Microwave power splitter/combiner

A microwave, power splitter/combiner (20) is formed as part of a multilayer laminate (27, 28, 29, 33, 34) such that two ports (22, 23) are connected by plated vias (31, 32) to conductive pads (29, 30) connected across an isolation resistor (27). Furthermore, a microwave circuit is provided in the form of a multi-layer laminate including a substrate carrying a resistive layer which has been etched to define at least one resistor, a dielectric membrane covering the resistor, a conductive layer defining at least part of an electrical circuit, and said at least one resistor is electrically connected to the conductive layer by vias extending through the dielectric membrane.

This invention concerns microwave circuits and in particular, but not exclusively, the manufacture of a microwave power splitter/combiner either as a component, or as part of a manifold power splitter/combiner. More particularly, but not exclusively, the invention relates to the formation of a multi-layer laminate defining one or more microwave power splitter/combiners of the type originated by Ernest Wilkinson and commonly referred to as a Wilkinson splitter or a Wilkinson combiner.

The simplest form of Wilkinson splitter comprises a three port circuit which splits an input at a first port between two arms that constitute quarter-wave transformers each having a characteristic impedance of 1.414×Z° [=Z°√2], and terminate respectively in the second and third ports which are inter-connected by a 2×Z° isolation resistor; this configuration achieves equal split matching between all of the ports with low losses and a high isolation between the output ports. In operation as a splitter, an input signal entering the first port is split into equal-phase and equal-amplitude output signals at the second and third ports. The isolation resistor is decoupled from the input signal because its ends are at the same potential and no current passes through it.

The simplest form of Wilkinson combiner has the same structure but combines input signals at the second and third ports to produce an output signal at the first port. An input signal at either the second port or the third port has half of its power dissipated in the resistor in a manner well known in the art, with the remainder transmitted to the first port. The resistor therefore decouples the second and third ports.

Wilkinson splitters and combiners are well known to have a range of configurations all requiring the provision of at least one isolation resistor. Although some of these splitter and combiner designs have more than three ports, for instance 3:1 and 4:1 configurations, they all require a ported circuit defining at least three ports. The invention enables high insertion losses at microwave frequencies to be reduced.

According to one aspect of the invention, a microwave power splitter/combiner comprises a multi-layer laminate including a substrate carrying a resistive layer which has been etched to define a resistor, a dielectric membrane covering the resistor, a conductive layer defining at least part of an electrical circuit of the power splitter/combiner, and two ports of the power splitter/combiner are electrically connected across the resistor by vias extending through the dielectric membrane.

The resistive layer is preferably formed from a nickel-phosphorus alloy.

The resistive layer may have been etched to define a profile similar to the microwave circuit, the conductive layer defining the microwave circuit has been deposited on the etched profile of the resistive layer, and the two ports are electrically connected by the vias to the microwave circuit.

Alternatively the resistive layer may define a discrete resistor, conductive pads are secured to the resistor, the conductive layer is formed on the opposite side of the dielectric membrane to the discrete resistor, and the two ports are electrically connected by the vias one to each of the conductive pads.

The conductive pads are preferably formed of copper. The multi-layer laminate preferably includes a copper foil covering the resistive layer, the copper foil having been etched to define the conductive pads.

The dielectric membrane is preferably formed from expanded poly-tetra-flouro-ethelyene impregnated with a thermoset resin. The conductive layer is preferably formed from copper.

According to another aspect of the invention, a manifold power splitter/combiner comprises a multi-layer laminate defining a plurality of microwave power splitters/combiners each as hereinbefore specified, the conductive layer being etched to define the electrical connections between the microwave circuits of the power splitters/combiners.

According to another aspect of the invention, a method of manufacturing a microwave power splitter/combiner comprises forming a laminate including a substrate carrying a resistive layer, a conductive layer carried by the resistive layer, a dielectric membrane covering the conductive layer, and at least three ports arranged on the opposite side of the dielectric layer to the conductive layer, including etching the resistive layer and the conductive layer to define a microwave circuit for the microwave power splitter/combiner with an integral resistor, and forming electrically conductive vias through the dielectric membrane to connect the ports to the microwave circuit.

