GB2040103A - Slow wave coupling circuit - Google Patents

Abstract

A circuit for coupling a first electromagnetic circuit to a second electromagnetic circuit includes an inner slow wave structure (Fig. 6) and an outer slow wave structure (Fig. 3) coaxial with the inner structure. The outer structure is preferably a strapped bar line and the inner structure is preferably an interdigital line. The phase velocities for the two structures are proportional to the circumferences of the structures so that the rate of circulation of a wavefront is the same for a wave propagating about the inner structure and a wave propagating about the outer structure. The structures are terminated in the characteristic impedances to provide for wave propagation in one direction without reflections. Elliptical structures are described. The spacing between the two parts of the interdigital line may be adjustable to adjust the inner structure phase velocity correctly. The device is used to process a test signal which is compared with the output of a cross-field amplifier, the error signal being used as a measure of the ageing of the amplifier. <IMAGE>

Description

SPECIFICATION Slow wave coupling circuit The invention relates to a circuit for coupling a signal of electromagnetic energy between a first circuit and a second circuit and, more particularly, for simulating the effect of a slow wave device such as a crossed field amplifier upon a signal.

Devices employing slow wave structures are utilized in radar as well as communication systems for amplifying signals, such devices including crossed field amplifiers and travelling wave tubes. Such slow wave devices tend to modify the signal by introducing a phase shift and amplitude shift as a function of frequency. The foregoing modification of the signal varies with ageing of the crossed field amplifiers and travelling wave tubes.

To correct for the foregoing modification of the signal, it is desirable to have a reference channel wherein a reference signal is made available to the radar signal processing equipment. However, in the past, such reference signals have often been obtained by extracting a sample of the output signal from the slow wave device. However, the effect of ageing, associated with decreased electron emission from a cathode of a crossed field amplifier or travelling wave tube goes undetected in the reference signal in the foregoing example wherein the reference signal is obtained at the output port of the slow wave device. When the signal is obtained prior to the input port of the slow wave device, the reference signal does not show any effects of the foregoing modification of the signal.Thus, no data is available as to the effect of ageing of the slow wave device, and no corrective action can be taken by a radar signal processerto compensate for the ageing effect on the signal produced by the slow wave device.

According to the present invention there is provided a coupling circuit comprising inner and outer coaxial, cylindrical slow wave structures having phase velocities proportional to their circumferences.

The rate of circulation of a wavefront about the common axis of the two structures is the same for waves propagating about the inner structure and the outer structure.

This circuit can be used to simulate the coupling of electromagnetic energy which occurs in the slow wave structures of crossed field amplifiers and travelling wave tubes.

In the preferred arrangement, the inner structure has two ports, one port serving as an input port to be coupled to an input signal from the first electromagnetic circuit while the second port is coupled to a load matched to the characteristic impedance of the inner structure. The matched load ensures the propagation of a wave in one direction without reflections and the attendant presence of a standing wave or a resonance. Similarly, the outer structure is provided with two ports one of which is terminated in a matched load and the second of which serves as an output terminal for coupling an output signal to the second electromagnetic circuit. The matched load of the outer structure provides for the propagation of a wave in one direction without reflections and the attendant presence of a standing wave or a resonance.The slow waves of the inner and outer structures circulate about the axis in the same direction. Minimal coupling and maximum attenuation are attained between a signal at the input terminal and a signal at the output terminal under the foregoing conditions wherein no standing waves are present.

In a preferred embodiment of the invention, both slow wave structures have a circular cylindrical shape. The two structures are spaced apart without electrical contact to provide the desired attenuation and isolation. The inner slow wave structure is constructed in the form of an interdigital line while the outer slow wave structure has the form of a strapped bar line. Means are included for altering the relative positions of elements of the interdigital line to accomplish a change in capacitance of the line for adjusting the phase velocity of the slow wave. This permits a precise alignment of the phase velocity of the wave of the inner structure with the phase velocity of the wave on the outer structure. The spacing between the two structures is approximately equal to the width of a member of the interdigital line to give an attenuation of approximately 30 db.The diameter of the outer structure is typically one foot (30 centimetres) at a signal frequency of approximately 900 MHz.

