Compact phase shifter circuit using coupled lines

The present invention provides a switched-line phase shifter (300) for creating a differential phase shift between switched transmission paths (302, 304). A switched-line phase shifter (300) incorporates Schiffman sections (306, 308) of lengths which are non-integer multiples of quarter-wavelength. The lengths of the Schiffman sections (306, 308) are chosen such that no isolation points, which result from frequencies at which the effective electrical length of one of the transmission paths (302, 304) is an integer multiple of .lambda./2, occur over the operating frequency range of the phase shifter (300). The present invention also provides a space-efficient implementation of a switched-line phase shifter (600) which utilizes switches (640, 642, 644) between Schiffman subsections (610, 630) to alternately combine and isolate the Schiffman subsections (610, 630) thereby alternately creating effective Schiffman sections of greater and lesser length respectively.

BACKGROUND OF THE INVENTION
 The present invention relates generally to phase shifter circuitry. More
 specifically, the present invention relates to switched-line phase
 shifters using parallel coupled line sections.
 A basic component in microwave/millimeter wave circuits is the differential
 phase shifter. Differential phase shifters are commonly implemented using
 a switched-line configuration in which switching devices are used to
 switch a signal between alternate transmission paths. The alternate
 transmission paths, in turn, have different electrical lengths, and thus
 there is a difference in relative signal phase between signals propagated
 through the alternate transmission paths. For example, if a first
 transmission line has an electrical length of .lambda./2 (where .lambda.
 is the wavelength of the signal) and a second transmission line has an
 electrical length of .lambda./4, the differential phase shift between the
 two transmission paths is .lambda./4 (or 90.degree.).
 One problem with conventional switched-line phase shifters incorporating
 non-coupled transmission lines is that the differential phase shift varies
 with signal frequency. For example, a first transmission line with an
 electrical length of .lambda..sub.1 /2 at a frequency of 3 GHz may have an
 electrical length of .lambda..sub.2 /4 at 1.5 GHz. Likewise, a second
 transmission line with an electrical length of .lambda..sub.1 /4 at a
 frequency of 3 GHz may have an electrical length of .lambda..sub.2 /8 at
 1.5 GHz. Thus, while the differential phase shift between the two
 transmission lines at 3 GHz is .lambda./4, the differential phase shift
 between the same two transmission lines at 1.5 GHz is .lambda./8.
 In response to the need to maintain a single phase shift over a range of
 frequencies, switched-line phase shifters utilizing parallel
 coupled-transmission lines (hereinafter "Schiffman sections") have been
 developed. Such phase shifters are described by B. M. Schiffman in the
 paper entitled "A New Class of Broad-Band Microwave 90-Degree Phase
 Shifters," IRE Transactions on Microwave Theory and Technique, April 1958,
 pages 232-237.
 One problem with switched-line phase shifters, including Schiffman-type
 phase shifters using series switches, is that when the effective
 electrical length of the switched-off transmission path is an integer
 multiple of 180.degree. (half the wavelength of the operating frequency),
 a resonance is established in the switched-off path. The resonance results
 from the practical implementation of switching devices that have leakage
 capacitance. Although the switched-off path is theoretically isolated from
 the external network, in actuality the switched-off path is capacitively
 coupled to the external network. Since the switched-off path is coupled to
 the rest of the network (including the switched-on transmission path), the
 effects of the switched-off path resonance are seen in the performance of
 the switched-on path as well. In particular, the resonance results in
 phase shifter operating points of high signal attenuation (also known as
 isolation points) at the operating frequencies associated with the points
 of resonance.
 Various techniques have been proposed to reduce the resonance effect. For
 example, the use of transfer switches instead of standard single pole
 double throw (hereinafter "SPDT") switches to switch between transmission
 paths has been explored. In a transfer switch implementation, when a
 transmission path is switched off, it is connected to a load with a
 matching characteristic impedance. Although the resonance problem can be
 reduced through the use of transfer switches, the performance of a
 switched-line phase shifter using this loading technique suffers the
 disadvantage of considerable driver circuit complexity. In addition, the
 bandwidth of such a phase shifter is limited due to the RF properties of
 the associated switching devices and load circuitry.
