Quadrature hybrid low loss directional coupler

[0001] The present invention relates to microwave radio frequency (RF) couplers, and more specifically, to addressing the losses inherent in such couplers.

[0002] A hybrid directional coupler is a four port electromagnetic device that is configured to provide an output that is proportional solely to the power incident from a source. For a given bandwidth, a hybrid directional coupler will divide the incident power that is input to one port between two other output ports at quadrature phase. The relative power at each other port with respect to the incident power at the input port will be known based upon the impedances coupled to the various output ports of the device.

[0003] Quadrature hybrid directional couplers are used in communications equipment. Such couplers allow a sample of a communications signal that is input at an input port and output at an output or “direct” port, to be taken from the signal at third or “coupled” port. Similarly, there will be no appreciable signal at the fourth or “isolated” port. When appropriately designed, a directional coupler may discern between a signal input at the input port and a signal input at the direct or output port. Such ability to discern the signals is particularly useful when, for example, a coupler is coupled intermediate an RF amplifier and an antenna. In such a configuration, the output of the RF amplifier may be monitored independently from that of a signal reflected from a mismatched antenna. Moreover, such a monitored signal may be used to control the gain (e.g., automatic gain control (AGC)), or reduce the distortion of the RF amplifier.

[0004] Directional couplers have been constructed in a variety of different designs. Initially, directional couplers were constructed by sandwiching conductive copper strips or traces between pieces of dielectric material, such as polyolefin or Teflon. Directional couplers were also constructed by locating the inner conductors of two coaxial cables in close proximity with each other, and surrounding them with a common outer conductor. Directional couplers constructed using conductive traces deposited on dielectric materials also included metal containers for housing the dielectrics with, coaxial connectors mounted to the containers to provide connections to the traces. Today such construction techniques are typically used only for high power applications, and may or may not use dielectric materials.

[0005] Subsequently, directional couplers were developed without bulky metal housings and coaxial connectors, thereby reducing the size, weight, and cost, and improving the manufacturing of the couplers, as the well as the products using these couplers. These miniaturized directional couplers, often referred to as “filmbrids”, are laminated stripline assemblies that may be bonded together by fusion or by thermoplastic or thermoset films, and are often dispensed from reels and wave soldered onto land areas on circuit boards.

[0006] Many ways of constructing directional couplers have been developed; however, practically all of these designs suffer from insertion losses. Insertions losses may be generally attributed to the conductors and dielectric materials used in the construction of many couplers upon which the conductors are deposited, etched or otherwise placed. For example, dielectric materials absorb some of the power applied to a coupler, resulting in throughput or insertion losses. Such losses are particularly troublesome when a coupler is coupled intermediate an RF amplifier and an antenna, since such losses require more amplifier output to overcome the losses inherent in the coupler.

[0007] The relative propensity of the dielectric materials in couplers to absorb energy is generally designated by tan(d), the dielectric absorption factor or constant, and is related to air (tan(d)=0). The higher the loss tangent (tan(d)) or loss factor, and the more dielectric material used, the greater the amount of energy absorbed and the greater the losses.

[0008] For example, the dielectric absorption factor for pure Teflon (PTFE) is on the order of 0.0006, however pure Teflon is typically unworkable and impractical for use in couplers. As a result, a material, such as fiberglass, may be added to Teflon to provide strength and workability, the dielectric absorption factor there being on the order of 0.001. Other materials typically have dielectric absorption factors on the order of 0.03 or greater. Thus, for a like sized dielectric, a material with a higher dielectric absorption factor will absorb more energy than a material with a lower dielectric absorption factor, resulting in greater insertion loss for a directional coupler constructed using the material with the higher dielectric absorption factor.

[0009] Ideally, to minimize losses, a coupler would be constructed with an air dielectric. However, currently available air dielectric couplers have a sheet metal housing, or outer conductor, that is expensive to manufacture and is difficult to surface mount due to co-planarity issues between the housing and attached connections, or leads. Further, the leads are fragile and easily damaged.

[0010] Therefore, there is still a need for improving couplers. Particularly, there is a need for a low loss coupling device or coupler that is easy and relatively inexpensive to manufacture and mount.

[0018] The present invention provides a low loss coupling device or coupler that is easy and relatively inexpensive to manufacture and mount by fabricating the device using a thin dielectric septum supported by the top surface of a host circuit board, having a cavity, with first and second shields on opposing sides of the septum. Such an arrangement allows air gaps of precise proportions, and reduces losses commonly inherent in coupling devices.