The invention will be described in more detail, by way of example, with reference to the accompanying drawings, wherein: Figure 1 is a perspective view of a coupling circuit embodying the invention, Figure 2 is a perspective view of a housing of Figure 1, the housing.being partially cut away to show an outer slow wave structure comprising a pair of conductors which are periodically loaded by a set of bars, Figure 3 is an isometric view of the outer slow wave structure of Figure 2 with electrical connections from the input and output terminals thereof to coaxial cables being shown schematically, Figure 4 is a graph of frequency versus wave number for explaining the operation of wave propagation about the slow wave structures, Figure 5 shows a slow wave structure fabricated in the manner of that shown in Figure 3 but being modified to provide two input ports and two output ports, Figure 6 shows an inner assembly including an inner slow wave structure of the coupling circuit of Figure 1, the structure having the form of an interdigital line, -Figure 7 is an isometric view of the inner slow wave structure of Figure 6 with electrical connections from the input and output terminals thereof to coaxial cables being shown schematically.

Figure 8 shows an elevation view of an alternative embodiment of the inner slow wave structure of Figure 7, this embodiment providing for a variation in the gap size between portions of the interdigital structure to permit a variation in the speed of propagation of a wavefront around the slow wave structure; Figure 9 is a sectional view of the structure of Fig ure 8 taken along the lines 9-9 of Figure 8, Figure 9 also showing an electric motor for adjusting the gap size, the electrical connection to the motor and to a shaft angle encoder being shown schematically; Figure 10 is a plan view of the structure of Figure 8 taken along the lines 10-10 of Figure 8; Figure 11 shows a block diagram of a pass band controller shown connected to the coupling circuit of Figure 1, Figure 11 furthershowing circuitry for activating the motor of Figure 9;; Figure 12 shows the use of the coupling circuit of Figure 1 in a radar system orcommunication system for monitoring the output signal of a crossed field amplifier; Figure 13 shows the use of the coupling circuit of Figures 1 and 5 for coupling a signal to a plurality of utilization devices; and Figure 14 shows a diagrammatic view of a modification of outer slow wave structures have ellipsoidal cross sections.

Referring now of Figures 1-4, there is seen a coupling circuit 20 for the coupling of electromagnetic energy in accordance with the invention. The coupling circuit 20 comprises a cylindrical housing 22 having first and second ports 24 and 26 thereon for the coupling of electromagnetic energy to an outer slow wave structure 28 affixed to the interior of the housing 22. Referring also to Figure 6, upper and lower cover plates 30 and 32 secure an inner slow wave assembly 34 at the center of the housing 22. The upper cover plate 30 is provided with first and second ports 36 and 38 for coupling electromagnetic energy to an inner slow wave structure 40 of the assembly 34.Aterminal 42 is also provided in the upper cover plate 30 for use in an alternative embodiment of the circuit 20, as will be described hereinafter, for coupling electrical signals to control the size of a gap 44 between interdigital members 46 of the inner slow wave structure 40.

The outer slow wave structure 28 comprises upper and lower conductors 48 and 50 positioned circumferentially along the interior of the housing 22, a first end of the conductor 48 and of the conductor 50 serving as one terminal of the slow wave structure while a second end of the conductor 48 and of the conductor 50 serves as a second terminal of the slow wave structure 28. The first and second terminals of the structure are coupled respectively to the ports 24 and 26 as shown schematically in Figure 3. The conductors 48 and 50 provide a line along which a wave is guided, the line being loaded by transverse bars 52 of which opposite ends are alternatively mounted by pedestals 54 to the conductors 48 and 50, the bars 52 providing a periodic loading of the line to slow down the phase velocity of a wave travelling along the line.The upper and lower ends of each of the bars 52 are bent over and secured to upper and lower rims 56 and 58 of the housing 22. The width of each bar 52 is approximately equal to the spacing between the bars 52.

