG. 10 is a view along the lines 10--10 of FIG. 9. Each of these irises is supported from one wall of the waveguide by an arrangement such as is shown in FIG. 10, for example. This includes insulators 136, 138 and diodes 140, 142, for example. The switching arrangement is the same as is shown in FIG. 5. This system may also use switches as described in connection with FIGS. 7 and 8. The purpose of these switches, as has been described previously, is to short-out or alter the value of the capacitance between the iris and the side of the waveguide from which it is suspended. Other capacitances are established between the other sides of the rectangular irises and the walls of the waveguide opposite which they are adjacent. It is also within the scope of this invention to also insert switches of the capacitance changing type (as is described later herein) but not of the completely-closing type, between the iris and these other walls.

FIG. 11 illustrates by an end view in section a variation of the arrangement shown in FIGS. 9 and 10. The variation consists of using pairs of irises, 135, 137 in a waveguide 133 instead of single irises. Here the capacitance C.sub.1 is established between the opposite inner edges of the two irises and the capacitance C.sub.2 between the outer edges of the pair of irises and the waveguide wall.

FIG. 12 illustrates, in an end view in section, an embodiment of the invention using a circular coaxial waveguide. A ring-shaped iris 139 is employed at each iris location, which has a hole in the center which is large enough to permit the central conductor 141 to pass therethrough and to provide for space whereby C.sub.1 is established. Each iris is insulatingly supported from the walls by spacers, such as 143. A plurality of switches, such as 144, are provided to short or alter the value of the capacitance C.sub.2, which is established between the outer edge of the iris and the waveguide wall.

FIG. 13 illustrates, in an end view in section, an embodiment of the invention using a circular waveguide without inner conductor. Here, the iris 146 is circular or ring-shaped also, has a central aperture 148, and is insulatingly and centrally supported, spaced from the wall 150 of the waveguide. Support is by spaced insulators 152, for example. A plurality of spaced switches 154 for example, are used for connecting the outer edge of the iris to the wall.

Another type of switch which may be used with any or all of the embodiments of the invention which are described and shown herein is a bistable capacitor. FIG. 14 is a cross-sectional view of such a device. FIGS. 15 and 16 are simplified cross-sectional views of the device illustrating two operating positions. A glass or ceramic hollow capsule 160 contains a permanent magnet shuttle 162. This should have a gold coating which is a good RF conductor. The length of the shuttle is slightly more than half the length of the capsule and the remainder of its interior is filled with a lubricating dielectric gas or fluid. One end of the capsule is fitted in a socket 164 which extends along the axis of the capsule and has a flange 166 which extends at right angles to the socket 164, adjacent its open end. This flange constitutes R.F. ground and is connected to the waveguide wall 168. The socket 164 terminates in an iron core 164 terminates in an iron core 168 around which there is wound a solenoid winding 170.

There is another conductive socket 172, which fits over the other end of the capsule 160. This socket is fitted into intimate contact with the flexible strap (copper braid) 174 which connects to an iris. The socket 172 has an iron plug 176.

The magnetic slug 162 is pulled to the end of the capsule plugged into socket 164 (ground end) when a pulse having one polarity is applied to the solenoid winding 170 from a pulse source 178. The magnetic shuttle is pushed to the other end of the capsule ("hot" end) in response to a pulse of the opposite polarity applied to the solenoid winding. Magnetic forces hold the shuttle in one or the other of its end positions after the solenoid pulse is ended. The capacitor is thus bistable and requires very little driving power. Its switching speed should be comparable to that of the mercury switch of comparable size, mentioned earlier, but the losses due to the mercury and the oxides in the metal/glass seals of the latter are eliminated. To those skilled in the art, variations on the magnetic circuitry illustrated are seen not to alter the spirit of the invention, which is to provide a multi-iris-tuned, high-frequency resonator in which the capacitance associated with each iris is altered from one value to another by a bistable mechanical capacitor, rather than shorted-out by a contact-to-contact switch. (The use of a solid-state variable capacitor, or varactor, for this function is not preferred because that device would not allow high RF power levels to be used with the filter or resonator and would introduce distortion of the RF waves).

The capacitance between the sockets of the bistable capacitor shown in FIG. 14 goes between capacitance values C.sub.A and C.sub.B where C.sub.A << C.sub.B. Because C.sub.B would have a typical value of only a few picofarads, its use is at high frequency RF. C.sub.A is obtained when the magnetic shuttle is at the ground end of the capsule (FIG. 15) and C.sub.B when it is at the "hot" end of the capsule (FIG. 16). When this device is used in the gap between an iris and a waveguide wall, the design equations become:

C.sub.2A = C.sub.2 + nC.sub.A (shuttles clearing gap between sockets)

or

C.sub.2B = C.sub.2 + nC.sub.B (shuttles closing gap between sockets)

C iris.sub.(min) = C.sub.1 C.sub.2A /C.sub.1 +C.sub.2A

C iris.sub.(max) = C.sub.1 C.sub.2B /C.sub.1 +C.sub.2B

where n is the number of devices in parallel.

