BYPASSABLE WILKINSON DIVIDER

Abstract: The invention relates to a modified Wilkinson power divider used in high-frequency engineering which can be installed either as a power divider or as a lossless transmission path with small stray impedances and with a smaller number of bridging devices (R, B1 to B3) in view of the prior art. The invention is based on that a lossless transmission path is formed by a parallel connection of two non-symmetrical transmission lines (2, 4), of which one is a branch (2) present in the Wilkinson divider and the other is an extra branch (4) formed inside the divider, the impedance of which branch is dimensioned so that the impedance produced by the parallel connection of these two transmission lines is equal to the impedance of an input port (IN) and an output port (OUT1), or theeir geometric mean if they are unequal.

Description: Bypassable Wilkinson divider The invention relates to high-frequency engineering, more exactly to power dividers used in microwave and radio engineering. On high frequencies, especially in microwave and radio engi¬ neering, it is often necessary to split a signal into two or more output ports or combine several signals into one output port. In some solutions the same switching device has to be used either as a power divider from one input port into two output ports or as a lossless transmission line from one input port into one output port as required at each time. This is conventionally imple¬ mented by selection devices, such as bridges, placed on circuit boards. For example, a surface-mounted resistor of zero ohms can operate as a bridging component suitable for industrial mass production. Standard junction lines can also be used. One generally used passive switching device is a so-called Wilkinson divider. The operation of a standard Wilkinson divider appears from Figure 1A. The figure shows a situation in which the signal splits from one input port into two output ports. With respect to the present invention, the divider can be used also in the opposite way for combining a signal from two input ports into one output port. When operating as a power divider, the Wilkinson divider com¬ prises an input port IN, output ports OUT1 and OUT2, a T-junction 1 , a transmission line 2 connecting the input port IN and the output port OUT1 , and a transmission line 3 connecting the input port IN and the output port OUT2. The output ports OUT1 and OUT2 are further connected by a resistor R. The length of the transmission lines is a quarter of wavelength. The characteristic impedance of the input port IN is Z_. The char¬ acteristic impedances of the output ports OUT1 and OUT2 are Z and Z 2 , re¬ spectively. In a simple case, when Z 0 =Z 1 =Z 2l the characteristic impedance of the transmission lines is Z 0 V2 and the impedance of the resistor R is 2Z 0 . In a general case, when Z 0 =Zι=Z 2 does not necessarily hold true, the characteristic impedance of the transmission line 2 is 2Z0Z1 and, corre¬ spondingly, the characteristic impedance of the transmission line 3 is V2Z0Z2 The impedance of the resistor R is then 2VZ1Z2 . A known arrangement for transforming the Wilkinson divider into a lossless transmission line is disclosed in Figures 1A, 2A and 2B. The circuit in Figure 1 B comprises a transmission line 5 with respect to Figure 1A and bridging devices B1 to B5. Figure 2A shows how the Wilkinson divider thus transformed is transformed into a Wilkinson divider according to Figure 1A. In this case, the resistor R and the bridges B1 , B4 and B5 are installed, but not the bridges B2 and B3. The transmission line 5 has in this case no effect on the operation of the divider. It is shown in Figure 2B how the Wilkinson divider is bypassed, that is, transformed into a lossless transmission line. In this case, the resistor R is not installed, nor the bridges B1 , B4 and B5. When only the bridges B2 and B3 are installed, the circuit shown in Figure 2B is a lossless transmission line between the input port IN and the output port OUT1. A disadvantage of the circuit according to Figure 1B is e.g. the great number (5 in this embodiment) of bridging places operating as selection devices and the great number of installed bridges (3 in divider use, 2 as a transmission path). A further disadvantage of the prior art circuit is that the bridges B2 and B3 required for operating as a transmission path cannot be easily produced with small stray impedances since they combine wide lines. Another disadvantage of the prior art circuit becomes evident when the input port IN and the output ports OUT1 and OUT2 are not opposite to one another, especially when the Wilkinson divider is folded to reduce its size. It is more difficult to implement a low-impedance transmission line than a high- impedance transmission line on many substrates. Especially in cases such as this it is difficult or even impossible to fit a wide transmission line within the divider. Furthermore, the arrangement cannot be used at all when the divider is simultaneously being used as an impedance adapter, that is, Zι...Z 0 . The object of the present invention is to provide a power divider which can be installed flexibly, with small changes either as a divider or as a lossless transmission line and which does not share the problems associated with the prior art arrangement described above. The object is achieved with the arrangement