US 3146413 A
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Description (OCR text may contain errors)
J. L. BUTLER Aug. 25, 1964 PHASE SHIFTER 3 Sheets-Sheet 1 Filed Aug. 29, 1960 I PRIOR ART 3 b 320 2500 v 30d 40 2c Ou Jesse L.Bufler Fig.3
/N VE N TOR W ATTORNEY Aug. 25, 1964 J. BUTLER 3,146,413
. I PHASE SHIFTER Filed Aug.'29, 1960 I s Sheets-Sheet s Fig.6
' Jesse L.Bufler INVENTOR ATTORNEY United States Patent C) 3,145,413 PHASE SHIFTER Jesse L. Butler, Nashua, N.H., assignor to Sanders Associates, Inc, Nashua, N.H., a corporation of Delaware Filed Aug. 29, 196i Ser. No. 52,459 12 Claims. (Cl. 33331) This invention relates to high frequency transmission devices, generally called phase shifters, useful for varying the effective length of a transmission line. More specifically, it relates to a phase shifting device comprising a pair of directional couplers connected in tandem along a pair of transmission lines. By means of the couplers, energy entering on one line is transferred to the other line, from which it leaves the device. The effective length of the device is varied by changing the positions along the transmission lines where the coupling action takes place. The invention is particularly adaptable to construction in strip transmission line form.
Phase shifters are used to vary the time required for a signal to travel between two points in a transmission system. In other words, they effectively vary the length of the transmission path joining the two points. At high frequencies, one method of achieving this result is, of course, to change the length of the transmission path by splicing in or removing appropriate lengths of transmission line. This process involves considerable work and is particularly impractical where frequent adjustments of relative phase are desired. Another device commonly referred to as a line stretcher, has overlapping conductors capable of translation with respect to each other in a telescoping action. Unreliable electrical contact between the sliding contacts is an inherent problem in this structure. Another method of varying the relative phase of a signal is to change the velocity of propagation in the transmission line, for example, by inserting a dielectric material into the interconductor space. The phase delay for a material having a given dielectric constant is proportional to the thickness and length of the dielectric material. The I incremental phase delay due to the presence of the dielectric material, however, is generally only a small fraction of the free space delay; thus, a long transmission path and a large amount of dielectric material are required to obtain a delay variable over a wide range. Hence, the amount of incremental phase delay is substantially limited by the bulk and weight of a practical device.
Accordingly, it is an object of this invention to provide an improved transmission line phase shifitng device, i.e., a device capable of varying the effective length of a transmission path.
A further object of the inventon is to provide a compact phase shifter capable of imparting a phase delay adjustable over a wide range.
Another object is a phase shifter in which the transmission line properties, such as characteristic impedance, are substantially independent of the phase adjustment.
Still another object of the invention is to provide a phase shifting device which is free of the contact problems inherent in prior devices having sliding contacts.
A further object of the invention is to provide a phase shifter of relatively simple construction.
Other objects of the invention will in part be obvious and will in part appear hereinafter.
The invention accordingly comprises the features of construction, combinations of elements, and arrangements of parts which will be exemplified in the constructions hereinafter set forth, and the scope of the invention will be indicated in the claims.
In general, the invention consists of a pair of transmission lines field coupled together by a pair of directional couplers connected in tandem along the lines. The cou- 3,l45,4l3 Patented Aug. 25, 1964 plers are preferably parallel line couplers, and, therefore, the two transmission lines may be moved parallel to each other to vary the effective length of the unit without affecting energy transfer between them. As described below, the couplers are so arranged that energy entering the unit on one line is coupled in the same direction on the other line. Furthermore, the distances along the lines between the couplers are preferably equal. With this arrangement, all the energy entering the unit on one line is transferred to the other line, and thus the unit behaves as a single transmission line having a variable electrical length.
A phase shifter of this type may be easily fabricated with simple adaptations of strip transmission line. In this form, it provides a maximum range of phase variation with minimum size and weight.
