Single toroid hybrid mode RF phase shifter

Abstract: A miniaturized waveguide mode ferrite RF phase shifter is efficiently transitioned to a matched impedance microstrip transmission line mode at either end to result in an ultra small, efficient and lightweight essentially "planar" phase shifter device having a single ferrimagnetic toroid.

Description: FIELD OF THE INVENTION This invention relates generally to controllable RF phase shifters. It is particularly concerned with very high performance yet extremely small-sized phase shifters especially useful in phased RF radiator arrays at higher RF frequencies where available space between arrayed radiator elements is quite limited and essentially "planar" microstrip circuits are most effectively utilized. The invention has special utility for realizing small size phasers, switches, polarization networks and the like in the microwave industry. RELATED APPLICATIONS This application is related to the following prior art patent documents: EP-A-0 389 672 and EP-A-0 389 673. The former document describes a non-reciprocal radio frequency phase shifter having dual toroids with a central high dielectric slab positioned between the toroids. The latter document describes a reciprocal hybrid mode RF coupling circuit using both microstrip and waveguide modes of RF signal transmission. BACKGROUND AND BRIEF DESCRIPTION OF THE INVENTION Other relevant prior art patent documents are: US 3 277 401 which describes microwave phase shifting devices; US 4 881 052 which describes a microstrip non-reciprocal latching phase shifter having a ferrite rod between ramp-shaped dielectric waveguide members; and US 4 445 098 which describes a method and apparatus for a fast-switching dual-toroid microwave ferrite phase shifter. Ideally, a controllable RF phase shifter should have minimum size, minimum insertion loss, minimum weight, minimum cost and complexity, substantial immunity from all adverse ambient environmental factors (including physical and electrical) and an ability to produce any desired phase shift accurately and instantly upon demand. Unfortunately, in spite of many years of effort by those in the art, the truly ideal phase shifter has yet to be realized. A waveguide mode twin slab ferrite phase shifter (e.g. of the type described in commonly assigned U.S. patent No. 4,445,098 - Sharon et al) is one of the most accurate phase shifters known to date. However, in prior realizations, such waveguide mode phase shifters are large and expensive. If unswitched reciprocity is desired, this waveguide unit used in conjunction with circulators is too large for two dimensional phased arrays (where inter-radiator dimensions on the order of 0.6 wavelength are involved). The Sharon et al type of dual toroid ferrite phase shifter has been greatly miniaturized and incorporated serially with a microstrip transmission line to produce a novel, ultra-miniaturized, essentially planar, phase shifter of superior structure and performance. A miniaturized dual toroid phase shifter is disclosed in the application entitled "Hybrid Mode Phase Shifter" identified above. There has been a need for a miniaturized single toroid phase shifter. Accordingly, the invention provides a radio frequency phase shifter having: a latching reciprocal RF phase shifter with a toroid and a dielectric slab disposed along a longitudinal axis between opposite ends of a conductive waveguide, the phase shifter being disposed serially with a microstrip RF transmission line via an impedance-matched transition located adjacent at least at one of the ends of the waveguide, where the transition is effected without extending into a toroid wall, wherein the phase shifter is characterized as having just one ferrimagnetic toroid, and the toroid and slab of the phase shifter are asymmetrically mounted within the waveguide, and the transition is offset from the toroid axis. We have successfully converted the miniaturized dual toroid phase shifter into a single toroid phase shifter. Some of the advantages that the single toroid phase shifter has over the dual toroid phase shifter are that it is less complex, more economical to produce and more compact. The present invention may, in some respects, be described as a single toroid, side slab miniaturized waveguide phase shifter inserted serially between interrupted matched-impedance microstrip transmission lines. Some embodiments may position the waveguide portion into an underlying ground plane structure while others dispose at least a portion of the waveguide above the top level of a microstrip substrate. In a presently preferred embodiment, the waveguide portion is butted between terminated ends of the microstrip substrate so that the maximum thickness of the whole device is merely that of the central waveguide