If C.sub.B >> C.sub.A and nC.sub.B >> C.sub.1, the use of a bistable capacitor of the type shown has very little effect on the design of a high-frequency resonator or filter system from that based on open/closed switches. However, the resonator Q would be much higher and the filter would have a narrower bandwidth and/or lower insertion loss.

There has accordingly been described and shown herein a novel and useful electronically-tunable VHF/UHF/microwave resonator, such as would be used in a bandpass or other filter. The filter parameters, such as the range limits, loaded and unloaded Q, bandwidth, VSWR, and insertion loss, are determined almost solely by the losses in the more significant irises. As a result, a filter is provided whose basic design is independent of the ultimate tuning increment desired. Less-significant irises can be added or deleted at any time, giving a smaller or larger tuning increment, without affecting the performance for any one channel.

Claims

1. An electronically tunable waveguide resonator filter comprising:

a rectangular coaxial waveguide having a longitudinally extending central conductor therein,

a plurality of pairs of irises,

means for insulatingly supporting each of said pairs of irises at predetermined distances from an end of said waveguide along the length of said waveguide,

each of a pair of irises being supported in a common plane on opposite sides of said central conductor, and from opposite walls of said waveguide,

switch means for selectively altering the impedance between one edge of each member of said pairs of irises and an adjacent waveguide wall for changing the frequencies of the resonator filter in a predetermined manner, and

means for electronically actuating said switch means for effectuating said

2. An electronically tunable waveguide resonator filter as recited in claim 1 wherein there is included a plurality of identical fine tuning irises:

means for supporting each fine tuning iris, in the same plane as a pair of irises, and insulatingly from a wall other than the two opposite walls from which said pair of irises are insulatingly supported, and

switch means for selectively altering the impedance between each of said plurality of fine tuning irises and the wall from which they are

3. An electronically tunable waveguide resonator filter as recited in claim 1 wherein said switch means comprises:

a plurality of diodes connected between one edge of each of a pair of irises and the wall from which it is insulatingly supported,

a current source, and

means for connecting said current source to a plurality of diodes for a pair of irises to bias said diodes in a forward direction for establishing one impedance value between said pair of irises and said waveguide wall, or in a reverse direction for establishing a second impedance value

4. An electronically tunable waveguide resonator filter as recited in claim 1 wherein said switch means comprises:

a plurality of capacitor means for each pair of irises, each of which has a large capacitance value when in one condition, a small capacitance value when in a second condition, and means for switching between said two capacitance values,

means for switching all of the capacitive means for a pair of irises to have their small capacitive value when it is desired to establish one impedance value between a pair of irises and said waveguide wall, and

means for switching all of the capacitive means to have their large capacitive value when it is desired to establish a second impedance value

5. A tunable electromagnetic resonator comprising

a waveguide, said waveguide has one end thereof short-circuited,

said waveguide has N irises insulatingly supported, spaced from one another within said waveguide, and perpendicular to the longitudinal axis of said waveguide at locations along said waveguide as follows:

sin.sup.2 .pi./2 .sup.. L.sub.1 /L = 2 sin.sup.2 .pi./2 .sup.. L.sub.2 /L = 4 sin.sup.2 .pi./2 .sup.. L.sub.3 /L = 8 sin.sup.2 .pi./2 .sup.. L.sub.4 /L = . . . = 2.sup.N.sup.-1 sin.sup.2 .pi./2 .sup.. L.sub.N /L,

where L.sub.1, L.sub.2, L.sub.3, L.sub.4 . . . L.sub.N are each locations of an iris measured from said short-circuited end of said waveguide, and L is the distance from the short-circuited end to the mid-plane when the waveguide is operated at half wavelength resonance, and is the distance to the other end of said waveguide when said waveguide is operated at quarter wavelength resonance, and

means for selectively altering the impedance between each iris and a wall of said waveguide to thereby alter the resonance frequency of said

6. A tunable electromagnetic resonator as recited in claim 5 where L.sub.1 = 0.5L, L.sub.2 = 0.333L, L.sub.3 = 0.23, L.sub.4 = 0.161L, . . . ,

