Abstract:
A circuit is provided where impedance converters ( 41-48 ) are provided such that active components (P 1  to P 8 ) can be switched off without power from the other active components leaking away through those components which are switched off.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a signal combiner, a signal splitter and to a circuit using these components. 
     BACKGROUND OF THE INVENTION 
     It is known to combine the outputs of a number of devices, such as high power amplifiers. This can be advantageous as it may deliver a method of obtaining higher output powers than might otherwise conveniently be obtainable, for example in a radio base station, or it may enable efficiencies of amplifiers to be increased by running a plurality of amplifiers of different power outputs in combination in order to achieve a desired power output. There may also be other instances in which it is desirable to combine the outputs of electronic devices. 
     Combiners are known. For example a Wilkinson Combiner System will work acceptably to combine the outputs of a plurality of amplifiers provided that all of the amplifiers are operating. However, should one amplifier fail, then the loss of output power is many times greater than the original contribution of that amplifier. This is due to the fact that not only is the contribution from that amplifier lost, but the amplifier can now become a load and the power from the other amplifiers is now wasted as it flows through the output impedance of the amplifier plus the balancing loads present as part of the combiner. The balancing loads therefore need to be adequately rated to cope with the possibility of amplifier failure and hence can become very large, possibly needing to be rated at several hundred watts in some cases. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention, there is provided a signal combiner having a plurality of signal input paths, wherein each signal path comprises an input impedance transforming device and switching means for selectively connecting the input of the impedance transforming device to a predetermined impedance, the output of the impedance transforming devices being connected together in a summing configuration. 
     By providing an input impedance transforming device, the effect of failure of a device connected to an input of the signal combiner can be reduced or alleviated. 
     Advantageously the switching means associated with one or more of the impedance transforming devices is arranged to connect the input of the associated impedance transforming device to ground. The advantage of this is that a switching mean does not have to be rated to handle the full amount of power which might be provided to the signal input path. For example, if the combiner was used to combine the outputs of several RF amplifiers, each rated at several hundred watts, the switch itself is not involved in connecting the output of the amplifier to an input of the signal combiner, and therefore does not have to be rated to handle several hundred watts of power and neither can it degrade the signal path. The switch merely serves to ground the input of the impedance transforming device when the amplifier is switched off or in a fault mode. 
     Preferably the or each impedance transforming device is implemented using a transmission line. 
     When the input of the impedance transforming device, formed from a transmission line, is shorted to ground, the transmission line becomes mismatched. This mismatch causes the impedance presented at the output of the transmission line to be transformed. Thus for a circuit having a nominal centre operating frequency F 0  having an electrical wavelength δ, the impedance presented by the output of a transmission line having a characteristic impedance Z 0  and a length 
       λ   4       
 
is given by:
 
 Z  out= Z   0   2   ÷Z   load 
 
     When the input of the transmission line is shorted to ground, the impedance at its output rises to infinity for the ideal (loss less) transmission line. 
     For a combiner having N input channels, and consequently N impedance converters, each nth converter, where n is a integer in the range 1-N, has a output connected to a nth node, and the nth node is connected to the (n−1)th node by an (n−1)th linking element. 
     Advantageously each linking element is also a transmission line. Transmission lines of an odd number of ¼ wavelengths, ie 
         λ   4     ,           ⁢       3   ⁢   λ     4     ,           ⁢       5   ⁢   λ     4         
 
and so on are preferred.
 
     According to a second aspect of the invention, there is provided a device comprising plurality of channels, and a combiner according to the first aspect of the present invention. 
     Advantageously the electronic device further comprises a splitter and a plurality of signal processing devices. As used herein the term signal processing device is intended to include a device which changes the frequency, amplitude or phase of a signal passing therethrough. Advantageously the signal processing devices are active devices, such as amplifiers. 
     Advantageously the splitter has a plurality of outputs. An input signal to the splitter may be divided equally amongst the outputs. Advantageously each of the outputs is in phase with each other. 
     Preferably the splitter has N outputs and the phase difference between an nth and an (n−1)th output is 
         λ   4     ,       
 
where n is a positive non zero integer in the range 1 to N. Advantageously the delay applied to the nth output is 
           (     n   ⁢           ⁢   λ     )     4     .       
 