according to the characterizing part of claim 1 in which the configuration of the power divider into a lossless transmission line is realized by a parallel connection of two non-symmetrical transmission lines which usually have different impedances. One of the transmission lines is a branch 2 present in the Wilkinson divider and the other is an extra branch 4 formed 5 inside the divider. The preferred embodiment of the invention is explained by means of figures, in which Figure 1A shows a standard Wilkinson divider; Figure 1 B shows a prior art way of transforming the Wilkinson di- vider into a lossless transmission path; Figure 2A illustrates a switching device shown in Figure 1 B in¬ stalled as a Wilkinson divider; Figure 2B illustrates a switching device shown in Figure 1 B in¬ stalled as a lossless transmission path; Figure 3A shows a modified Wilkinson divider according to the invention; Figure 3B shows a modified Wilkinson divider according to the invention folded into as small a space as possible; Figure 4A shows a switching device according to the invention installed as a Wilkinson divider; Figure 4B shows a switching device according to the invention installed as a lossless transmission path. The solution according to the invention is shown in Figure 3A. It is assumed herein that Zι=Z_, but the circuit operates in the same way if The idea of the invention is to implement a transmission line with a characteristic impedance Z 0 by a parallel connection of two narrow high- impedance transmission lines: one transmission line 2 with an impedance Z 0 2, which is already present in a standard Wilkinson divider, and another transmission line 4 with an impedance 2Zo/(2-V2). When Z 0 =50S, the imped¬ ance of the transmission line 2 should be about 70S and the impedance of the transmission line 4 about 170S. The latter impedance cannot be produced on most substrates without special procedures. One such procedure is to etch ground plane from under the 170S line 4. Another way is to place the 170S line 4 very close to the 70S line 2, whereby the interaction between the lines 2 and 4 will raise the impedance of the line 4. It would not be very harmful if the impedance was not exactly at its optimum value. For example, on a 1.6 mm FR-4 substrate or a 0.76 mm teflon substrate, the maximum obtainable characteristic impedance is between 140 and 150S. With this impedance the standing wave ratio (VSWR) will be about 1.1. The operation of the invention is further examined on the basis of Figure 4A. Only the bridge B1 and the resistor R are installed for the splitting operation. As the bridges B2 and B3 are not present, the both branches 2 and 3 of the Wilkinson divider are passages of the signal. The circuit operates now as a standard Wilkinson divider. Non-splitting operation is studied in Figure 4B. The bridge B1 and the resistor R are not installed but the bridges B2 and B3 are installed. In this case, the signal meets the parallel connection of the transmission lines 2 and 4 of a quarter of wavelength, the impedances of which are ZoΛ/2 and 2Zo/(2 2), respectively. A quarter-wavelength long transmission line with the impedance Z 0 is produced by the parallel connection of the impedances. Figure 3B shows how a modified Wilkinson divider according to the invention can be folded in order to minimize the space it takes up on a circuit board. An advantage of the solution according to the invention is that the Wilkinson divider on the same circuit board can be used as required at each time both in splitting and non-splitting operation which will reduce the re¬ quired number of different circuit boards. Also, in the solution according to the invention a smaller number of bridges and places for bridges are needed than in prior art solutions. A further advantage of the solution according to the invention is that very little stray impedance is produced as bridges are needed only in high-impedance lines. Another advantage is that the extra line needed in the Wilkinson divider can easily be fitted into a limited space since the extra line is very narrow. The extra line 4 may also run close to the branch 2 in the Wilkinson divider, as long as the connection is taken into consideration in planning. See e.g. Matthaei, Young and Jones, Microwave filters, impedance- matching networks and coupling structures, Artech House Books, 1980, Fig¬ ure 5.09-1 , p. 219. By means of meandering, a quarter-wavelength long line can be placed into the available space. In the arrangement according to the invention, a power divider, such as a Wilkinson divider, can be configured so that the same component substrate, such as a circuit board, can be used either as a power divider from one input port into two output ports or as a lossless transmission path from one input port into one output port. The changes in the way of operation cause less alterations in the circuit than in conventional solutions. It is evident to those skilled in the art that the art according to the invention can be used in conjunction with other transmission lines, such as microstrips, suspended substrate microstrips, striplines, coaxial lines, copla- nar waveguides or combinations of the above mentioned. The production of transmission lines and bridging devices is not restricted to the example de¬ scribed above, but the field of the invention can vary within the scope of the claims.

