GB2115613A - Combiner network - Google Patents

Abstract

The present invention relates to a combiner network having n input ports P1, P2, P3,...Pn each port being connected to one of n resistors RD. The n resistors are connected in a star configuration to a common point. Each input port is also connected, by one of n transformers T1, T2, T3,...Tn to an output port for connection to a load RL such as an antenna. The transformers may be replaced by ???4 transmission lines (Fig. 3 not shown). The supply to each port may be a power amplifier with a centre tapped winding on the same core as the aforementioned transformer. <IMAGE>

Description

SPECIFICATION Combiner network The present invention relates to a hybrid combining network used for combining a plurality of RF power output stages. More particularly, the invention relates to a network which contains distributed transformers each only handling, in their secondary circuit, the current of a single output stage.

One embodiment contemplates the use of a series of wound, distributed transformers which provide a broadband system. Another embodiment is narrow band, however, the transformers can be replaced by inexpensive quarter-wave transmission lines. Both embodiments achieve isolation by a unique arrangement of resistors which are connected to a point of floating potential.

The advent of high power transistors has made it possible to construct solid state FF transmitters of high power. This is conveniently done by constructing a plurality of power amplifier stages each producing a power signal and then combining these power signals in a combining network so as to produce a single high power output signal which can be fed to a load, for example, a transmitting antenna. This arrangement is very reliable due to the sole use of solid state devices. In addition, when the combining network is of the isolation type, reliability is further enhanced by the redundancy of the power amplifier stages.

One embodiment of the combining system of the present invention provides total isolation between the input ports of the combining network. As a result, there is no effect on the remaining power amplifiers connected to the input ports of one or more input ports if one or more input ports to the combiner meet with, in the worst case, opened or short circiut conditions.

If one or more power amplifiers fail, the total power output will be reduced and some imbalance currents will flow in the above mentioned unique connection of resistors, however, the remaining operating power amplifiers will be unaffected.

U.S. Patent 4,156,212 which issued to Dennis H. Covill on May 22, 1979 illustrates a combining system wherein a plurality of transformers are lumped or wound around a single core. This transformer is expensive and difficult to manufacture. In that network, unlike the network of the present invention, the turns ratio of the windings of the transformers have to be dimensioned so that there is no net flow of flux in order to achieve isolation. Such constraints are not placed on the transformers of the present combiner.

In another embodiment of the invention, the transformers are replaced in the network by mere quarter-wave transmission lines. These transmission lines, over a fairly narrow frequency range, transform voltage sources at their inputs to current sources at their outputs. The current sources are then added together to form the composite signal. The unique resistive arrangement mentioned above can be connected to the input ports of the combiner, and by choosing the correct values of resistance for these resistors, as a function of the load resistance and the internal impedance of the quarter-wave transmission lines, total mutual isolation of the input ports is achieved.

As a result, it is one object of the present invention to provide a combining network which employs inexpensive transformers, which can be distributed throughout the network, and which do not require a flux cancellation to achieve mutual isolation of the various input ports.

In accordance with an aspect of the invention there is provided a combiner network having n input ports, where n is an integer greater than 1, each port being connected to one of n resistive means, said n resistive means being connected in a star configuration to a common point, each input port being connected, by one of n transformers to an output port for connection to a load.

The present invention will be described hereinbelow in detail with the aid of the accompanying drawings, in which: Figure 1 is a schematic diagram of a broadband embodiment of the present invention; Figure 2 is an alternate embodiment of the transformer used in the embodiment shown in Figure 1; and Figure 3 is a schematic diagram of a narrow band embodiment of the present invention.

Referring to Figure 1, n power amplifiers can be connected to n input ports P1, P2, P3,...., Pn.

Each input port is connected across a transformer T1, T2, T3, ...., Tn, each having a turns ratio of x:1. The secondary windings of the n transformers are series connected. This series connection is, in turn, connected to an output port PO. A load resistor RL is connected across output port P0 and could represent an antenna.

A resistive star is made of n resistors RD, with each connected to a respective input nort. The center of the star does not have to connect to any reference potential.

A power amplifier connected to input port P1 produces a voltage v1 and a current il. The current i1 can be broken down into two components iD1 and iS1 so that i1=iD1+iS1 (1) where iD1 is the component entering the star configuration and iS1 is the component which is combined.