portion. BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described by way of example and with reference to the accompanying drawings, in which: FIGURE 1 is a perspective bottom view of a first exemplary embodiment of the invention in which matched serial couplings are achieved by probes attached directly to microstrip transmission lines terminating and leading to the dielectric ends of a serially imposed waveguide phase shifter; FIGURE 2 is a perspective view of the metal end cap shown in FIGURE 1; FIGURE 3 is a top view of FIGURE 1; FIGURE 4 is a cross-sectional depiction of one end of the device along line 4-4 in FIGURE 3 illustrating the pin-type microstrip phase shifter coupling; FIGURE 5 is an approximate equivalent RF circuit of the microstrip and waveguide transmission media arrangement of FIGURE 1; FIGURE 6 is a perspective view of a presently preferred exemplary embodiment of this invention in which matched couplings between a waveguide phase shifter and abutting microstrip transmission line sections at either end is attained by a capacitance and metal ribbon; FIGURE 7 is an end view of the invention along line 6-6 in FIGURE 6; and FIGURE 8 is a side view of the invention shown in FIGURE 6. DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION In the perspective view of FIGURE 1, a parallel, elongated, rectangular ferrimagnetic toroid 2 has a slab 6 of high dielectric material affixed adjacent to one of its sides and metallized surfaces 8 on the outer sides of the composite toroid/slab structure to form a miniature waveguide internally thereof. A dielectric substrate 18, which also may be made of a ferrimagnetic material, has a metallized ground plane surface 20 on the side shown in FIGURE 1 as soldered to the metallized surfaces 8. Conductive microstrip lines 22 and 24 on the opposite side of substrate 18 are shown in dashed lines. They extend to or a little bit beyond the ends of the toroid 2 so to permit connection to a mode transmission pin or probe 32 located at each end of the toroid/slab. Although only one end of the toroid 2 is visible in FIGURE 1, the other end is the same. An aperture 30 in the metallized ground plane surface 20 extends, as better seen in FIGURE 4, through the substrate 18 at a location adjacent the end of the dielectric slab 6. A metal probe 32 is mounted on and electrically connected to the microstrip line 22. It extends through the aperture 30 without touching the metallized surface 20. The probe stands upright through the ground plane such that its axis aligns approximately with the junction between the toroid and slab. About one-half of the probe is in front of a wall of the toroid and the other half is in front of the slab. Routine experimentation is necessary to optimally align the probe in front of the toroid/slab. An L-shaped wire guide 34 is made of dielectric material and shaped with arm 36 that can be respectively inserted into the center space of toroid 2. Groove 42 on the outer sides of the arm 36 provides an ingress/egress passage for latching current wire 44. When the wire guide 34 is mounted in position, its base or bight 48 bears against the probe 32 as shown in FIGURE 4. As shown in FIGURES 1 and 2, a metal end cap 50 is designed to fit around the wire guide 34 and is soldered to the metallized surface 20 as well as to the metallized surfaces 8 along the tops and outer sides of the toroid 2 to complete an end for the waveguide mode structure. An end cap 50 at the other end of the toroid is mounted as just described. The resulting cavity housing assists in tuning the probe transition to a matched impedance condition. In the top view of FIGURE 3, the microstrip lines 22 and 24 are seen to provide a microstrip transmission line serially interrupted by the connection of the waveguide phase shifter via mode transmission probes 32. The bottoms of the solder connections 35 are just visible in FIGURE 3. Miniature coaxial transmission line connectors can easily be connected to a short length of the microstrip 22 or 24 (thus providing a highly compact coax-microstrip-waveguide-microstrip-coax RF mode sequence). Many possible alternate combinations and permutations are possible by omitting some of the modes from one or both ends. Thus, an overall coax-to-microstrip or microstrip-to-coax mode phase shifter device can be realized. FIGURE 4 shows the structure at the end of the toroid 2. The metal end cap 50 is soldered to the metallized surfaces 8 and to the metallized ground plane surface 20. Base 48 of the L-shaped wire guide is seen in section. The bottom of probe 32 is soldered 35 to microstrip line 22, and epoxy 52 is deposited along the line of contact between probe 32 and the end of the slab/toroid junction. FIGURE 5 is an approximate equivalent circuit for the matched coupling between microstrip transmission