 Another technique that has been explored, for example in the paper entitled
 "An Octave-Band Switched-Line Microstrip 3-b Diode Phase Shifter," IEEE
 Transactions on Microwave Theory and Techniques, Vol. MTT-21, No. 7, July
 1973, pages 444-449, is the use of shunt switches instead of series
 switched in the switched-line configuration. Though theoretically
 appealing, the performance of switched-line phase shifters utilizing shunt
 switches in practice is severely degraded due to the parasitic reactance
 present in practical switching devices. Furthermore, they use quarter-wave
 transformers to isolate the switched-off path at the input and output
 ports of the switched-line phase shifter. However, the quarter-wave
 transformers cannot be operated over a wide bandwidth, and thus limit the
 effective bandwidth of the phase shifter.
 A need has long existed for a wide-bandwidth switched-line phase shifter
 with relatively constant phase-frequency characteristics that eliminates
 isolation points over the design operating frequency range without
 incorporating complex circuitry or introducing substantial performance
 degradation.
 SUMMARY OF THE INVENTION
 It is an object of present invention to provide a switched-line phase
 shifter. It is another object of the present invention to provide a
 switched-line phase shifter with relatively constant phase-frequency
 characteristics which avoids isolation points in the design operating
 frequency range.
 It is a further object of the present invention to provide a switched-line
 phase shifter which avoids isolation points in the design operating
 frequency range by incorporating Schiffman sections of non-conventional
 length.
 It is a still further object of the present invention to provide a
 space-efficient implementation of a switched-line phase shifter
 incorporating an adjustable-length Schiffman section comprising several
 switchably connected Schiffman subsections.
 One or more of the foregoing objects is met in whole or in part by a
 preferred embodiment of the present invention that provides a compact
 switched-line phase shifter incorporating Schiffman sections of
 non-conventional length. The lengths of the Schiffman sections are chosen
 such that the effective electrical lengths of the individual transmission
 paths of the switched-line phase shifter do not become integer multiples
 of 180.degree. (half wavelength) over the design operating frequency range
 of the phase shifter. A space-efficient implementation of a switched-line
 phase shifter incorporating Schiffman sections is also provided. A
 plurality of Schiffman subsections are switchably connected to form a
 Schiffman section of variable length, thereby efficiently utilizing one or
 more Schiffman subsections in multiple switched transmission paths.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
 FIG. 1 illustrates a conventional switched-line phase shifter 100 with
 uniform transmission lines. Two SPDT switches 102, 104 are used to
 alternately connect two transmission paths 106, 108 between an input port
 110 and an output port 112. Differences in length between the first
 transmission path 106 and the second transmission path 108 result in
 different signal propagation times through the two paths 106, 108. The
 different signal propagation times, in turn, result in a differential
 phase shift between a signal propagated through the two paths 106, 108.
 For example, if the length of the first transmission path 106 is equal to
 the wavelength (.lambda..sub.o) of a signal of frequency f.sub.o being
 propagated through it, and the length of the second transmission path 108
 is equal to half of the wavelength (.lambda..sub.o /2) of the signal being
 propagated through it, the differential phase shift between the two
 transmission paths 106, 108 is 180.degree.=2 .pi./.lambda..sub.o
 (.lambda..sub.0 -.lambda..sub.o /2).
 As mentioned previously, a problem with conventional switched-line phase
 shifters incorporating uniform transmission lines is that the realized
 differential phase shift is a function of the frequency of the signal
 applied to the phase shifter. For example, considering the example of the
 previous paragraph, if a signal of frequency f=1.2f.sub.o (and thus
 wavelength .lambda.=.lambda..sub.o /1.2) is applied to the phase shifter,
 the effective electrical length of the first transmission path 106 becomes
 1.2.lambda., and the effective electrical length of the second
 transmission path 108 becomes 1.2 .lambda./2. The differential phase shift
 between the two transmission paths 106, 108 becomes
 2.pi./.lambda.(1.2.lambda./2) (or approximately 216.degree.).
 Turning now to FIG. 2, that figure shows an example of a Schiffman-type
 phase shifter 200 that includes a Schiffman section 202 in a first
 transmission path 204 and an uncoupled uniform transmission line in a
 second transmission path 206. In accordance with the conventional series
 switched-line phase shifter configuration, two switches 208, 210
 alternately connect the first transmission path 204 and the second
 transmission path 206 between the input port 212 and the output port 214
 of the phase shifter 200.
 In the Schiffman phase shifter 200, the difference in physical transmission
 path lengths is still a determining factor in the realized differential
 phase shift. However, the coupling between the parallel coupled
 transmission line segments of the Schiffman section may, for a finite
 frequency bandwidth, result in a phase-frequency relationship for the
 first transmission path 204 that closely resembles the phase-frequency
 relationship for the second transmission path 206. Thus, over the finite
 frequency bandwidth, a substantially constant differential phase shift
 between the switched transmission paths 204, 206 may be obtained.