The current flow in the secondary circuit of the series connected transforms is given by the expression v 1 v2 v3 vn -±± +... + x x x x isec= (2) R, Current flowing in the primary of any transformer is merely the current in the secondary divided by the turns ratio x. As a result, from equation (2) the current iS1 can be expressed as v1 +v2+v3+ . . . vn isl= (3) R,X2 The voltage v, appearing at the center of the star is merely the average voltage from the sources v1, v2, v3,.. vn and is given by the equation vl+v2+v3+...vn VD= (4) n The current flowing through R, from P1 is |D1 and is the voltage drop across RD divided by the resistance RD. As a result, iD1 is given by vi-vD iD 1 = (5) Ro Substituting the value of vD from equation (4) into equation (5) (n-1) (v2+v3+ . .. +vn) vi- n n iD1= R0 (6) If y=v2+v3+ ... vn, then from equations (1), (3) and (6) (n-1) Y .v1- - n n v1 y i1= - + ----- + R0 RLX2 RLX2 (7) From equation (7), it can be seen that if Y Y (8) nR D REX2 il would be free of any terms involving v2, v3,....vn and therefore total mutual isolation would be achieved.

From equation (8) it can be seen that when RD=x2RL/n, all input ports are mutually isolated from one another and i1=nv1/x2RL or in general in=nvn/x2R,.

When v1=v2=v3=....=vn then y=(n-1 )v.

Under these conditions 1 D1 =0 and all the power from each power amplifier is combined at the output port PO.

The input impedance at each port is zn=vn/in which is equal to x2 RL/n ohms.

Referring now to Fig. 2, it can be seen that it is possible to combine signals and achieve isolation in a typical push-pull power amplifier configuration by using a 3 winding transformer.

The supply voltage vcc is fed to the push-puii power amplifier stage via a first center tapped winding. The turns ratio between this winding and the other windings is not important with respect to isolation. Isolation is achieved when the turns ratio between the center winding and the secondary winding is x:1 and the value of the resistors RD is x2R,/n.

A narrow band version of the isolating combiner of the present invention is shown in Figure 3. The transformers T1, T2 ...., Tn are replaced by n quarter-wave transmission lines, each having an impedance ZO. One end of each transmission line is connected to a power output stage. The other end of each transmission line is connected to a common point which is connected to a load and forms the output port. The load is defined by the resistance RL and is most commonly an antenna. As in the first embodiment, n resistors RD are connected so that one end of each resistor is connected to an associated output stage and the other end of each resistor is connected to a common point, thereby forming a star configuration.

In the circuit shown in Figure 3, the quarterwave length transmission lines function, at a center frequency, exactly as the transformers in the circuit of Figure 1 and serve to series connect v1, v2,...., vn, as though through a transformer of ratio Zo/RL:1 .

Therefore, if ZJRL is substituted for x in the above equations, the current in is given by n vn RL RD= (9) Z02 Zp, the impedance of the output port is vn/in and equals z02 Zp= (10) nR, The value of resistance RD is given by z02 RD =Zp (11) nR, The hybrid networks shown in Figure 1 and 3 are normally used to combine the output power of a plurality of power output stages to form a single high power output for an antenna, while at the same time, providing mutual isolation between the various input stages. However, the system could be equally used in reverse and in such a configuration it would provide a signal splitter with each output port having a signal whose magnitude was the same. Each output port would also be mutually isolated.

Claims (9)

Claims

1. A combiner network having n input ports, where n is an integer greater than 1, each port being connected to one of n resistive means, said n resistive means being connected in a star configuration to a common point, each input port being connected, by one of n tranformers to an output port for connection to a load.

2. The combiner of claim 1, wherein each of said n transformers has a primary winding and a secondary winding with each primary winding being connected across an associated input port and each of said n secondary windings being connected in series to form first and second terminal ends which represent said output port.

3. The combiner of claim 2, wherein said load has a resistance of R, ohms and each transformer has a primary to secondary turns ratio of x:1, and wherein mutual isolation exists between each input port when the resistance of each of said n resistive means is x2Rin ohms.

4. The combiner of claim 1, wherein each of said n transformers has a first, second and third winding and wherein said first winding of each of said n transformers is connected across one of said n input ports wherein each of said n input ports is connected to one of said n resistive means via said second winding and wherein said third winding of said n transformers are connected in series to form first and second terminal ends which represent said output port.

5. The combiner of claim 4, wherein each input port is for use in connection with a push-pull output stage of a power amplifier and wherein power for said push-pull output stage is supplied via a center tap on said first winding.

6. The combiner of claim 5, wherein said load has a resistance of R, ohms and wherein the turns ratio of said second to said third winding is x:l, and wherein mutual isolation exists between each input port when the resistance of each of said n resistive means is x2RJn ohms.

7. The combiner of claim 1, wherein each of said n transformers are quarter-wave transmission lines each having one end connected to an associated input port and another end connected to a common point which forms, in conjunction with a ground connection, said output port.

8. The combiner of claim 6, wherein said load has a resistance of R, ohms and wherein each of said n quarter-wave transmission lines has an internal impedance of ZO, and wherein mutual isolation exists between each input port when the resistance of each of said n resistive means is ZOVnR, ohms.

9. A combiner network constructed and arranged to operate substantially as herein described with reference to and as illustrated in the accompanying drawings.