By way of example, in the construction of the coupling circuit 20 for electromagnetic waves having a frequency of approximately 900 megahertz, the diameter of the outer slow wave structure 28 is approximately one foot (30 centimeters), and each of the bars 52 has a length of approximately one-half wavelength between the pedestals 54. The outer slow wave structure 28 is fabricated of an electrically conducting material such as copper. Details in the construction of such strapped bar lines as well as of the interdigital inner slow wave structure 40 are described in a book entitled "Microwave Filters, Impedance-Matching Networks, and Coupling Structures" by G. L. Matthaei, L. Young and E. M. Jones published by McGraw-Hill Company in 1964. For example, an interdigital line is described on page 621 thereof.The operation of a slow wave structure may be explained by reference to the graph of Figure 4 wherein the vertical axis is in terms of radian frequency, cl), and the horizontal axis is is terms of wave nurnber, p, designating phase shift per pitch, the pitch being the distance between successive bars 52.

The graph shows that the wave propagating along the structure 28 is a backward wave. The phase shift between successive bars 52 is approximately 1500 at the operating frequency. In an embodiment of the invention which has been built, twenty-three bars 52 were utilized in constructing the coupling circuit 20.

Referring now to Figure 5, there is seen an alternative embodiment of the outer slow wave structure 28, this alternative embodiment being identified by the legend 28A. The structure 28A is bifurcated and is composed of strapped bar lines for coupling to two sets of ports. Each of the bar lines extends approximately halfway around the circumference of the inner surface of a housing such as the housing 22 to permit the coupling of electromagnetic energy from the inner slow wave structure 40 of Figure 6 to each section of a bifurcated slow wave structure rather than to a slow wave structure such as a structure 28 having but one pair of terminals. Additional ports (not shown) such as the ports 24 and 26 are to be provided on the housing 22 when the structure 28A of Figure 5 is utilized.

Referring now to Figure 6, the inner slow wave assembly 34 is seen to comprise the aforementioned cover plates 30 and 32, and the inner slow wave structure 40. The cover plates 30 and 32 are secured respectively to the top and bottom edges of the inner slow wave structure 40 by bolts 60 which pass through both of the plates 30 and 32 and are secured by nuts 62. Both the inner slow wave structure 40 and the outer slow wave structure 28 of Figure 1 have a cylindrical form with the outer diameter of the inner slow wave structure 40 being less than the inner diameter of the outer slow wave structure 28 to permit the placement of the structure 40 within the structure 28. The spacing between the two structures 40 and 28 is approximately equal to the width of a bar 52 of the outer slow wave structure 28. The plates 30 and 32 are provided with flanges 64 for nesting within the upper and lower rims 56 and 58 of the housing 22 of Figure 2.

Referring also to Figure 7, there is seen an isometric view of the inner slow wave structure 40, the structure 40 having a cylindrical format with the central portion of the structure 40 comprising an interdigital line cut into a cylindrical wall 68 to produce the aforementioned members 46 spaced apart by the gap 44. The graph of Figure 4 also applied to the structure 40, the structure 40 supporting a slow wave propagating in the backward mode. A slot 70 defines first and second ends of the interdigital line which are coupled respectively to the coaxial lines of the first port 36 and the second port 38. The operating point on the graph of Figure 4 provides for a phase shift of approximately 1500 between succeeding members 46 of the interdigital line.The structure 40 as well as the upper and lower plates 30 and 32 are fabricated of an electrically conducting material such as copper. As can be seen in Figure 7, the perforation of the gap 44 progresses completely around the wall 68 to the slot 70 thereby dividing the wail 68 into the upper and lower sections. The upper section is secured to the upper cover plate 30 and the lower section secured to the lower cover plate 32 by a press fit along a mating surface on the respective plates 30 and 32. The bolts 60, by securing the plates 30 and 32 in intermediate contact with the housing 22, provide for maintaining the desired width of the gap 44. The speed of propagation of the slow wave about the interdigital line is dependent of the capacitance between the terminal of the members 46 and the wall 68.