 The phase shift through a Schiffman section is presented by E. M. T. Jones
 and J. T. Bolljahn in the paper entitled "Coupled Strip Transmission Line
 Filters and Directional Couplers," IRE Transactions on Microwave Theory
 and Techniques, vol. MTT-4, April 1956, pp. 75-81. The phase .phi. shift
 is:
 ##EQU1##
 where .rho. is a ratio of the even and odd mode characteristic impedances
 of the transmission line (.rho.=Z.sub.oe /Z.sub.oo) and .theta. is the
 electrical length of the transmission line (.theta.=.beta.l), where .beta.
 is a phase constant, and l is the length of the transmission line.
 As mentioned previously, a resonance problem exists in both conventional
 switched-line phase shifters 100, as illustrated in FIG. 1, and
 Schiffman-type phase shifters 200, as illustrated in FIG. 2. Since
 practical switching devices have finite parasitic capacitances, energy is
 coupled to the switched-off transmission path. When the frequency of the
 signal input to a series switched-line phase shifter is such that the
 effective electrical length of the switched-off transmission path is an
 integer multiple of half the wavelength of the input signal, a resonance
 is established in the switched-off path. The capacitive coupling between
 the input and output ports and the switched-off path allows energy to flow
 into the switched-off path to maintain the resonance. In addition, the
 capacitive coupling also allows energy from the resonance in the
 switched-off path to interfere with the signal propagating through the
 switched-on path. Thus a resonance established in either transmission
 path, when switched off, results in severe performance degradation for the
 switched-line phase shifter.
 The points of resonance (also referred to as isolation points) cause severe
 performance degradation at one or more frequencies in the operating
 frequency range of the phase shifter. Either the performance degradation
 at the isolation points must be accepted, additional circuitry must be
 added to compensate for the performance degradation at the isolation
 points, or the isolation points must be avoided.
 The present invention effectively eliminates isolation points in the design
 operating frequency band by relocating the isolation points out of the
 design operating frequency band. As explained in more detail below, the
 present invention utilizes Schiffman sections of non-conventional length
 to accomplish the relocation of isolation points.
 In the past, lengths of Schiffman sections have been chosen to be integer
 multiples of quarter-wavelength (i.e. integer multiples of a quarter
 wavelength) at the design primary operating frequency for the phase
 shifter. Unfortunately, the use of Schiffman sections of lengths which are
 integer multiples of quarter-wavelength typically results in the creation
 of isolation points in the design operating frequency band.
 For example, if the Schiffman section length for a transmission line is
 chosen to be .lambda./4 at the primary operating frequency for the phase
 shifter, the effective electrical length of the section will be close to
 .lambda./2 (or 180.degree.). Adding the effects of the parasitic coupling
 capacitances of the switching devices may, for example, result in a
 transmission path with an effective electrical length of approximately
 200.degree.. A change in operating frequency f from f.sub.primary to
 0.9*f.sub.primary would then result in operation at an isolation point.
 Thus while the example phase shifter may work well at the primary
 operating frequency, a relatively small shift in operating frequency
 results in severely degraded performance. In addition, the practical
 electrical length of a transmission path may also vary unpredictably due
 to non-constant and inconsistent switching device characteristics and
 manufacturing process variances.
 FIG. 3 illustrates a series switched-line phase shifter 300 incorporating
 Schiffman sections 306, 308 of non-conventional length according to a
 preferred embodiment of the present invention. Each transmission path 302,
 304 contains a Schiffman section 306, 308. The switching devices 320, 322
 alternately connect the transmission paths 302, 304 between the input port
 324 and the output port 326.
 The phase shift through a Schiffman section was given earlier in equation
 (1). Equation (1) may be applied to the first Schiffman section 306 to
 arrive at the first Schiffman section 306 phase .phi..sub.1 and to the
 second Schiffman section 308 to arrive at the second Schiffman section 308
 phase .phi..sub.2. Assuming similar lengths for the non-coupled
 transmission line sections 310, 312, 314, 316, the differential phase
 shift between the two transmission paths 302, 304 is calculated as the
 difference between the first Schiffman section 306 phase .phi..sub.1 and
 the second Schiffman section 308 phase .phi..sub.2 (or
 .DELTA..phi.=.phi..sub.2 -.phi..sub.1)
 Design parameters and resulting bandwidths for 45.degree. and 90.degree.
 switched-line phase shifters according to a preferred embodiment of the
 present invention are shown in Table 1. The bandwidths in Table 1 were
 measured about the primary operating frequency to points of 2.degree.
 phase shift error. Note that the resulting operating frequency bands of
 each of the phase shifters do not contain isolation points.
