Abstract:
A coupled oscillator arrangement is provided comprising first and second VCOs (voltage controlled oscillators), and a bi-directional interconnect which injection locks the first and second VCOs to each other. Advantageously, this provides a mechanism which effectively eliminates the phase jitter between the phase of first and second VCOs which makes them suitable for use in a diversity receiver, while at the same time providing a redundancy arrangement in which the failure of one of the VCOs does not effect the functionality of the other. The use of a bi-directional interconnect minimizes the complexity and cost of coupling the two VCOs. Preferably, the bi-directional interconnect injects a first signal representative of an output signal of the first VCO to the output of the second VCO and injects a second signal representative of an output signal of the second VCO to the output of the first VCO.

Description:
FIELD OF THE INVENTION 
     The invention relates to a method and apparatus for the injection lock of redundant oscillators, such as local oscillators in multichannel diversity receivers. 
     BACKGROUND OF THE INVENTION 
     Dual-diversity reception systems feature a pair of receivers configured to provide both diversity reception functionality, and redundancy such that should one of the receivers fail, the remaining receiver can take over thereby avoiding any loss of service. Such dual-diversity arrangements are common in wideband receivers. 
     There is a common relatively low frequency reference oscillator used by each receiver to drive the frequency of a respective much higher frequency local oscillator through a phase locked loop. Two independent higher frequency local oscillators are required for redundancy. 
     Diversity outputs are often combined using maximum ratio combining. It is important that the relative phase of the diversity outputs be somewhat constant within a burst for this maximum ratio combining to be effective. Using conventional phase locked loops avoids the requirement for complex custom multiloop phase locked loops. However, these phase locked loops utilize dividers with large local oscillator-to-reference frequency ratios, for example as large as 50000. This means that the high frequency oscillators in the two receivers are updated relatively infrequently, for example every 0.2 μs. The relative phases of the two local oscillators are thus allowed to drift relative to one another. The result is considerable phase jitter between the two oscillators&#39; output frequencies even though they are derived from the same reference. 
     While it would be possible to have both receivers configured to exist on the same circuit board using a common oscillator, this defeats one of the advantages of having a diversity reception system, namely that protection is provided against single point failure. If one receiver fails, the remaining of the two receivers can continue to be used with some reduction in sensitivity. The fact that the two receivers are implemented on separate circuit boards further complicates the task of getting the oscillators to operate in phase. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to obviate or mitigate one or more of the above-identified disadvantages. 
     A broad aspect of the invention provides a coupled oscillator arrangement comprising first and second VCOs (voltage controlled oscillators), and a bi-directional interconnect which injection locks the first and second VCOs to each other. Advantageously, this provides a mechanism which effectively eliminates the phase jitter between the phase of first and second VCOs which makes them suitable for use in a diversity receiver, while at the same time providing a redundancy arrangement in which the failure of one of the VCOs does not effect the functionality of the other. The use of a bi-directional interconnect minimizes the complexity and cost of coupling the two VCOs. 
     Preferably, the bi-directional interconnect injects a first signal representative of an output signal of the first VCO to the output of the second VCO and injects a second signal representative of an output signal of the second VCO to the output of the first VCO. 
     Preferably, the bi-directional interconnect has a first coupler into which is coupled a first coupled signal which is a coupled portion of the output signal of the first VCO, a second coupler into which is coupled a second coupled signal which is a portion of the output signal of the second VCO, and a conductive path which conducts the first coupled portion to the output of the second VCO and simultaneously conducts the second coupled portion to the output of the first VCO. 
     The conductive path might for example be a first conductive portion from a first end of the first coupler to a first end of the second coupler, a second conductive portion from a second end of the first coupler to the output of the first VCO, and a third conductive portion from a second end of the second coupler to the output of the second VCO. 
     Signal isolation is preferably provided with a buffer arrangement consisting of a first buffer between the output of the first VCO and the first coupler, a second buffer between the output of the second VCO and the second coupler, a third buffer which buffers the signal connected to the output of the first VCO and a fourth buffer which buffers the signal connected to the output of the second VCO. 
     Typically, the first and second VCOs form part of respective receiver paths and have their respective support structures, for example respective printed circuit boards. The interconnect in such cases may include an interconnecting wire which physically connects the two separate printed circuit boards. 
     Furthermore, the first VCO would typically be part of a first phase locked loop and the second VCO would typically be part of a second phase locked loop. In such embodiments, a reference oscillator is connected to function as a reference for each of the first and second phase locked loops, typically through a divider circuit. 