lines 22, 24 and the waveguide mode phase shifter (i.e. the toroid 2, slab 6 and the metallized surfaces 8). The beyond cutoff waveguide cavity is represented by shunt inductance 54, and the capacitance coupling provided by gap G between the distal end of a probe 32 and the opposite end cap 50 is represented by shunt capacitance 56. Capacitances 58 and 60 represent series capacitances associated with the probe. Thus, the single toroid design, as shown in FIGURES 1-5 includes a toroid 2 adjacent a slab of high dielectric material 6 (ε'=80). The high dielectric slab 6 functions similarly to a dielectric center core in any other single toroid. Additionally, the slab provides a thermal path to remove heat from the toroid generated by RF power dissipation. The toroid and slab are secured together (e.g. epoxy) and metallized. The RF fields are thus concentrated towards the slab side of the toroid. The most RF-active ferrite is located on the side of the toroid adjacent the dielectric slab. The other side of the toroid is relatively inactive and serves merely to complete a magnetic path and allow latching operations (as is explained more fully in Sharon et al). This other side of the toroid decreases the efficiency (differential phase per unit length) of the phase shifter, because the dielectric material (the ferrite) at the waveguide walls is magnetized in a direction to subtract from the primary differential phase shift obtained by the wall adjacent the slab. This effect is minimized by using a high dielectric slab. A unique transition impedance matching scheme is used in FIGURES 1-5 to match the single toroid waveguide phase shifter section to the RF input and output microstrip transmission line structures. This matching technique may possibly be explained by considering the boundary between the toroid loaded waveguide structure and waveguide (operated beyond cutoff) cavity section. The boundary at the toroid and cavity section looks like a shunt inductance. The probe 32 protruding from the microstrip line appears as a shunt capacitance and a small series capacitance (as shown in the equivalent circuit of FIGURE 5). The distance from the back plane of the cavity to the probe (i.e. space occupied by section 48 of the L-shaped dielectric member 34) and the probe gap distance G to the opposite side of the waveguide changes the shunt capacitance. Variable match-tuning capacitance, once the probe depth is fixed, is achieved from back plane adjustment of end caps 50. This technique permits broad frequency operation because the matching occurs, for all practical purposes, in the same plane as the impedance discontinuity. The return loss was measured over the frequency band of 9.575 to 10.46 GHz. The return loss was a minimum of approximately 15 dB over the frequency band. The return loss was limited due to the OSM to microstrip adapters at each end. From measurements made on a straight section of microstrip 50 ohm line with the OSM to microstrip connectors, it has been calculated that the hybrid mode phase shifter has a return loss greater than 23 dB over the same frequency band. Another preferred embodiment of the invention is illustrated in FIGURES 6-8. A microstrip line 68 is butted against a toroid end 70. The exposed sides of the toroid as well as the top and bottom of the high dielectric slab 74 are metallized 75 to form a miniaturized rectangular waveguide. The metallized lower ground plane surface 66 of the microstrip structure makes electrical contact with the lower metallized surface 75. Mechanical rigidity as well as good electrical contact is provided by soldering a metal plate 76 (or plated dielectric substrate) to the metal ground plane surface 66 (at one end) and to an abutting lower end portion of the metallized surface 75. The height of the microstrip dielectric 62, e.g. about 1.4 mm (0.055 inch), is less than the height of the toroid 70, e.g. about 2.54 mm (0.100 inch), so that the microstrip 68 butts against slab 74 at a point near its vertical center. The microstrip line is about 0.76 mm (0.030 inch) wide and 5 µm (0.0002 inch) thick. The microstrip is aligned in a horizontal direction such that its axis is approximately centered on the junction between the slab and toroid wall. The optimal position of the strip with respect to the slab/toroid junction is used as a tuning mechanism. One side of a capacitance 78 (e.g. a chip capacitor) is mounted in electrical contact with the microstrip line 68, and a metal ribbon 80, e.g. gold bonding ribbon 0.64 mm (0.025 inch) wide and 25 µm (0.001 inch) thick, is suspended in electrical contact (e.g. by soldering) between the other side of the capacitance 78 and a location on the top metallized surface 75 that is immediately above slab 74. In the alternative, the ribbon 80 can be conductively attached