[0019] With reference to FIGS. 1 through 4, there is shown one embodiment 10 of a quadrature hybrid low loss coupler in accordance with principles of the present invention. Coupler 10 includes a circuit board, such as a radio frequency (RF) circuit board 12, a coupler board 14, a first metal shield 16, and a second metal shield 18.

[0020] RF circuit board 12 may be constructed of fiberglass, or some other suitable circuit board material well known to those skilled in the art. RF circuit board 12 has a first surface 20, seen in FIGS. 1 and 2, and a second surface 22, not appearing in FIG. 1; but, seen in FIG. 3. Formed or cut in circuit board 12, intermediate first surface 20 and second surface 22, is an opening or cavity 24, forming a wall 26 around cavity 24 between first surface 22 and second surface 24.

[0021] Deposited or otherwise placed on first surface 20 and on second surface 22 is ground plane 28. Ground plane 28 may be copper, a copper alloy or some other suitably conductive material. Connectivity between portions of ground plane 28 on the first surface 20 and on the second surface 22 may be provided by vias 30 and/or plated through holes 32 that may be used in the assembly.

[0022] Etched away or otherwise formed from ground plane 28 on at least one side of board 12, and leading up to cavity 24, are stripline traces 34a-d as illustrated in FIG. 1. Proximate the ends of stripline traces 34a-d are respective land areas 36a-d. The width of traces 34a-d and the relative spacing between the traces 34a-d and the proximity to ground plane 28, as well as the thickness of RF circuit board 12, determine the characteristic impedance of the stripline, and may be varied as desired by those skilled in the art to realize a desired characteristic impedance. It will be appreciated that RF circuit board 12 may include additional traces and land areas (not shown) for purposes of mounting other components to the board 12, and thereby realize practically any desired circuit.

[0023] Coupler board 14 may be constructed of dielectric material having a dielectric constant as desired or advantageously approaching that of air (∈_r=1). For example, coupler board 14 may be constructed of a material having a dielectric constant of nominally 2.2-2.3. Moreover, the thickness of coupler board 14 may be reduced such that the amount of dielectric material capable of absorbing RF power is reduced. Coupler board 14 may also advantageously include notches 38, corresponding to holes 32 in RF circuit board 12, for use in the assembly of coupler 10.

[0024] Referring also to FIG. 5, wherein the dielectric material is depicted as translucent, RF power is transmitted through conductive traces 40a and 40b which are deposited, etched or otherwise formed on opposing sides of coupler board 14, as illustrated in FIG. 5. Proximate the ends of the conductive traces 40a and 40b are connecting elements 42a-d. As shown, the traces 40a, 40b on opposites side of board 14 are overlapped in certain regions along their length. The relative distance between the traces 40a, 40b, controlled by the thickness of the dielectric material, and the amount of overlap of the traces 40a, 40b generally determine the capacitive coupling between the traces. Such coupling parameters may be adjusted by those skilled in the art. As seen in FIG. 5, each trace 40a, 40b has a portion which angles away from the portion that overlaps with the other trace. The angle of approach of traces 40a and 40b to portions where the traces overlap determines electrical parameters such as the voltage standing wave ration (VSWR), and may also be adjusted those skilled in the art.

[0025] First metal shield 16 may be stamped from sheet metal, and bent to form top 44 and side walls 46. The side walls raise the top and form a cavity area 47 (see FIG. 4). Alternatively, first metal shield 16 could be cast or otherwise formed. First metal shield 16 may advantageously include tabs 48, corresponding to holes 32 in RF circuit board 12 and notches 38 in coupler board 14, for use in assembly of the coupler 10. First metal shield 16 may also includes notches 50a-d (notches 50a, 50b and 50d shown in dashed lines), corresponding respectively to traces 34a-d on the first side of RF board 12, so as not to short traces 34a-d to ground plane 28. First metal shield 16 may also include perforations 52.

[0026] Second metal shield 18 may also be stamped from sheet metal, or otherwise formed, and include perforations 52. Second metal shield 18 may also advantageously include holes 54, corresponding to holes 32 in RF circuit board 12, notches 38 in coupler board 14, and tabs 48 of first metal shield 16, for use in the assembly of coupler 10.