For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings, in which:
FIGURE 1 is a transverse section of a strip transmission line, showing the configuration of the electric and magnetic fields between the conductors,
FIGURE 2 is a schematic representation of a phase shifter embodying the principles of my invention,
FIGURE 3 is a plan view, partly in section, of a phase shifter constructed in strip line form, showing in detail the arrangement of the inner conductors,
FIGURE 4 is a longitudinal vertical section of the phase shifter of FIGURE 3,
FIGURE 5 is a schematic representation of another embodiment of the invention,
FIGURE 6 is a longitudinal vertical section of a strip line construction of the phase shifter of FIGURE 5, and
FIGURE 7 is a schematic representation of the phase shifter of FIGURE 2, modified to include impedance compensation.
Some dimensions in the drawings are shown out of proportion for the purpose of clarity.
In FIGURE 1 there is illustrated the field distribution in a typical strip transmission line. The line has an inner conductor 19 situated between and parallel to a pair of outer ground plane conductors 12 and 14. The conductors ll), 12, and 14 are flat and may be quite thin. For example, they may be formed of foil bonded to dielectric material (not shown) filling the space between them. At an instant of time when the conductor It is positive with respect to the ground planes 12 and I4 and the current in the conductor 10 is in the direction of the arrow 15, the field distribution in the transmission line is as shown in FIGURE 1, with the solid arrows representing the electric field E and the dash lines representing the magnetic field H.
The field configuration of FIGURE 1 is indicative of the TEM propagation mode. However, it is possible to transmit other modes on the line under certain conditions. For example, if the inner conductor It is offset from its nominal position midway between the ground planes 12 and 14, the ground planes will be at somewhat different potentials. This difference in voltage will support a parallel plate mode. Accordingly, the ground planes are shorted together by a plurality of pins 16 spaced along both edges of the inner conductor. The pins impose an equipotential condition on the planes and thereby suppress this mode. For effective suppression, the spacing of the pins in the lengthwise direction of the line should be less than a half wavelength. Ordinarily, this spacing is on the order of one eighth wavelength or less.
Another limitation on pin spacing results from the desirability of avoiding a resonant condition in any loop defined by the ground planes and a pair of adjacent pins.
ameale 3 A resonant loop will distort the transmission characteristics of the line as Well as facilitate radiation of energy therefrom. Resonance occurs when the length of the loop is an integral number of wavelengths, and, accordingly, the distance between adjacent pins should be considerably less than the spacing providing a wavelength loop.
If either of the transverse dimensions, i.e., ground plane to ground plane or pin to pin spacing is greater than a half wavelength, a transverse electrical Waveguide mode may be excited. Therefore, both these dimensions should be less than a half wavelength. There is also a restriction on the length of the circumferential path around the inner conductor and passing midway between the inner conductor and the ground planes 12 and 14 and pins 16. This path should be less than a wave length. Otherwise, the line will support a higher order transverse electric transmission line mode.
Referring to FIGURE 2, a phase shifting device on"- bodying the present invention comprises a pair of strip transmission lines whose inner conductors are generally indicated at 30 and 32. The conductors 3t) and 32 are disposed between a pair of ground plane conductors not shown in this figure. Although the device is reciprocal, for convenience, an input terminal is indicated at 34 and an output terminal at 36.
Inner conductor 30 consists of an input section 30a and coupling sections 30b and 380 separated by an intermediate section 3d. The conductor 32 is similarly arranged, having an output section 32a and coupling sections 32b and 320 separated by an intermediate section 32d. Coupling sections 30b and 320 form a parallel line directional coupler, generally designated at 38, which will be described below. Coupling sections 3th and 32b form a similar coupler generally indicated at 40. In a manner described below, coupling sections 32c and 3217 are moved longitudinally along inner conductor 30 to vary the length of the transmission path between the terminals 34 and 36.
In strip line form, a parallel line directional coupler comprises the inner conductors of two transmission lines disposed parallel and in close proximity to each other, preferably for a length equivalent to a quarter wavelength at the operating frequency. Coupling between the inner conductors is achieved by the electric and the magnetic fields developed by an input current in one of the inner conductors. In effect, part of the input energy is transferred to the other conductor and travels in the opposite direction thereon; the remainder of the input energy is transmitted through the coupler on the first conductor. The proportion of the input current coupled to the second conductor increases as the space between the two conductors is decreased. Of particular interest in the present invention is a 3 db directional coupler, in which one-half of the input current is coupled to the second conductor. When the length of the coupler is a quarter wavelength, the signal appearing at the output of the coupler on the input conductor is delayed in time with respect to the signal on the coupled conductor by a quarter wavelength, i.e., a phase lag of 90 degrees.