7. A tunable electromagnetic resonator as recited in claim 5 wherein said waveguide is a coaxial waveguide having a central conductor:

each iris of said plurality of said irises comprises a pair of irises, one iris of a pair being positioned between one side of said central conductor and the waveguide wall opposite to said one side, and a second iris of a pair being positioned between the other side of said central conductor and the wall opposite to said other side, and

said means for selectively altering the impedance between each iris and a wall of said waveguide includes

means for simultaneously connecting or disconnecting a pair of irises to

8. A tunable electromagnetic resonator as recited in claim 7 wherein there are provided a plurality of fine tuning irises:

means for insulatingly supporting each fine tuning iris in the same plane as each pair of irises that are positioned on either side of said central conductor, and

means for selectively altering the impedance between said fine tuning irises and the adjacent waveguide wall for altering the resonance

9. A tunable electromagnetic resonator as recited in claim 5 wherein there is included a plurality of fine tuning irises, each of said fine tuning irises being smaller then each of said plurality of irises:

means for insulatingly supporting said fine tuning irises spaced from one another within said waveguide and perpendicular to the longitudinal axis of said waveguide, and

means for selectively altering the impedance between each fine tuning iris and a wall of said waveguide to thereby alter the resonance frequency of

10. A tunable electromagnetic resonator as recited in claim 5 wherein said waveguide is a circular coaxial waveguide, and

each of said plurality of irises has the shape of a ring with a central opening larger than the central conductor of said circular coaxial

11. A tunable electromagnetic resonator as recited in claim 5 wherein said

12. A tunable electromagnetic resonator as recited in claim 5 wherein said waveguide is rectangular and each iris of said plurality of pairs of irises comprises a pair of irises,

one iris of a pair being insulatingly supported from one wall of said waveguide,

the other iris of a pair being insulatingly supported from the opposite wall of said waveguide, there being a space between the opposite ends of said irises, and

said means for selectively altering the impedance between each iris and a wall of said waveguide being connected between each iris of a pair and the

13. A tunable electromagnetic resonator as recited in claim 5 wherein said means for selectively altering the impedance between each iris and a wall of said waveguide includes:

a plurality of diodes,

means for biasing said diode in reverse direction when it is desired to increase the impedance between an iris and an adjacent wall, and

means for biasing said diodes in a forward direction when it is desired to

14. A tunable electromagnetic resonator as recited in claim 6 wherein said means for selectively altering the impedance between each iris and a wall of said waveguide comprises:

a plurality of switches for each iris,

means for closing said switches for each iris when it is desired to lower the impedance between an iris and an adjacent wall, and

means for opening the switches for each iris when it is desired to increase

15. A tunable electromagnetic resonator as recited in claim 5 wherein said means for selectively altering the impedance between each iris and a wall of said waveguide for each iris comprises:

a plurality of capacitor means each of which has a large capacitance value when in one condition, a small capacitance value when in a second condition, and means for switching between said two capacitance values,

means for switching all of the capacitor means to their large capacitance value when it is desired to reduce the impedance between an iris and a wall of said waveguide, and

means for switching all of the capacitor means to their small capacitance value when it is desired to raise the impedance between said iris and the

16. A tunable resonator filter comprising

a waveguide having a central conductor extending longitudinally therethrough,

said waveguide having one end thereof short-circuited,

N iris means within said waveguide distributed at predetermined spaced positions along the length of said waveguide for establishing a first capacitance between each of said iris means and one wall of said spaced waveguide, and a second capacitance between each of said iris means and the central conductor,

said N iris means being insulatingly supported at locations along said waveguide as follows: L, 0.5L, 0.333L, 0.23L, 0.161L, . . . , 2L/.pi. arc sin 1/2.sup.N.sup.-1, where L is the distance from the short-circuited end of said waveguide to its other end when the resonator filter length is a quarter wavelength and L is the distance from the short-circuited end of said waveguide to its mid-plane when the resonator filter length is one half length, and

means for selectively altering the value of one of the capacitance established by said plurality of iris means for varying the signal

17. A tunable resonator filter as recited in claim 16 wherein said waveguide is rectangular, and

each of said iris means comprises a pair of rectangular diaphragms mounted in a common plane one between one wall and said central conductor, and the other between the wall opposite to said one wall, and the central

18. A tunable resonator filter as recited in claim 16 wherein said waveguide is circular, and

each of said iris means comprises a ring-shaped diaphragm having a central

19. A tunable resonator filter as recited in claim 16 wherein said means for selectively altering the value of one of the capacitances established by said plurality of iris means comprises:

a plurality of diode means coupling each iris means to a wall of said waveguide, and

means for applying bias to said diodes for altering the value of one of the

20. A turnable resonator filter as recited in claim 16 wherein said means for selectively altering the value of one of the capacitances established by said plurality of iris means comprises:

a plurality of bistable capacitance means coupling each iris to a waveguide wall, and

means for driving each plurality of bistable capacitance means to one or the other of its stable states.