Claims: Claims 1. A switching device to be used on high frequencies comprising: - a first port (IN), a second port (OUT1) and a third port (OUT2); - a first quarter-wavelength transmission line (2), when connected between the first port (IN) and the second port (OUT1), combines said ports to one another; - a second quarter-wavelength transmission line (3), when con¬ nected between the first port (IN) and the third port (OUT2), combines said ports to one another; - first selection devices (R) to be installed and second selection devices (B1) to be installed so that only when the first and second selection devices (R, B1 ) are installed, they connect at least one of the transmission lines (2, 3) between the port (IN) and the ports (OUT1 , OUT2); characterized in that it further comprises - a third quarter-wavelength transmission line (4); and - third selection devices (B2, B3) to be installed which, when in¬ stalled, connect the third transmission line (4) in parallel with the first trans¬ mission line (2). 2. A switching device according to claim 1 in which the first port (IN) has a first characteristic impedance Z_, the second port (OUT1 ) has a second characteristic impedance Z and the third port (OUT2) has a third characteristic impedance Z 2 , characterized in that the characteristic im¬ pedance of the third quarter-wavelength transmission line (4) is dimensioned so that the impedance produced by the parallel connection of the first trans¬ mission line (2) and the third transmission line (4) is essentially equal to ZoZi . 3. A switching device according to claim 1 or 2, characterized in that the sum of the numbers of the first, second and third selection devices is less than 6. 4. A switching device according to claim 1 or 2, characterized in that the sum of the numbers of the first, second and third selection devices is 4. 5. A switching device according to any one of claims 2 to 4, characterized in that the first selection device (R) is a resistor whose re- sistance is essentially equal to and that the resistance of the second and third selection devices (B1 to B3) is essentially zero. 6. A switching device according to any one of claims 2 to 5, characterized in that the third transmission line (4) is formed by etching ground plane from under the third transmission line (4). 7. A switching device according to any one of claims 2 to 6, characterized in that the third transmission line (4) is placed especially close to the first transmission line (2), whereby the interaction between the first transmission line (2) and the third transmission line (4) will raise the im- pedance of the third transmission line (4). 8. A switching device according to any one of the preceding claims, characterized in that the switching device is folded by placing both convex and concave curves into the transmission lines (2, 3, 4). 9. A switching device according to any one of the preceding claims, characterized in that the switching device is folded so that the transmission lines (2, 3, 4) are not situated on the same plane. 10. A switching device according to any one of the preceding claims, characterized in that the switching device is implemented by a stripline placed on the surface of a circuit board.

Inventors: DEKKER ANDRE, DEKKER, ANDRE
Applicants: DEKKER ANDRE, NOKIA TELECOMMUNICATIONS OY, DEKKER, ANDRE
Application year: 1996
Main classification: H01P5
IPC codes: H01P5