 TABLE 1
 Desired Resulting
 Phase Design Parameters Bandwidth (in
 Shift .theta..sub.1 .rho..sub.1 .theta..sub.2 .rho..sub.2 terms
 of .theta..sub.1)
 45.degree. 140.degree. 1.8 118.degree. 1.8 105.degree. to 180.degree.
 (0.614 octaves)
 90.degree. 140.degree. 1.9 99.6.degree. 1.7 126.degree. to 180.degree.
 (0.428 octaves)
 As an example, choosing 3 GHZ as an example primary operating frequency
 f.sub.o for a 45.degree. phase shifter, the corresponding full wavelength
 .lambda..sub.o is 0.1 meters. The electrical length for the first
 transmission path is chosen to be 140.degree. (approximately 0.038889
 meters), and the electrical length for the second transmission path is
 chosen to be 118.degree. (approximately 0.032778 meters). Notice that
 neither electrical length of either transmission path is relatively near
 an isolation point at the primary operating frequency (0.05 meters being
 the closest integer multiple of .lambda..sub.o /2)
 The electrical length for the first transmission path is an integer
 multiple of 180.degree. when the operating frequency is approximately 3.86
 GHz, and the electrical length of the second transmission path is an
 integer multiple of 180.degree. when the operating frequency is
 approximately 4.58 GHz. According to Table 1, the operating frequency band
 in which an absolute phase error of less than 2.degree. is experienced
 ranges from f.sub.min =0.75f.sub.o to f.sub.max =1.287f.sub.o (or
 f.sub.min =2.25 GHz to f.sub.max =3.86 GHz for the example chosen). Thus,
 no isolation points exist within the operating frequency range.
 FIG. 4a illustrates a plot 400 showing performance of a 45.degree. phase
 shifter designed in accordance with the design parameters presented in
 Table 1. The plot line 402 illustrates the calculated differential phase
 shift as a function of normalized electrical length.
 Likewise, FIG. 4b shows a plot 450 showing performance of an example
 90.degree. phase shifter designed in accordance with the design parameters
 shown in Table 1. The plot line 452 illustrates the calculated
 differential phase shift as a function of normalized electrical length.
 Note that the inventive concept also applies to switched-line phase
 shifters incorporating multi-subsection Schiffman sections. FIG. 5 shows a
 switched-line phase shifter 500 incorporating a multi-subsection Schiffman
 section 502 of non-conventional length according to an alternative
 embodiment of the present invention. The Schiffman section 502 may
 comprise multiple Schiffman subsections 506, 508 which when combined, form
 a total effective electrical length that is not an integer multiple of
 quarter-wavelength. The paper, "A New Class of Broad-Band Microwave
 90-Degree Phase Shifters," gives an equation for calculating the phase for
 coupled line all-pass cascaded sections having unequal lengths and
 coupling factors as:
 ##EQU2##
 where .rho..sub.1 =Z.sub.oe1 /Z.sub.oo1 of the first subsection, where
 ##EQU3##
 and where .theta..sub.1 is the electrical length of the first subsection
 506, .theta..sub.c is the electrical length of the cascaded subsection
 508, and Z.sub.ooc is the odd mode characteristic impedance of the
 cascaded subsection 508. The switching devices 510, 512 alternately
 connect the first transmission path 514 and the second transmission path
 516 between the input port 518 and the output port 520.
 Incorporating Schiffman sections into multiple transmission paths of a
 switched-line phase shifter typically results in increased circuit space
 requirements. Because, in switched-line phase shifters, a significant
 portion of the conductor is typically duplicated in alternate transmission
 paths, an implementation of a switched-line phase shifter which
 efficiently shares conductor length, and thus circuit space, between
 alternate transmission paths may result in the realization of substantial
 circuit space savings. FIG. 6 illustrates a space-efficient implementation
 of a multi-subsection Schiffman-type switched-line phase shifter 600
 according to a preferred embodiment of the present invention.
 The phase shifter 600 has an input port 602 connected to the left end of a
 first non-coupled conductor 603 and an output port 604 connected to the
 right end of a second non-coupled conductor 605. A first Schiffman section
 610 has a left conductor 612, the lower end of which is connected to the
 right end of the first non-coupled conductor 603, and a right conductor
 614, the lower end of which is connected to the left end of the second
 non-coupled conductor 605. A transition section 620 has a left conductor
 622, the lower end of which is connected to the upper end of the left
 conductor 612 of the first Schiffman section 610, and a right conductor
 624, the lower end of which is connected to the upper end of the right
 conductor 614 of the first Schiffman section 610. A second Schiffman
 section 630 has a left conductor 632, the upper end of which is connected
 to the left end of an end conductor 636, and a right conductor 634, the
 upper end of which is connected to the right end of the end conductor 636.