According to a further aspect of the invention, a method of manufacturing a microwave power splitter/combiner comprises forming a laminate including a substrate carrying a resistive layer, a first conductive layer carried by the resistive layer, a dielectric membrane covering the first conductive layer, and a second conductive layer covering the dielectric membrane, and includes etching the resistive layer and the first conductive layer to define a discrete resistor having conductive pads, etching the second conductive layer to define a microwave circuit of the power splitter/combiner, and forming electrically conductive vias through the dielectric membrane to connect two ports of the microwave circuit one to each of the conductive pads.

According to yet another aspect of the invention, a method of manufacturing a manifold power splitter/combiner comprises forming a laminate including a substrate carrying a resistive layer, a first conductive layer carried by the resistive layer, a dielectric membrane covering the first conductive layer, and a second conductive layer covering the dielectric membrane, and includes etching the resistive layer and the first conductive layer to define a plurality of discrete resistors each having conductive pads, etching the second conductive layer to define an equivalent plurality of ported microwave circuits of power splitters/combiners together with electrical interconnections, and forming electrically conductive vias through the dielectric membrane to connect two ports of each ported microwave circuit one to each of the conductive pads of one of the discrete resistors.

The method may also include testing the value of each resistor before placing the dielectric membrane over the conductive pads.

The method may further include adjusting the value of any resistor to a specified value before placing the dielectric membrane over the resistor.

According to yet another aspect of the invention, the invention resides in a microwave circuit in the form of a multi-layer laminate including a substrate carrying a resistive layer which has been etched to define at least one resistor, a dielectric membrane covering the resistor, a conductive layer defining at least part of an electrical circuit, and said at least one resistor is electrically connected to the conductive layer by vias extending through the dielectric membrane.

In a preferred embodiment, the resistive layer defines a discrete resistor, conductive pads are secured to the resistor, the conductive layer is formed on the opposite side of the dielectric membrane to the discrete resistor, and the vias extend one to each of the conductive pads.

In preferred embodiments of the present invention, the use of a separate resistive layer eliminates resistive elements from the main circuit layer which has the advantage that losses otherwise associated with resistors provided in the main circuit layer are reduced or substantially eliminated. Furthermore, during manufacture of the circuit, DC testing of the resistors can be carried out separately from testing of the main circuit.

In the following description, preferred embodiments of the present invention are described with reference to the manufacture of a particular microwave circuit component—a Wilkinson power splitter/combiner. However, all preferred embodiments described below may be applied to microwave circuits of a general nature having one or more resistors, not necessarily including a Wilkinson power splitter/combiner, and to a method of their manufacture. In particular, preferred embodiments of the present invention may be directed to microwave circuits in general, and to techniques for their manufacture, in the form of a multi-layer laminate having a separate resistive layer to that carrying the main circuit.

With reference toFIGS. 1 and 2, a Wilkinson power splitter/combiner20defines three ports21,22and23which are interconnected by a conductive layer24defining a pair of arms25,26constituting quarter-wave transformers each having a characteristic impedance of 1.414×Z° [or Z°√2] in a well-known manner. The ports22and23are also interconnected by a discrete 2×Z° isolation resistor27carried by a substrate28. Conductive pads29,30are conductively secured to the ends of the discrete resistor27, as shown inFIG. 2, and are electrically connected to the ports22and23by respective plated vias31and32.

As will be described later in detail, the resistor27has been etched, to the size and shape illustrated inFIGS. 1 and 2, from a resistive layer that originally covered the upper surface of the substrate28. The conductive pads29,30are formed from copper that has been plated onto surfaces defined by the ends of the resistor27as illustrated, and then covered by a dielectric membrane33carrying a conductive layer34, for instance of copper, which is etched to define the ported circuit of the Wilkinson splitter/combiner20including ports21,22and23, and the pair of arms25and26. The vias31,32are formed in any convenient manner, for instance by using an excimer laser, followed by electro-plating to provide good electrical connections between the conductive pad29and the port22, and between the conductive pad30and the port23, a plated layer35also being deposited on top of the entire upper profile of the copper sheet34. It should be noted that, whilst the copper sheet34is positioned on top of the dielectric membrane33, the resistor27and its associated conductive pads29and30are encased between the substrate28and the dielectric membrane33.