To illustrate the operation of the phase shifting device of FIGURE 2, there is included a generator 42 connected to the input terminal 34 and a load 44 connected to the output terminal 36. Energy from the generator 42 is in part coupled to the transmission line comprising the inner conductor 32 by coupler 38. The remaining energy on the conductor 30 travels to coupling section 30c Where part of it is coupled to conductor 32 and delivered to terminal 36 and load 44. Part of the energy originally coupled into conductor 32 at 32c is transferred to conductor 30 at coupling section 32b, and the remaining energy is transmitted through the coupler 49 to the output terminal 35.
Preferably, the directional couplers 38 and 4%) are 3 db couplers. Then, assuming that the lengths of the intermediate sections 30d and 32d are equal, or differ by m, where n is any integer and A is the wavelength at the operating frequency, the energy delivered to terminal 36 from conductor 32 will be equal in amplitude and phase to the energy delivered to this terminal from conductor 30 by way of the coupler 40. On the other hand, the energy arriving at the remote end Site of the conductor 30 by transmission along this conductor sufiers a degree phase delay in each of the couplers 38 and 40 compared to energy first coupled to the conductor 32 in the coupler 3S and transferred back to the conductor 30 in the coupler 40. Thus, the energy arriving at Site along the conductor 36 is degrees out of phase and equal in amplitude to the energy arriving there from the conductor 32. Accordingly, the two signals cancel each other and no resultant energy is delivered to the end 3%. It will be apparent that no energy will leave the coupler 40 by way of the intermediate sections 30d and 32d, and, therefore, substantially all the input energy is transmitted to the output terminal 36. It will be noted that if the lengths of the intermediate sections 36d and 32d are equal, i.e., 11:0, the above-described operation is largely independent of frequency.
When the characteristics of the preferred embodiment, as described above, are not fulfilled, the energy components delivered to the remote end Site of the conductor 34) do not completely cancel. Accordingly, a matched termination 46 may be provided at we to absorb the excess energy and thus prevent undesirable reflections. A matched termination 48 may also be provided at the otherwise unterminated end 326 of coupling section 320 to absorb energy reflected along inner conductor 32 and to absorb energy which under some conditions may be coupled in that direction by the couplers 38 and 40.
When conductor 32 is longitudinally displaced to the position indicated by the dashed lines of FIGURE 2, the input section 3% and output section 32a are each lengthened by the amount of the displacement. The distance between the couplers 38 and 40 along each of the conductors 3t and 32 is reduced by the amount of the displacement. Hence, the total length of the unit is increased by the amount of the displacement. In other Words, if the conductor 32 is displaced a distance x in the direction shown from the solid line to the dashed line, the sections 30a and 32a, which are in series, are then lengthened by x, for a total of 2x. The intermediate sections 30d and 32d, which are essentially in parallel, each decrease by x. Hence, the net change in the effective length of the device is: 2xx=x. It will be apparent that a shift of the conductor 32 in the opposite direction will serve to shorten the electrical length of the unit.
The changes imparted in each of the conductors 3t} and 32 when the transmission length is varied are symmetrical, and the line lengths along each conductor remain equal. Accordingly, since adjustment of the phase shifter does not introduce any line sections of un equal length, which inherently are frequency sensitive, the phase shifter may have a wide frequency range.