 The conductors 612, 614 of the first Schiffman section 610 run parallel to
 each other along a main longitudinal axis 650 separated by a first spacing
 615. The conductors 632, 634 of the second Schiffman section 630 run
 parallel to each other and preferably along the main longitudinal axis 650
 separated by a second spacing 635 which may be different than the first
 spacing 615. The conductors 622, 624 of the transition section 620 are
 positioned between the upper ends of the conductors 612, 614 of the first
 Schiffman section 610 and the lower ends of the conductors 632, 634 of the
 second Schiffman section 630. The conductors 622, 624 of the transition
 section 620 provide a conductive spacing transition between the conductors
 612, 614 of the first Schiffman section 610 and the conductors 632, 634 of
 the second Schiffman section 630.
 A first single pole single throw (hereinafter "SPST") switching device 640
 is connected between the upper ends of the left conductor 612 and the
 right conductor 614 of the first Schiffman section 610. A second SPST
 switching device 642 is connected between the upper end of the left
 conductor 622 of the transition section 620 and the lower end of the left
 conductor 632 of the second Schiffman section 630. A third SPST switching
 device 644 is connected between the upper end of the right conductor 624
 of the transition section 620 and the lower end of the right conductor 634
 of the second Schiffman section 630.
 To form a first transmission path between the input port 602 and the output
 port 604, the switching devices 640, 642, 644 assume a first switch state
 forming a first Schiffman section. In the first switch state, the first
 switching device 640 conductively connects the upper ends of the
 conductors 612, 614 of the first Schiffman section 610, and the second and
 third switching devices 642, 644 break the connection between the upper
 ends of the conductors 622, 624 of the transition section 620 and the
 lower ends of the conductors 632, 634 of the second Schiffman section 630.
 A conductive path is thereby created from the input port 602 to the output
 port 604 through the first non-coupled conductor 603, the left conductor
 612 of the first Schiffman section 610, the first switching device 640,
 the right conductor 614 of the first Schiffman section 610 and the second
 non-coupled conductor 605.
 To form a second transmission path between the input port 602 and the
 output port 604, the switching devices 640, 642, 644 assume a second
 switch state forming a second Schiffman section. In the second switch
 state, the first switching device 640 breaks the connection between the
 upper ends of the conductors 612, 614 of the first Schiffman section 610.
 The second switching device 642 conductively connects the upper end of the
 left conductor 622 of the transition section 620 to the lower end of the
 left conductor 632 of the second Schiffman section 630. The third
 switching device 644 conductively connects the upper end of the right
 conductor 624 of the transition section 620 to the lower end of the right
 conductor 634 of the second Schiffman section 630. A conductive path is
 thereby created from the input port 602 to the output port 604 through the
 first non-coupled conductor 603, the left conductor 612 of the first
 Schiffman section 610, the left conductor 622 of the transition section
 620, the second switching device 642, the left conductor 632 of the second
 Schiffman section 630, the end conductor 636, the right conductor 634 of
 the second Schiffman section 630, the third switching device 644, the
 right conductor 624 of the transition section 620, the right conductor 614
 of the first Schiffman section 610 and the second non-coupled conductor
 605.
 Note that both the first transmission path and the second transmission path
 include the first non-coupled conductor 603, both conductors 612, 614 of
 the first Schiffman section 610 and the second non-coupled conductor 605.
 The sharing of conductor length, and hence circuit space, results in the
 realization of substantial circuit space savings.
 The present invention provides a switched-line phase shifter with
 continuous and effective operation throughout the operating frequency
 band. The removal of isolation points from the operating frequency band
 results in more reliable phase shifter operation. The phase shifter is
 more robust with regard to varying operating conditions and variances in
 electrical component characteristics and manufacturing processes. In
 addition, the space-efficient implementation of the present invention
 results in the realization of substantial circuit space savings.
 While particular elements, embodiments and applications of the present
 invention have been shown and described, it will be understood that the
 invention is not limited thereto since modifications may be made by those
 skilled in the art, particularly in light of the foregoing teachings. It
 is therefore contemplated by the appended claims to cover such
 modifications as incorporate those features which come within the spirit
 and scope of the invention.