In use as a microwave power splitter, a microwave input entering port21will be split into equal-phase and equal amplitude outputs at ports22and23.

In use as a microwave power combiner, microwave inputs entering the ports22and23will be combined to produce an output signal at port21.

Although the Wilkinson splitter/combiner20illustrated inFIGS. 1 and 2could be a single electronic component mounted on its own area of laminate27,28,33,34, a plurality of Wilkinson splitters/combiners20could be formed on the same laminate, for instance as illustrated inFIG. 3.

InFIG. 3an eight-way manifold combiner40comprises seven Wilkinson combiners20formed on the same laminate in the manner described with reference toFIGS. 1 and 2, the combiners20having their ports interconnected as shown such that inputs entering the eight input ports41will be combined at the single output port42. By changing the ports so that port42is the input and ports41are the outputs, the eight-way manifold40becomes a splitter. Manifold splitters are used, for instance, as components in the construction of microwave radiating elements, whilst manifold combiners are useful as components in the construction of microwave antennas. AlthoughFIG. 3illustrates an eight-way manifold combiner, different configurations of Wilkinson splitters or combiners can be interconnected to provide different configurations, for instance a six-way manifold combiner or splitter.

The Wilkinson splitter/combiner20, described with reference toFIGS. 1 and 2, can be formed using the method that is now described with reference toFIGS. 4 to 16which diagrammatically show the sequential formation and attachment of the ports22and23to their respective ends of the discrete isolation resistor27. The reference numerals used inFIGS. 1 to 3are used, wherever appropriate, inFIGS. 4 to 16and denote the same features unless stated to the contrary.

The method of manufacture utilises a laminated sheet50, as shown inFIG. 4, comprising a thin layer of resistive material51laminated between a copper foil52and a dielectric sheet defining the substrate28. The layer of resistive material can comprise either a thin-film nickel-phosphorous alloy of about 0.1 to 0.4 microns thick supplied by Ωhmega Technologies Inc. under their trade mark Ohmega-Ply, or a thin film embedded resistor of the type supplied by Gould Electronics Inc. under their trademark TCR.

As shown inFIG. 5, two areas53and54of photoresist are applied to the copper foil52, then exposed and developed. The uncovered area of the copper foil52is then etched, as indicated inFIG. 6, to expose the resistive material51except where it is covered by the photoresist areas53and54and the intervening area of copper foil which will define the conductive pads29and30.

The next stage is shown inFIG. 7and involves stripping the photoresist areas53and54to expose the conductive pads20and30.FIG. 8shows the application of photoresist55to the upper surface of the resistive material51between the conductive pads29and30. An etching solution that does not attack copper is then used to strip the exposed area of the resistive material51as shown inFIG. 9, thereby leaving an area of the resistive material51defining the discrete isolation resistor27.

The next step is to strip the photoresist55to achieve the structure shown inFIG. 10in which the discrete isolation resistor27is carried by the substrate28and carries the conductive pads29and30. At this point in the process it is possible to check the value of the resistor27by applying an appropriate gauge across the pads29and30. If the value of the resistor27is outside acceptable tolerances, the process can either be terminated to save further manufacturing costs, or the resistor27can be adjusted to fall within such tolerances. If the value of the resistor is too low, the portion between the pads29and30can have its surface abraded or pared until an appropriate resistance is achieved. On the other hand, if the value of the resistor is too high, its effective length can be shortened by adding copper to the inwardly-facing end of one of the pads29or30.