A phase shifter incorporating the above arrangement of inner conductors is physically embodied in the construction illustrated in FIGURES 3 and 4. As shown in these figures, the inner conductors 30 and 32 are bonded to dielectrics 5t and 52, respectively, and are disposed between a pair of ground plane conductors formed by the inner surfaces 54a and 56a of a pair of housing members 54 and 56, to which the dielectrics 5t) and 52 are attached. The members 54- and as are in sliding contact along surfaces 54b and 56b, the surfaces being sufliciently large to assure reliable electrical contact between the housing members. The ground plane conductors are thus effectively shorted together and, as discussed with regard to the pins 16 of FTGURE l, undesirable modes are suppressed. The area between the ground plane conductors may be reduced to further suppress undesirable modes; for example, housing member 54 is shown gen.-
(E erally conforming to the shape of the intermediate section 30d. Member 56 is formed to conform similarly to section 320..
A knob 58, attached to the housing member 56, is provided to facilitate longitudinal displacement of the coupling sections 32b and 32c along the conductor 30. The conductors 30 and 32 are shown in register with each other in the couplers 38 and 40. However, it will be noted that the exact positions of the conductors required for 3 db coupling may vary, depending on such factors as ground plane to ground plane spacing, width of the inner conductors, etc.
The terminals of the transmission lines are shown in FIGURE 4 as coaxial connectors generally indicated at 60 and 62. The connector 60 consists of an inner conductor 60a coaxially disposed within an outer conductor 60]). Outer conductor 60b is connected directly to the housing member 54, and inner conductor 60a extends through the member 54 to connect to the inner conductor 30 of the phase shifter. The conductors 62a. and 62b of the connector 62 are similarly connected to the inner conductor 32 and housing member 56.
Matched terminations, not shown in FIGURES 3 and 4, may be connected at the remote ends 30e and 32a of conductors 30 and 32, respectively.
When the housing member 56 is moved right or left along the member 54, there is longitudinal movement of the conductor 32 along the conductor 30. This increases or decreases the electrical length of the phase shifter in the manner described above. In other words, it alters the phase delay imparted to a signal transmitted through the device. The maximum variation in transit time available with this phase shifter is determined by the lengths of the conductor sections along which the coupling sections 30c and 320 can be moved.
A schematic representation of a folded phase shifter is shown in FIGURE 5. It consists of two phase shifters of the type shown in FIGURE 2 connected in series. A pair of strip transmission line inner conductors, generally indicated at '72 and 74, form a phase shifter generally indicated at 75, and inner conductors 76 and 78 constitute a second phase shifter generally indicated at 77. A conductor 80 connects the conductors 74 and 76; other than that there is no coupling or connection between them. The conductors 74, 76 and 80 are longitudinally movable (left or right) as a unit.
When conductors 74 and 76 are displaced to the right, away from the terminals 82 and 84 connected to the conductors 72 and 78, the length of each phase shifter 75 and 77 increases by the amount of the displacement. Thus, the total change in electrical length is twice the displacement of the conductors. Another advantage of the folded phase shifter is that both the terminals 82 and 84 may remain stationary during adjustment of the device.
A vertical cross section of the folded phase shifter is shown in FIGURE 6. The bottom half of this construction is substantially identical to the structure shown in FIGURES 3 and 4. The construction of the top half is a mirror image of the bottom half.
More specifically, the conductors 72 and 74 are bonded to dielectrics 86 and 88, respectively, and are spaced between ground plane conductors formed by the inner surfaces 90a and 92a of housing members 90 and 92. The conductors 76 and 78 are similarly afiixed to dielectrics 94 and 96 secured to ground plane surfaces 92b and 98a of housing members 92 and 98. The conductor 80, joining the conductors 74 and 76, passes through and is insulated from member 92.
Housing member 92 can be slidably moved by a knob 99 along the inner surfaces 90b and 98b of housing members 90 and 98, with electrical contact maintained between these surfaces and surfaces 92c and 92d of the housing member 92.
It is apparent that by stacking additional phase shifters 6 in the above manner an even greater variation in phase delay can be obtained for a given mechanical displacement.
To maintain a substantially constant characteristic impedance in the phase shifter for all values of phase delay, the ratio of inner conductor inductance to inner conductor to ground plane conductor capacitance should remain reasonably constant. In each directional coupler, two inner conductors are in close proximity to each other. The presence of each inner conductor increases the capacitance between the other inner conductor and the ground plane conductors, and thus decreases the characteristic impedance of each transmission line section involved in the coupler. Furthermore, movement of the couplers along the transmission lines during adjustment of the phase shifter moves the positions where the low impedance appears. The low impedance that results from interconductor coupling can be substantially compensated by reducing the width of each inner conductor in the directional couplers. As shown below, variation in impedance caused by movement of the coupler can be substantially eliminated by providing compensating conductors that move in synchronism with the coupling sections.