     In some embodiments, it may be necessary or convenient to provide a phase adjust circuit, wherein the divided reference oscillator signal is passed directly to the first phase locked loop, and is passed to the second phase locked loop through the phase adjust circuit. 
     Embodiments also provide a receiver and a wide band receiver including such an oscillator arrangement. 
     Another embodiment provides a coupled oscillator arrangement comprising first and second VCOs (voltage controlled oscillators), and a first unidirectional interconnect which injection locks the first VCO to the second VCO and a second unidirectional interconnect which injection locks the second VCO to the first VCO. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the invention will now be described with reference to the attached drawings in which: 
     FIG. 1 is a circuit diagram of a coupled oscillator arrangement featuring the injection lock of redundant oscillators, according to a first embodiment of the invention; and 
     FIG. 2 is a circuit diagram of a coupled oscillator arrangement featuring the injection lock of redundant oscillators, according to a second embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring firstly to FIG. 1, shown is a coupled oscillator system provided by an embodiment of the invention designed to provide two independent local oscillator signals which have frequencies based on the frequency of a common reference oscillator, with very little phase jitter between the two local oscillator signals. A reference oscillator  10  is shown connected to a divider circuit  12  the output of which is connected directly to a first phase locked loop  14 . The divider circuit  12  output is also connected to a second phase locked loop  16  through a phase adjust block  18 . An oscillator coupling block  18  is connected to each of the phase locked loops  14 , 16 , and has first and second outputs  20 , 22  for producing output signals A and B. 
     Each of the phase locked loops  14 , 16  features a respective phase comparator/mixer  30 , 32  connected to the divider  12  either directly or through the phase adjust block  18 . Within each phase locked loop  14 , 16  each phase comparator/mixer  30 , 32  has an output connected to a respective loop filter  34 , 36  which is in turn connected to a respective local voltage controlled oscillator  38 , 40 . The output  42 , 44  of each local oscillator  38 , 40  is connected through a respective divider  46 , 48  to the respective phase comparator/mixer  30 , 32 . 
     The reference oscillator  10  might for example be a 4.8 MHz oscillator, and the divider circuit  12  might divide this frequency by 24 to produce a 200 KHz divided reference signal which is input to the two phase locked loops  14 , 16 . The nominal output frequency produced by the phase locked loops  14 , 16  might for example be 1740 MHz. The dividers  46 , 48  divide the divided local oscillator signal each having a frequency which nominally matches the frequency of the signals produced by the respective local oscillator  38 , 40  to produce a respective divided reference frequency. For our example in which the nominal local oscillator frequency is 1740 MHz, and the divided reference frequency is 200 MHz, the dividers  46 , 48  divide the nominal local oscillator frequency of 1740 MHz by 8700 to produce a signal with a nominal frequency of 200 KHz. 
     The coupling block  18  has first and second buffer amplifiers  50 , 52  both having gain G 1  which are connected to the outputs  42 , 44  of the local oscillators  38 , 40 . Output traces  54 , 56  carry output signals of the buffer amplifiers  50 , 52  which constitute the output signals A,B of the coupled oscillator arrangement as a whole which are passed on at outputs  20 , 22  to other components in the receivers, for example to mixer circuitry. The coupling block  18  has third and fourth buffer amplifiers  60 , 62  both having gain G 2  each having an output  64 , 66  connected to the local oscillator outputs  42 , 44  in the phase locked loops  14 , 16 . A first coupling circuit trace  70  is shown which has a first portion  72  providing a connection to an interconnect  75 , a second “coupling” portion  74  running parallel to the output trace  54  for the first phase locked loop  14 , and a third portion  76  connected to the input of the third buffer amplifier  60 . Similarly, a second circuit trace  78  is shown which has a first portion  80  providing a connection to the interconnect  75 , a second “coupling” portion  82  running parallel to the output trace  56  for the second phase locked loop  16 , and a third portion  84  connected to the input of the fourth buffer amplifier  62 . In a typical application, the first phase locked loop  14 , buffer amplifiers  50 , 60 , output circuit trace  54  and circuit trace  70  would be located on a first printed circuit board  71 . The second phase locked loop  16 , buffer amplifiers  52 , 62 , output circuit trace  56  and circuit trace  72  would be located on a second printed circuit board  73 , and the interconnect  75  provides an interconnection between the two printed circuit boards  71 , 73 . Typically, the reference oscillator is on a separate shelf supplying multiple shelves of equipment. 