     Advantageously the output of each channel can be selectively connected to the signal processing device, which signal processing device presents a matched load, or to a terminating load. Thus the active signal processing device can be switched out of the channel without its removal causing substantial changes to the performance of any other of the channels, as the terminating load prevents reflections. 
     In an arrangement where the active devices are amplifiers, the combined electronic device which has a plurality of amplifier channels can be left operating whilst a amplifier in any one channel is switched off for maintenance or removal, thereby giving rise to a circuit which allows “hot swapping” of active components. 
     According to a third aspect of the present invention, there is provided a combined splitter and phase compensation device for use in combination with the combiner according to the first aspect of the present invention, wherein the combined splitter and phase compensation device has N output channels and the phase difference between a nth channel and an (n−1)th channel of the splitter and phase compensator matches he path length difference between a nth channel and an (n−1)th channel of the combiner, where n is an integer in the range 1 to N. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will further be described, by way of example, with reference to the accompanying FIGURE which discloses the multi-channel amplifier constituting an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The arrangement shown in the accompanying FIGURE comprises a eight way 0° splitter  2 , a phase compensation section indicated generally as  4 , eight power amplifiers P 1  to P 8  and an eight way combiner indicated generally as  6 . The circuit is designed to amplify an RF input having a carrier frequency F 0  which has a wave length δ 0  in a transmission line. The splitter  2  accepts an RF signal at its input  10  and splits this equally in amplitude and without introducing phase shifts between its eight outputs  11  to  18 . The first output is connected to a first delay line  21  introducing a phase delay of 
         λ   4     .       
 
The second output  12  is connected to a delay line  22  introducing an delay of 
           2   ⁢   λ     4     .       
 
The third output  23  is connected to a third delay line  23  introducing a delay of 
           3   ⁢   λ     4     ,       
 
and so on up to the 8th output which is connected to a delay line  28  introducing a delay of 
           8   ⁢   λ     4     .       
 
The output of each delay line can then be connected to an associated amplifier via a switch. Thus the output of the first delay line  21  can be connected to the input of amplifier P 1  via a switch  31 . Similarly the output of the second delay line  22  is connected to the amplifier P 2  via a second switch  32 , and so on for each channel. Each of the switches  31  to  38  is a two position switch, such that if the delay line is not connected to the associated amplifier, then the output of the delay line is connected to ground via a terminating load having the same characteristic impedance as the transmission lines used to form the delay lines.
 
     The output of each of the amplifiers P 1  to P 8  is connected to an associated impedance transforming element  41  to  48  which in this example is constituted by a transmission line of wavelength λ/4. The output of the first impedance transforming element  41  is connected to the output of the second impedance transforming element  42  via a first linking element  61 . Similarly the output of the second impedance transforming element  42  is also connected to the output of a third impedance transforming element  43  via a second linking element  62 , and so on. Each of the linking elements  61  to  67  is, in this example, constituted by a transmission line having a wavelength of λ/4. 
     Each of the impedance transforming element  41  to  48  is also associated with a switch  71  to  78  which can operate to short the input of the impedance transforming element to ground. 
     Node  51  represents an output node which is connected to an input of a band pass filter  80 . 
     In use, should any of the amplifiers fail or need replacing, the switches associated with that amplifier are operated to remove the amplifier from the circuit. Thus, for example, if amplifier P 5  fails, then switch  35  is operated to connect the output of the delay element  25  to the associated matched load, and switch  75  is operated to short the input of the ¼ wavelength transmission line  45  to ground. The impedance transforming property of the transmission line means that, at the operating frequency F 0 , the short circuit to ground at its input is transformed into an open circuit (high impedance) at node  55 . Thus the outputs of the other amplifiers do not see a leakage path to ground via the impedance converter  45 . Consequently if an amplifier is removed from operation then the power loss from the output is approximately equivalent to only the amount of power that the amplifier provides. Further power loss does not occur as in the prior art. 
     When each amplifier is working, its output is summed with the outputs of the other amplifiers. 
     Due to the topology of the circuit, the signals at the output of amplifier P 1  only have a short path to the output, whereas the signals at the output of the amplifier P 8  have a much longer path as these must travel through more ¼ wavelength transmission lines. It is therefore necessary to compensate the phases of the signals supplied to the input of the amplifier. The variable length delay lines  21  to  28  provide this compensation such that the delay paths in the splitter match the delay paths in the combiner. 
     It will be appreciated that the layout of the combiner needs to be carefully controlled in order to ensure that losses or reflections are kept to a minimum. 
     In the above description, it has been assumed that each of the amplifiers contributes equally. However this need not be the case and the amplifiers may, for example, be binary weighted such that the output of the second amplifier P 2  is twice that of the first, the output of the third amplifier P 3  is twice that of the second and so on. 
     The implementation of the switches has not been described in detail, but these could be fabricated from any suitable switching technique, such as PIN diodes, FET series switches or coaxial relays. The loss in these switches is relatively unimportant since they are only present in the low power part of the system. It will be appreciated that the number of paths or channels may be increased or decreased, and that the output node may also be moved to a different point in the circuit. The switches may be automatically activated by a monitoring device (not shown) which monitors the performance of each one of the amplifiers in order to determine that it is working correctly.