Patent Description:
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 <NUM> x Z° [= Z°✔<NUM>], and terminate respectively in the second and third ports which are inter-connected by a <NUM> x 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 <NUM>:<NUM> and <NUM>:<NUM> configurations, they all require a ported circuit defining at least three ports. The invention enables high insertion losses at microwave frequencies to be reduced.

This paper describes a Wilkinson combiner/divider circuit implemented in a <NPL>.

This paper provides a quantitive assessment of the variation in resistance of a NiP layer and formation of these think films: <NPL>.

This paper describes a Wilkinson power divider implemented using a LTCC process: <NPL>.

According to one aspect of the invention there is provided a microwave power splitter/combiner according to claim <NUM>. 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.

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 second 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 according to claim <NUM>, the conductive layer being etched to define the electrical connections between the microwave circuits of the power splitters/combiners.

According to a further aspect of the invention there is provided a method according to claim <NUM>.

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.

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.

The invention is now described, by way of example only, with reference to the accompanying drawings, in which:-.

With reference to <FIG>, a Wilkinson power splitter/combiner <NUM> defines three ports <NUM>, <NUM> and <NUM> which are interconnected by a conductive layer <NUM> defining a pair of arms <NUM>, <NUM> constituting quarter-wave transformers each having a characteristic impedance of <NUM> x Z° [or Z°✔<NUM>] in a well-known manner. The ports <NUM> and <NUM> are also interconnected by a discrete <NUM> x Z° isolation resistor <NUM> carried by a substrate <NUM>. Conductive pads <NUM>, <NUM> are conductively secured to the ends of the discrete resistor <NUM>, as shown in <FIG>, and are electrically connected to the ports <NUM> and <NUM> by respective plated vias <NUM> and <NUM>.

As will be described later in detail, the resistor <NUM> has been etched, to the size and shape illustrated in <FIG>, from a resistive layer that originally covered the upper surface of the substrate <NUM>. The conductive pads <NUM>, <NUM> are formed from copper that has been plated onto surfaces defined by the ends of the resistor <NUM> as illustrated, and then covered by a dielectric membrane <NUM> carrying a conductive layer <NUM>, for instance of copper, which is etched to define the ported circuit of the Wilkinson splitter/combiner <NUM> including ports <NUM>, <NUM> and <NUM>, and the pair of arms <NUM> and <NUM>. The vias <NUM>, <NUM> are formed in any convenient manner, for instance by using an excimer laser, followed by electro-plating to provide good electrical connections between the conductive pad <NUM> and the port <NUM>, and between the conductive pad <NUM> and the port <NUM>, a plated layer <NUM> also being deposited on top of the entire upper profile of the copper sheet <NUM>. It should be noted that, whilst the copper sheet <NUM> is positioned on top of the dielectric membrane <NUM>, the resistor <NUM> and its associated conductive pads <NUM> and <NUM> are encased between the substrate <NUM> and the dielectric membrane <NUM>.

In use as a microwave power splitter, a microwave input entering port <NUM> will be split into equal-phase and equal amplitude outputs at ports <NUM> and <NUM>.

In use as a microwave power combiner, microwave inputs entering the ports <NUM> and <NUM> will be combined to produce an output signal at port <NUM>.

Although the Wilkinson splitter/combiner <NUM> illustrated in <FIG> could be a single electronic component mounted on its own area of laminate <NUM>,.

<NUM>, <NUM>, <NUM>, a plurality of Wilkinson splitters/combiners <NUM> could be formed on the same laminate, for instance as illustrated in <FIG>.

In <FIG> an eight-way manifold combiner <NUM> comprises seven Wilkinson combiners <NUM> formed on the same laminate in the manner described with reference to <FIG>, the combiners <NUM> having their ports interconnected as shown such that inputs entering the eight input ports <NUM> will be combined at the single output port <NUM>. By changing the ports so that port <NUM> is the input and ports <NUM> are the outputs, the eight-way manifold <NUM> becomes 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.

Although <FIG> illustrates 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/combiner <NUM>, described with reference to <FIG>, can be formed using the method that is now described with reference to <FIG> which diagrammatically show the sequential formation and attachment of the ports <NUM> and <NUM> to their respective ends of the discrete isolation resistor <NUM>. The reference numerals used in <FIG> are used, wherever appropriate, in <FIG> and denote the same features unless stated to the contrary.

The method of manufacture utilises a laminated sheet <NUM>, as shown in <FIG>, comprising a thin layer of resistive material <NUM> laminated between a copper foil <NUM> and a dielectric sheet defining the substrate <NUM>. The layer of resistive material can comprise either a thin-film nickel-phosphorous alloy of about <NUM> to <NUM> microns thick supplied by Qhmega 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 in <FIG>, two areas <NUM> and <NUM> of photoresist are applied to the copper foil <NUM>, then exposed and developed. The uncovered area of the copper foil <NUM> is then etched, as indicated in <FIG>, to expose the resistive material <NUM> except where it is covered by the photoresist areas <NUM> and <NUM> and the intervening area of copper foil which will define the conductive pads <NUM> and <NUM>.

The next stage is shown in <FIG> and involves stripping the photoresist areas <NUM> and <NUM> to expose the conductive pads <NUM> and <NUM>. <FIG> shows the application of photoresist <NUM> to the upper surface of the resistive material <NUM> between the conductive pads <NUM> and <NUM>. An etching solution that does not attack copper is then used to strip the exposed area of the resistive material <NUM> as shown in <FIG>, thereby leaving an area of the resistive material <NUM> defining the discrete isolation resistor <NUM>.