FIG. 11shows the addition of further laminates comprising an expanded polytetrafluoroethane (PTFE) dielectric membrane60and a low melting point bonding film61carrying a copper layer62. These layers are pressed against the pads29and30with an appropriate force and at an appropriate temperature until they are completely embedded in the dielectric membrane60. A suitable material for the dielectric membrane60is a sheet of expanded PTFE impregnated with thermosetting resins, such as that manufactured by W L Gore and Associates Inc. of Newark, Del., USA under their trade mark SPEEDBOARD. A suitable material for the bonding film with copper layer is the laminate manufactured by Arlton, Inc. of Lancaster, United Kingdom under their trade mark CuClad 6700.

FIG. 12shows the formation of via holes63and64extending vertically through the copper layer62, the bonding film61and the dielectric membrane60, into the conductive pads29and30. The next step is a plating process, as indicated inFIG. 13, to fill the via holes63,64with a conductive material, such as copper, to form the plated vias31,32, thereby electrically connecting the conductive pads29and30to the copper layer62. During this plating process the surface of the copper layer62becomes covered with a plated layer65thereby enhancing electrical conductivity between the copper layer62and the plated vias31,32.

As shown inFIG. 14, the next step is to apply an area of photoresist66to the plated layer65. Although this area of photoresist66is shown as two separate areas, the actual area is the plan of the splitter or combiner and any associated connections. The two areas of photoresist66are effectively the ports22and23of the splitter or combiner and would, of course, be connected to an adjacent area of photoresist defining the port21and the arms25and26.

Photoresist66is then exposed and developed, and the exposed portions of the plated layer65and the copper layer62are etched away to produce the configuration shown inFIG. 15. The final step is stripping the photoresist66to leave the complete splitter/combiner as shown inFIG. 16.

Although the method of manufacture described with reference toFIGS. 4 to 16is preferred, it may be modified to suit the selection of materials and their associated formation processes.

In an alternative method of manufacture, the area of photoresist55inFIGS. 8 and 9can be increased to cover the entire outline of the Wilkinson power splitter/combiner20illustrated inFIG. 1. In this manner the area of resistive material51will be enlarged to the same size as the outline of the power splitter/combiner20.

Removal of all parts of the layer of resistive material51that are not required for defining the, or each, discrete resistor27produces a splitter/combiner having minimal resistor parasitics.

FIG. 17illustrates the construction of a second embodiment of a single Wilkinson power splitter/combiner. The same reference numerals as those used inFIGS. 1-16are employed to indicate equivalent components and features, and only the ports of difference are described.

The substrate28and the dielectric membrane33are omitted for clarity so that the entire microwave circuit is clearly seen. The multi-layer laminate comprises the unshown substrate28which carries a resistive layer70covered by a first conductive layer71in the form of a 17 um copper foil, the first conductive layer71being covered by an unshown dielectric membrane covered with the conductive layer34constituting a second conductive layer.

This multi-layer laminate has been etched, for instance by using the aforesaid “Gould Process”, or any convenient variant thereof, to leave the illustrated structure. FromFIG. 17is will be noted that the first conductive layer71has been etched to define the pair of arms25and26constituting the quarter-wave transformers, and indeed most of the microwave circuit. The resistive layer70has been etched to the same profile as the first conductive layer71, except that an additional area has been left un-etched to define the resistor27. The second conductive layer34has largely been etched away, leaving only three conductive connectors defining the ports21,22and23. In this manner the unshown substrate28will underlie the resistive layer70, and the unshown dielectric membrane33will be positioned between the upper surface of the first conductive layer71and the lower surface of the second conductive layer34.

Plated vias72,31and32respectively connect the ports21,22and23to the appropriate points of the first conductive layer71as shown. These vias are formed in any convenient manner, for instance by using an excimer laser, followed by electro-plating as for the first embodiment.

It will be noted that these vias72,31and32are hollow. This form of via may also be used in the embodiment illustrated inFIGS. 1-16.

The microwave power splitter/combiner ofFIGS. 1-16has the advantage of minimising the number of vias, but can incur higher resistor parasitics.

On the other hand, the microwave power splitter/combiner ofFIG. 17has the advantage of avoiding asymmetry and discontinuities near the resistor27, but requires an additional via.

While particular materials have been suggested for use in preferred embodiments of the present invention, it will be clear that other materials may be selected without departing from the scope of the invention.