to the microstrip line 68 and capacitively coupled to the metallized surface 75 adjacent to the slab 74. As better seen in the side view of FIGURE 8, ribbon 80 may form a roughly triangular opening 82. An identical mode transition structure at the other end of the toroids is generally shown in FIGURE 8. The gap dimension G between the ribbon 80 and the dielectric slab 74 is a tuning mechanism to impedance match between the microstrip transmission line and the phase shifter. Exact values for a given design are best obtained by routine experimentation. G is not a critical parameter, for instance, when the dielectric substrate is positioned co-planar with the top of the phase shifter, G becomes zero. At a frequency of about 6 to 11 GHz, good operating results have been attained with the chip capacitor 78 (e.g. simply a suitable length of ribbon 80 insulated from microstrip line 68 by dielectric tape which results in a capacitance of about 0.3 pF), a mean gap distance G between the ribbon and the end of the slab 74 of about 0.38 mm to 10.2 mm (0.015 to 0.40 inch) and a height of the slab 74 above the microstrip 68 of about 1.27 mm (0.050 inch). In the FIGURES 6-8 technique for achieving the microstrip to ferrite toroid transition, as earlier stated, one key element of the matching technique is the realization of a series capacitive element in the microstrip line to toroid connection. The transition shown in FIGURES 6-8 is capable of achieving a low insertion loss and a good impedance match. The assumed principle of operation can be explained in terms of an equivalent one stage LC ladder circuit. Here, a shunt ladder inductance represents the shunt inductance of the basic microstrip to toroid junction. The capacitance is chosen to represent the required impedance for impedance matching between the microstrip and toroid waveguide characteristic impedances.

Claims: A radio frequency phase shifter having: a latching reciprocal RF phase shifter with a toroid (2,70) and a dielectric slab (6,74) disposed along a longitudinal axis between opposite ends of a conductive waveguide, the phase shifter being disposed serially with a microstrip RF transmission line (22,24,68) via an impedance-matched transition (32,78,80) located adjacent at least at one of the ends of the waveguide, where the transition is effected without extending into a toroid (2,70) wall, wherein the phase shifter is characterized by having just one ferrimagnetic toroid (2,70), and the toroid and slab (6,74) of the phase shifter are asymmetrically mounted within the waveguide, and the transition is offset from the toroid axis. A radio frequency phase shifter as in claim 1, wherein: the conductive waveguide is formed by metallization (8,75) of the outermost surfaces of the composite toroid/slab structure; and a conductive latch wire (44) is threaded through an open center of the toroid (2,70) for use in setting remnant magnetic flux within said toroid to predetermined values. A radio frequency phase shifter as in claim 1 or claim 2, wherein each of the impedance-matched transitions includes: a conductive link (32,78,80) capacitively coupled between said microstrip line and said waveguide at a point proximate the junction between said dielectric slab (6,74) and toroid (2,70). A radio frequency phase shifter as in claim 3, wherein: the conductive link includes a ribbon member (80) capacitively (78) coupled at one end to said microstrip line (68) and conductively coupled at its other end to said waveguide (70,74,75). A radio frequency phase shifter as in claim 3 or claim 4, wherein: the waveguide (70,74) is disposed with its ends between abutting ends of dielectric substrates (62) having first conductive ground plane surfaces (66) and second ground plane surfaces with said microstrip transmission line formed on the second surface; the first conductive ground plane surfaces of the substrates being conductively coupled with each other and with one side of said abutting waveguide ends; the substrates (62) being of lesser thickness than the waveguide (70,74); and the conductive link defining a predetermined gap G between it and the exposed respective end of said dielectric slab. A radio frequency phase shifter as in claim 5, wherein said gap G is of approximately triangular shape. A radio frequency phase shifter as in claim 6, including a chip capacitor (78) affixed to each microstrip transmission line (68) at a distance away from said junction between the slab and the toroid. A radio frequency phase shifter as in claim 7, wherein each capacitor (78) has a capacitance of approximately 0.3 pF.

Inventors: ROBERTS ROGER C, SHARON THOMAS E, ROBERTS, ROGER C., SHARON, THOMAS E.
Applicants: EMS TECHNOLOGIES INC, EMS TECHNOLOGIES, INC.
Application year: 1992
Main classification: H01P1
IPC codes: H01P1, H01P5