Referring now to Figures 8,9 and 10, there is shown an alternative embodiment of the inner slow wave assembly, this embodiment being identified by the legend 34A. The cover plates 30 and 32 have been deleted in Figures 9 and 10 to simplify the drawings. A wall 68A is provided with a sleeve 72 having an overlapping edge 74 which slidably nests within an edge 76 of the lower portion of the wall 68A. A channel 78 circumscribing the wall 68A permits a vertical displacement of the lower portion of the wall 68A while the upper portion of the wall 68A remains rigidly secured to the upper cover plate 30 and the sleeve 72 remains rigidly secured to the lower cover plate 32.By vertically displacing the lower portion of the wall 68A, the members 46 of the lower portion of the wall 68A move past the members 46 of the upper section of the wall 68A thereby altering the width of the gap 44. Since the capacitance between the termini of the members 46 and the wall 68A are dependent of the width of the gap 44, the sliding of the lower portion of the wall 68A along the sleeve 72 permits adjustment of the capacitance and thereby a selection of the speed of propagation of a wavefront of the slow wave about the inner slow wave structure 40. Such adjustment is useful for equalizing the rate of circulation of a wavefront about the inner slow wave structure with the rate of circulation of a wavefront about the outer slow wave structure 28.

A mechanical supporting structure, to be referred to hereinafter as a spider 80, positions the upper portion of the wall 68A, the lower portion of the wall 68A and the sleeve 72 relative to each other. The spider 80 comprises a cylindrical wall 82 with a floor 84 at the bottom end thereof, the wall 82 having legs 86 extending outwardly therefrom contacting the upper section of the wall 68A and the sleeve 72. The wall 82 is also provided with slots 88 through which legs 90 are directed from a central block 92 to the lower portion of the wall 68A. The block 92 is slidably positioned along a rail 94 by a worm drive 96 rotated by a motor 98 with the amount of angular rotation of the drive 96 being monitored by a shaft angle encoder 100. The rail 94 is secured by a boss 102 to the floor 84.By way of example, the motor 98 may be a stepping motor with electrical drive pulses being provided thereto along the electric wires shown schematically as line 104 for driving the motor 98 clockwise or counterclockwise. Electric signals designating the angle of rotation of the drive 96 are transmitted by the encoder 100 via line 106.

Rotation of the worm drive 96 advances the position of the block 92 and thereby alters the position of the lower portion of the wall 68A relative to the upper portion of the wall 68A and to adjust the gap 44 of the interdigital line. Accordingly, the signal on line 106 also represents the width of the gap 44.

Returning to Figure 1, the coupling circuit 20 is operated as follows. A source of signal, such as a signal generator 108 is coupled to the first port 36 while a load 110 which is matched to the characteristic impedance of the inner slow wave structure 40 is coupled to the second port 38 on the upper cover plate 30. A second load 112 which is matched to the characteristic impedance of the outer slow wave structure 28 is coupled to the first port 24 on the housing 22 while the second port 26 is coupled to a utilization device 114. With reference also to Figure 2 and 6, the signal from the generator 108 is coupled via the port 36 to the first end of the interdigital line, and proceeds to travel as a slow wave around the inner slow wave structure 40. Upon reaching the second end of the interdigital line, the signal is coupled via the second port 38 to the matched load 110.

Absorption ofthe signal in the matched load 110 insures that there are substantially no reflections of a wave from the second end of the interdigital line. As a result, the strength of the electric field is maintained at substantially lower values along the inner slow wave structure 40 than would be the case if the interdigital line were notterminated by the matched load in which case the standing wave due to reflections at the ends of the line would appear. Such standing waves have substantially increased values of electric field strength.Due to the relatively small amount of electric field strength obtained by terminating the line in its characteristic impedance, the inner and outer structures 40 and 28 are partially isolated electrically from each other so that the coupling is consistently maintained at a steady value independently of the utilization device. In a preferred embodiment of the invention, one-thousandths of the input power, -30 dB, is coupled to the utilization device.

In a similar manner, a wave induced on the outer slow wave structure 28 by the coupling from the inner slow wave structure propagates around the outer slow wave structure 28 without reflection at the end thereof due to the presence of a matched load 112. The wave on the inner structure 40 and the wave on the outer structure 28 propagate in the same direction about the common axis of the two structures 40 and 28. Preferably, the generator 108 and the utilization device 114 each have an impedance equal to the characteristic impedance of their respective slow structures 40 and 28 to ensure min imal reflections of waves at the terminals of the respective slow wave structures.