The next step is to strip the photoresist <NUM> to achieve the structure shown in <FIG> in which the discrete isolation resistor <NUM> is carried by the substrate <NUM> and carries the conductive pads <NUM> and <NUM>. At this point in the process it is possible to check the value of the resistor <NUM> by applying an appropriate gauge across the pads <NUM> and <NUM>. If the value of the resistor <NUM> is outside acceptable tolerances, the process can either be terminated to save further manufacturing costs, or the resistor <NUM> can be adjusted to fall within such tolerances. If the value of the resistor is too low, the portion between the pads <NUM> and <NUM> can 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 pads <NUM> or <NUM>.

<FIG> shows the addition of further laminates comprising an expanded polytetrafluoroethane (PTFE) dielectric membrane <NUM> and a low melting point bonding film <NUM> carrying a copper layer <NUM>. These layers are pressed against the pads <NUM> and <NUM> with an appropriate force and at an appropriate temperature until they are completely embedded in the dielectric membrane <NUM>. A suitable material for the dielectric membrane <NUM> is a sheet of expanded PTFE impregnated with thermosetting resins, such as that manufactured by W L Gore and Associates Inc. of Newark, Delaware, 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 <NUM>.

<FIG> shows the formation of via holes <NUM> and <NUM> extending vertically through the copper layer <NUM>, the bonding film <NUM> and the dielectric membrane <NUM>, into the conductive pads <NUM> and <NUM>. The next step is a plating process, as indicated in <FIG>, to fill the via holes <NUM>, <NUM> with a conductive material, such as copper, to form the plated vias <NUM>, <NUM>, thereby electrically connecting the conductive pads <NUM> and <NUM> to the copper layer <NUM>. During this plating process the surface of the copper layer <NUM> becomes covered with a plated layer <NUM> thereby enhancing electrical conductivity between the copper layer <NUM> and he plated vias <NUM>, <NUM>.

As shown in <FIG>, the next step is to apply an area of photoresist <NUM> to the plated layer <NUM>. Although this area of photoresist <NUM> is shown as two separate areas, the actual area is the plan of the splitter or combiner and any associated connections. The two areas of photoresist <NUM> are effectively the ports <NUM> and <NUM> of the splitter or combiner and would, of course, be connected to an adjacent area of photoresist defining the port <NUM> and the arms <NUM> and <NUM>.

Photoresist <NUM> is then exposed and developed, and the exposed portions of the plated layer <NUM> and the copper layer <NUM> are etched away to produce the configuration shown in <FIG>. The final step is stripping the photoresist <NUM> to leave the complete splitter/combiner as shown in <FIG>.

Although the method of manufacture described with reference to <FIG> is 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 photoresist <NUM> in <FIG> and <FIG> can be increased to cover the entire outline of the Wilkinson power splitter/combiner <NUM> illustrated in <FIG>. In this manner the area of resistive material <NUM> will be enlarged to the same size as the outline of the power splitter/combiner <NUM>.

Removal of all parts of the layer of resistive material <NUM> that are not required for defining the, or each, discrete resistor <NUM> produces a splitter/combiner having minimal resistor parasitics.

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

The substrate <NUM> and the dielectric membrane <NUM> are omitted for clarity so that the entire microwave circuit is clearly seen. The multi-layer laminate comprises the unshown substrate <NUM> which carries a resistive layer <NUM> covered by a first conductive layer <NUM> in the form of a <NUM> copper foil, the first conductive layer <NUM> being covered by an unshown dielectric membrane covered with the conductive layer <NUM> constituting 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. From <FIG> it will be noted that the first conductive layer <NUM> has been etched to define the pair of arms <NUM> and <NUM> constituting the quarter-wave transformers, and indeed most of the microwave circuit. The resistive layer <NUM> has been etched to the same profile as the first conductive layer <NUM>, except that an additional area has been left un-etched to define the resistor <NUM>. The second conductive layer <NUM> has largely been etched away, leaving only three conductive connectors defining the ports <NUM>, <NUM> and <NUM>. In this manner the unshown substrate <NUM> will underlie the resistive layer <NUM>, and
the unshown dielectric membrane <NUM> will be positioned between the upper surface of the first conductive layer <NUM> and the lower surface of the second conductive layer <NUM>.

Plated vias <NUM>, <NUM> and <NUM> respectively connect the ports <NUM>, <NUM> and <NUM> to the appropriate points of the first conductive layer <NUM> as 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 vias <NUM>, <NUM> and <NUM> are hollow. This form of via may also be used in the embodiment illustrated in <FIG>.

The microwave power splitter/combiner of <FIG> has the advantage of minimising the number of vias, but can incur higher resistor parasitics.

Claim 1:
A microwave power splitter/combiner (<NUM>), having three ports (<NUM>, <NUM>, <NUM>), comprises a multi-layer laminate including a substrate (<NUM>) carrying a resistive layer (<NUM>) defining a resistor (<NUM>); the multi-layer laminate comprising:
a first conductive layer (<NUM>) defining conductive pads (<NUM>, <NUM>);
a dielectric membrane (<NUM>, <NUM>) covering the first conductive layer (<NUM>);
a second conductive layer (<NUM>) covering the dielectric membrane (<NUM>,<NUM>), the second conductive layer (<NUM>) defining at least part of a microwave circuit (<NUM>,<NUM>) that interconnects the three ports; and
electrically conductive vias (<NUM>,<NUM>) extending through the dielectric membrane (<NUM>,<NUM>) between the conductive pads (<NUM>,<NUM>) and the second conductive layer (<NUM>) to electrically connect two of the three ports (<NUM>, <NUM>, <NUM>) across the resistor (<NUM>); and
the first conductive layer (<NUM>) is carried by the resistive layer (<NUM>), and the resistive layer (<NUM>) comprises a nickel-phosphorus alloy.