More specifically, as shown in FIGURE 7, an input section 30a and an output section 32a are oriented perpendicular to coupling sections 30b and 32b to isolate or space the former sections from variations in capacitance caused by movement of inner conductors 32. The widths of the inner conductors of coupling sections 30b, 30c, 32c and 32]) are reduced to compensate for capacitance coupling between the coupling sections forming the directional couplers.
Two compensating conductors 102 and 103, and 104 and are mechanically connected to each of the coupling sections 32c and 30c, respectively. They are bonded to the same dielectrics as the conductors 30 and 32 (FIGURE 4)conductors 102 and 103 to dielectric 52 and conductors 104 and 105 to dielectric 50. They are isolated from the coupling sections to which they are attached by gaps 106, 107, 108 and 109.
The widths of the compensating conductors 102 and 103 are such as to have substantially the same effect on the capacitance per unit length of the section 30b as the section 320 has on it. The widths of the compensating conductors and the section 320 combine with the reduced width of the section 3% to provide a capacitance for the latter section equal to that of the other sections of the conductor 30. The same effect is provided in the section 320' by its width and the proximity of the section 30!). The conductors 104 and 105 and coupling sections 32b and 30c have widths giving the same results in the coupler 40.
The conductors 102 and 103 are sufiiciently long to extend beyond the section 3017 in both directions over the entire range of movement of conductor 32'. Thus, assuming that the gaps 108 and 109 are made sufficiently narrow to have a negligible effect on the capacitance of the coupling section 3011, the capacitance between this section and the surrounding ground plane conductors is the same regardless of the setting of the phase shifter. The compensating sections 104 and 105 extend similarly beyond coupling section 3211, and, therefore, a substantially uniform capacitance is also maintained for the section 3212.
Thus, I have described an improved high frequency phase shifting device in which directional couplers are used to vary the effective length of a transmission path. With 3 db directional couplers connected in the manner disclosed above, the phase shifter has negligible loss and thus substantially all of the input energy is delivered to the output. With directional couplers having some other appropriate coupling ratio, some other predetermined portion of the input energy can be coupled to the output. By using couplers in which there is no direct contact between the coupled circuits, the disadvantages of sliding contacts in prior phase shifting devices are avoided.
The phase shifter may be provided with compensating conductors Which maintain a uniform characteristic impedance as the electrical length of the unit is varied. Accordingly, my invention can be used in transmission circuits having a very small standing wave ratio.
Directional couplers other than the parallel line type may be used without departing from the scope of my invention. For example, quarter-wave stub couplers, each consisting of two stubs placed a quarter wavelength apart, along one transmission line and capacitively coupled to another line, may be used.
It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efiiciently attained and, since certain changes may be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.
It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.
What is claimed is:
1. A transmission device of variable electrical length comprising, in combination,
(a) a pair of transmission lines,
(12) a pair of directional couplers, each directional coupler including a coupling section of each transmission line,
() said directional couplers being connected in overlapping relationship along the transmission lines so that energy entering said device on one of said transmission lines is coupled in the same direction on the other of said lines by both of the directional couplers,
(d) said coupling sections of each transmission line having an intermediate section of transmission line separating said sections, and
(e) the lengths of said intermediate sections differing by substantially m, where n is any integer including zero and A is the wavelength of the operating frequency.
2. The combination defined in claim 1 in which each of said directional couplers has a 3 db coupling ratio.
3. The combination defined in claim 1 in which the directional couplers are of the parallel line type.
4. The combination defined in claim 1 in which said intermediate sections have the same length.
5. The combination defined in claim 1 including compensating conductors associated with each directional coupler for maintaining constant the characteristic impedance of said transmission device when the electrical length thereof is varied.