In the event that the alternative embodiment of the inner slow wave assembly 34A of Figures 8-10 is utilized, a pass band controller 116, as will be described hereinafter with reference to Figure 11, is coupled via the terminal 42 of Figures 1,6 and 9 and the lines 104 and 106 to the motor 98 and the encoder 100 for adjusting the gap 44 of the inner slow wave structure 40. The coupling circuit 20 is responsive to the frequency of the signal provided by the generator 108 and provides a maximum amplitude of output signal to the utilization device 114 at that frequency. The bandwidth of the coupling circuit 20 is dependent on the bandwidth of the inner slow wave structure 40 and the bandwidth of the outer slow wave structure 28, these bandwidths being dependent on design criteria such as that of the frequency versus wave number relationship of Figure 4.Such bandwidth criteria are also described in the aforementioned book by Matthaei et al. A variation in the size of the gap 44 of the inner slow wave structure 40 alters the propagation speed of a wave around the structure 40 and also alters the pass band thereof. The pass band controller 116, by controlling the size of the gap 44, as will be described subse quentlywith reference to Figure 11,thereby controls the pass band of the coupling circuit 20.

Referring now to Figure 11, there is seen a block diagram of the pass band controller 116 connected to a coupling circuit 20A, the legend 20A indicating the coupling circuit is employing the inner slow wave assembly 34A of Figures 8-10. The controller 116 is seen to comprise an encoder 130 having a knob 132 thereon for manually setting a desired size of the gap 44, a memory 134 a clock pulse generator 138, a subtractor 140, and a gating circuit 142.

The controller 116 operates as follows. The desired speed of the propagation around the inner slow wave structure 40 is selected by the encoder 132, the knob 130 attached thereto permitting manual selection of the propagation speed. For each value of speed, there is a corresponding magnitude of the gap 44 as is known from the design of interdigital lines as is explained in the aforementioned book of Matthaie. The corresponding values of magnitude of the gap 44 are stored in the memory 134 and are addressed by signals from the encoder 130 representing the desired speed. The actual gap width as communicated via line 106 is compared with the required gap width of the memory 134 in the substractor 140. The substractor 140 forms the difference between the required gap width and the actual gap width, the difference appearing on line 148.The signal on line 148 includes a sign bit which indicates whether the actual gap width is larger or smaller than the required gap width. In response to the sign bit on line 148, the gating circuit 142 couples clock pulses from the generator 138 to one of the electric wires represented by the line 104 for driving the stepping motor 98 of Figure 9 in a clockwise or counterclockwise direction for enlarging or decreasing the width of the gap 44. When the signal on line 148 is of zero value, no clock pulses are transmitted by the gating circuit 142. Thereby, the actual gap width is made equal to the required width to produce t selected speed of the slow wave about the inner slow wave structure 40.

Referring now to Figures 12 and 13 there are seen systems demonstrating the use of the coupling circuits 20 and 20A. Assuming the more general case wherein it may be desirable to select a specific pass band of the coupling circuit, the coupling circuit 20A is shown. Figure 12 shows a transmitter 160 for use in a radar or communication system. The transmitter 160 comprises the aforementioned signal generator 108, the coupling circuit 20A with the matched load 110 and 112 connected thereto, and the controller 116. The transmitter 160 further comprises a crossed field amplifier 162 of conventional design and having a power gain of 10 dB, a 40 dB coupler 164, an antenna 166, a subtractor 168, a recorder 170, a correlator 172 and an alarm 174.

The coupler 164 provides a sample of the output signal of the amplifier 162 to the subtractor 168. In view of the 30 dB attenuation of the coupling circuit 20A and the 10 dB amplification of the amplifier 162, the 40 dB coupling of the coupler 64 produces a reference signal having an amplitude substantially equal to that at the output of the coupling circuit 20A.