[0027] In assembly, a first solder mask and paste is applied to the first surface 20 of RF circuit board 12. Coupler board 14 is then placed on and supported by the first surface 20 of RF circuit board 12, aligning connective elements 42a-d with land areas 36a-d, respectively. In doing so, a cavity 24 is formed on a side of the coupler board 14 based upon the thickness of the circuit board 12 and/or the depth of the opening/cavity 24 and walls 26. (See FIG. 4) Next, first metal shield 16 is placed over coupler board 12, passing tabs 48 past notches 38 and through holes 32, further locating coupler board 14 relative to RF board 12. A first soldering is then performed, soldering coupler board 14 and first metal shield 16 to RF circuit board 12. The metal shield 16 forms another cavity 47 on a side of the coupler board opposite cavity 24. A second solder mask and paste is then applied to the second surface 22 of RF circuit board 12. Second metal shield 18 is then placed against the second surface 22 of RF board 12 such that tabs 48 of first metal shield 16 pass through respective holes 54 of second metal shield 18. A second soldering is then performed, soldering second metal shield 18 to RF board 12 and first shield 16. Thus, assembly of circuit board 12, a dielectric septum 14, first metal shield 16, and a second metal shield 18 completes coupler 10 and forms cavities on either side of board 14.

[0028] When assembled, two cavities of air are formed on opposite sides of coupler board 14. One cavity is bounded by the coupler board 14 and the top 44 and walls 46 of first metal shield 16. The other cavity is bounded by coupler board 14, wall 26 around cavity 24 between first surface 22 and second surface 24 of RF board 12, and second metal shield 18. Thus, coupler board 14 acts, in effect, as a thin dielectric septum between the two cavities.

[0029] The formation of such cavities is particularly advantageous in the assembly of coupler 10. For example, when coupler board 14 is constructed of a material having a dielectric constant of nominally 2.2-2.3, coupler 10 may have an effective dielectric constant approaching 1.0, while still maintaining adequate mechanical separation. Thus, coupler 10 is easy to assemble and provides a reduction in losses by reducing the amount of dielectric material used in a coupler and by using air as a dielectric through the formation of the air cavities.

[0030] When coupled to a circuit such as the circuit on RF circuit board 12, coupler 10 may be configured as having an input port, an output or direct port, a coupled port, and an isolated port. For example, a signal may be coupled to trace 34a on RF board 12 as an input port of coupler 10. On coupler board 14, that input corresponds to trace 40b and connecting element 42b. One half of the input signal power emerges at the direct port, and is coupled to trace 34d through trace 40b and connecting element 42d. The other half of the signal power emerges at the coupled port, and is coupled through trace 40a to connecting element 42b and trace 34b. Little or no power emerges from the isolated port, at trace 34c corresponding to trace 40a and connecting element 42c.

[0031] Referring now to FIG. 6, a schematic block diagram of an embodiment 60 of an RF amplifier containing a quadrature hybrid low loss coupler 10 in accordance with principles of the present invention is illustrated. RF amplifier 60 comprises coupler 10, amplifier 62, a feedback circuit 64, and mixer 66. Some or all of these components may be mounted to a circuit board, such as RF circuit board 12 shown in FIGS. 1 through 4.

[0032] Coupler 10 is coupled intermediate amplifier 62 and antenna 68 via input and direct ports, respectively. The coupled port of coupler 10 is coupled to feedback circuit 64. Feedback circuit 64 is coupled to mixer 68. Mixer 68 is also coupled to amplifier 62, and serves as the input RF IN to RF amplifier 60.

[0033] In operation, a signal to be amplified is coupled to RF amplifier 60 at RF IN, and to amplifier 62 through mixer 68. Amplifier 62 amplifies the signal, coupling the signal through coupler 10 to antenna 70, as indicated at RF OUT. A portion of the power from amplifier 62 is coupled via the coupled port of coupler 10 to feedback circuit 64. Feedback circuit 64 processes the coupled power to develop a signal that may be mixed with the input signal RF IN, such that the operation of RF amplifier 60 is improved. Thus, coupler 10, with reduced insertion losses, reduces the amount of amplifier output power necessary to overcome losses typically found in many couplers for a given output at antenna 70.

[0034] While the present invention has been illustrated by the description of the embodiments thereof, and while the embodiments have been described in considerable detail, it is not the intention of the applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. For example, those skilled in the art may use principles of the present invention to construct a combiner or an N-way power splitter, where N denotes the number of ways power may be split, in additional to the embodiments of the couplers described herein. Further, those skilled in the art will appreciate that a coupling device may be constructed having practically any impedance as desired. Moreover, such a coupler, combiner or splitter may be either a stripline or a microstrip device. Therefore, the invention in its broader aspects is not limited to the specific details representative apparatus and method, and illustrative examples shown and described. Accordingly, departures may be made from such details without departure from the spirit or scope of applicants' general inventive concept.