6. A transmission device for varying the time required for a high frequency signal to be propagated through said device comprising, in combination, first and second transmission lines, first and second coupling sections in each of said transmission lines separated by an intermediate section, the difference between the lengths of said intermediate sections being nk, where n is any integer including zero and A is the wavelength at the operating frequency, first and second 3 db parallel-line directional couplers, each directional coupler including a coupling section of each of the transmission lines, said directional couplers having a 90 degree phase difference between their coupled and transmitted output signals, said directional couplers being so arranged that with energy input on said first line the first coupling section of said first line couples energy into the first coupling section of said second transmission line in the direction of the second coupling section of said second line, the second coupling section of said first transmission line couples energy in the same direction into the second coupling section of said second transmission line and said second coupling section of said second transmission line couples energy mto said second coupling section of said first transmission line in the direction opposite from the first coupling section of said first line; and means for moving the coupling sections of one of said transmission lines longitudinally with respect to the coupling sections of the other transmission line.
7. The combination defined in claim 6 including compensating conductors associated with one of the coupling sections of each of said directional couplers, said compensating conductors being arranged to compensate for variations in capacitance of the conductors of each of said transmission lines caused by movement of said coupling sections.
8. A transmission device of variable electrical length comprising, in combination,
(a) first and second transmission lines,
(In) each of said lines comprising first and second coupling sections separated by an intermediate section, said intermediate sections having equal electrical length,
(0) first and second parallel line directional couplers,
(d) each of said couplers including one coupling section of each of the transmission lines,
(e) said directional couplers arranged to couple high frequency energy from one of said transmission lines, in the same direction on the other of said lines,
(1) means for moving the coupling sections of one transmission line longitudinally with respect to the coupling sections of the other transmission line,
g) first and second compensating conductors disposed generally parallel to but isolated from said coupling sections in each of said directional couplers, and
(11) said compensating conductors being arranged to maintain the capacitance of each of said coupling sections uniform along the entire length of each of the coupling sections.
9. The combination defined in claim 8 in which each of said directional couplers has a 3 db coupling ratio.
10. The combination defined in claim 8 in which each of said transmission lines consists of a fiat inner conductor disposed between two ground plane conductors and insulated from said two ground plane conductors.
11. A transmission line phase shifter comprising, in combination, first and second transmission lines each consisting of a flat strip-like inner conductor disposed between the same two ground plane conductors, first and second coupling sections in each of said inner conductors separated by an intermediate section, first and second 3 db parallel line directional couplers, each of said directional couplers including one coupling section of each of said inner conductors, said directional couplers being connected to couple energy from the first transmission line in the same direction to the second transmission line, the electrical length of the intermediate sections differing by approximately m where n is any integer including zero and is the wavelength of the operating frequency, a translation mechanism for moving the coupling sections of one inner conductor with respect to the other inner conductor, and first and second pairs of compensating connectors, said first pair connected to move in synchronism with but spaced from one of the coupling sections in the first directional coupler, said first pair of compensating conductors disposed with respect to each of the coupling sections of said first coupler to maintain the reactance from each coupling section thereof to the ground plane conductors uniform along the entire length of each of said coupling sections, said second pair of compensating conductors similarly connected, to isolated from, and disposed with, the coupling sections of said second directional coupler.
12. The combination defined in claim 11 in which each of the inner conductors and the pair of compensating conductors connected with it are bonded to a flat dielectric member.
References Cited in the file of this patent UNITED STATES PATENTS Alford May 23, 1939 Bliss Sept. 23, 1952 Wolf Oct. 26, 1954 Seidel Oct. 18, 1955 Kostriza June 5, 1956 Englemann June 5, 1956 Tomiyasu et a1 June 19, 1956 Grieg June 19, 1956 10 Arditi Sept. 30, 1958 Robertson Oct. 6, 1959 Cohn Jan. 19, 1960 Cutler Feb. 16, 1960 Arditi Aug. 30, 1960 Englemann Nov. 29, 1960 Jacques June 27, 1961 Nigg Dec. 5, 1961 OTHER REFERENCES Strip-Line 3-db Directional Couplers, by J. K. Shimpages 4-15.