In operation, therefore, a signal produced by the generator 108 is amplified by the amplifier 162 and coupled via the coupler 164 to the antenna 166 for radiation therefrom. The pass band of the coupling circuit 20A is made equal to the pass band of the amplifier 162 by the controller 116. The gain of the amplifier 162 is monitored as a function of time by the recorder 170 which provides a record of the difference signal produced by the subtractor 168, the difference signal being the difference between the signals of the amplifier 162 and the circuit 20A. Thus, with aging of the amplifier 162, an effect of such aging being typically a loss of emissivity of electrons from the cathode thereof, the recorder 170 shows this loss of gain. Thereby, the time for replacement ofthe amplifier 162 can be readily observed.In addition, and by way of example in the use of the circuit 20A, the two input signals applied to the subtractor 168 may also be applied to a correlator 172. In the event that the frequency response of the amplifier 162 varies relative to the fixed response of the circuit 20A, signal pulses by the generator 108 may experience distortion which results in a loss of correlation between the two signals applied to the correlator 172. The alarm 174, being responsive to the magnitude of the correlation, sounds an alarm when the correlation falls off excessively, this indicating excessive distortion in the amplifier 162.

With reference to Figure 13, the coupling circuit 20A is understood to incorporate the bifurcated outer slow wave structure 28 of Figure 5 and, accordingly, has been further identified in Figure 13 by the legend 20A'. In addition to the utilization device 114, a second utilization device 176 is employed. The utilization device 114 may be coupled to an end of one section of the slow wave structure 28A via a port such as the port 26 of Figure 1 while the utilization device 176 is coupled via a similar port (not shown in Figure 1) to the corresponding end of the other section of structure 28A. Similarly, matched loads 1 12A and 112B, analogous to the matched load 112 of Figure 1 are coupled respectively to the remaining ends of the two sections of the structure 28A.In this way, a signal produced by the generator 108 is coupled to a plurality of utilization devices via the circuit 20A' while the controller 116 selects the pass band of the circuit 20A'.

Referring nowto Figure 14, there is shown schematically a plan view of a modification of the coupling circuit 20 of Figure 1 wherein the outer slow wave structure 28, a strapped bar line, and the inner slow wave structure 40, an interdigital line, are provided with elliptical cylindrical forms rather than the circular cylindrical forms of the circuit 20 of Figure 1.

In the embodiment of Figure 14, there are two cylindrical axes for each elliptical cylinder corresponding to the foci of each ellipse. All four axes are parallel.

The two sets of foci provide for a uniform spacing between the two ellipses for coupling electromagnetic energy from one slow wave structure to the other slow wave structure.

Claims (10)

1. A coupling circuit comprising inner and outer coaxial, cylindrical slow wave structures having phase velocities proportional to their circumferences.

2. A coupling circuit according to Claim 1, wherein the outer slow wave structure has the form of a strapped bar line.

3. A coupling circuit according to Claim 1 or 2, wherein the inner slow wave structure has the form of an interdigital line.

4. A coupling circuit according to Claim 3, further comprising means coupled to the inner slow wave structure for varying the spacing between members of the interdigital line for adjusting the speed of propagation of a wavefront about the inner slow wave structure.

5. Coupling circuit according to any of Claims 1 to 4, wherein the outer slow wave structure is bifurcated to provide a plurality of electrically isolated input ports and a plurality of electrically isolated output ports, electromagnet energy being coupled from the inner slow wave structure to individual sections of said outer slow wave structure.

6. A circuit according to any of Claims 1 to 5, wherein each of the slow wave structures is terms nated in its characteristic impedance to inhibit the formation of standing waves.

7. A coupling circuit comprising a first slow wave structure mounted adjacent a second slow wave structure for coupling electromagnetic energy from one of the structures to the other, means for applying energy to one of the structures and means for extracting energy from the other of the structures, each of the structures being terminated in its characteristic impedance, and wherein each structure is configured to provide a speed of propagation of a wavefront of the electromagnetic energy in step with a wavefront of radiant energy propagating around the other structure.

8. A circuit according to Claim 7, wherein a second and a third slow wave structure are positioned adjacent a first slow wave structure.

9. A circuit according to Claim 7 or 8, wherein one structure is in the form of an interdigital line and another structure is in the form of a strapped bar line.

10. A coupling circuit substantially as hereinbefore described with reference to and as illustrated in Figs. 1,2,3,6 and 7, or these Figures as modified by Fig. 5 or Figs. 8,9 and 10 of the accompanying drawings.