     The interconnect  75  is preferably a transmission line or at least has transmission characteristics having controlled impedance which is matched to the points on the two circuit traces  70 , 78 . 
     In the absence of the coupling circuit  18 , the divider circuit  46  in each phase locked loop  14 , 16  produces a divided version of the signal produced at local oscillator&#39;s output  42 , 44 , and this divided version is compared by the phase comparator/mixer  30 , 32  to the divided reference signal produced at the output of divider  12 . Any difference is translated by the loop filters  34 , 36  into an adjustment to the voltage driving the local oscillator devices  38 , 40 , either to increase or decrease the local oscillator frequency such that it more closely tracks the reference oscillator. This is conventional phase locked loop functionality. 
     According to an embodiment of the invention, a first directional coupling  75  is achieved by the proximal location of output trace  54  to coupling portion  74 . A second directional coupling  77  is similarly achieved by the proximal location of output trace  56  with coupling portion  94 . More specifically, the coupling portions  74 , 82  of the circuit traces  70 , 78  are directionally coupled to the output traces  54 , 56  in such a manner that there is a coupling coefficient C, and a directivity coefficient D. The coupling coefficient C is a measure of how much energy in a signal propagating in output traces  54 , 56  is coupled into the coupling portion  74 , 82  is a direction opposite to the propagation direction in the output traces  54 , 56 . The directivity coefficient D is a measure of how much energy is coupled into the coupling portion  74 , 82  in the same direction as the propagation direction in the output traces  54 , 56 . The coupling coefficient C is much larger than the directivity coefficient D. For example, the coupling coefficient C might be −20 dB, while the directivity coefficient D might by −40 dB. Thus, the coupling of the first coupling portion  74  to the first output trace  54  results in a coupled signal CxA  90  travelling in the coupling portion  74  in a direction opposite to the direction of propagation of signal A in the output trace  54 , and a directivity signal D×A  92  travelling in the coupling portion  74  in a direction the same as the direction of propagation of signal A in the output trace  54 . Similarly, the coupling of the second coupling portion  82  to the second output trace  56  results in a coupled signal C×B  94  and a directivity signal DxB  96  as indicated. The input the third buffer amplifier  60  is thus the directivity component D×A  92  from the first coupling portion  74  plus the coupling component CxB  94  from the second coupling portion  82  which is received through the interconnect  75 , i.e. CxB+DxA. Similarly, the input to the fourth buffer amplifier  62  is CxA+DxB. In both cases, the coupled components are orders of magnitude larger than the directivity components, and as such the inputs to the third and fourth buffer amplifiers  60 , 62  may be approximated by CxB and CxA respectively. 
     Preferably, the length of the coupling portions  74 , 82  have a length which is one quarter the wavelength of the signal being coupled. Shorter lines with impedance adjustments can achieve similar coupling and directivity over a smaller bandwidth. 
     After passing through the third buffer amplifier  60 , a signal effectively equal to CxG 2 xB is connected to the output  42  of the local oscillator  38  in the first phase locked loop  14 . Similarly, a signal effectively equal to CxG 2 xA is connected to the output  44  of the local oscillator  40  in the second phase locked loop  16 . It is a known fact that when a signal source is applied to the output of a voltage controlled oscillator, the voltage controlled oscillator will “injection lock” to the signal source if the frequencies are similar. Preferably, the combined signals are also similar in amplitude. Examining the signals being combined at the output  42  of the first VCO  38  by way of example, prior to passing the output signal through buffer amplifier  50 , the VCO produces a signal effectively equal to 1/G 1 xA. Similarly, the signal fedback from the other circuit board as indicated previously can be approximated by CxG 2 xA. In order for the amplitudes to be approximately equal, we require that 1/G 1  approximately to equal CxG 2 . This is not an exact requirement however. It is simply preferred that the amplitudes be similar when combined. In addition to having the appropriate gain, the buffer amplifiers should also provide a level of isolation, and preferably at least as much isolation as provided by the directional coupling between the output traces  54 , 56  and the coupling portions  74 , 82 . 
     With the instant circuit, when both oscillators  38 , 40  are working, the output of each oscillator is coupled and injection locked to the other resulting in the elimination of phase jitter between the two outputs. If either of the oscillators fails, the remaining oscillator can continue to function independently. Advantageously, this is achieved using a unique arrangement which requires only a single interconnect  75 . While in these examples, it has been assumed that there are two printed circuit board  71 , 73 , more generally the circuitry may be realized on one, two or any other suitable number of circuit boards or support structures. 
     Any suitable conducting interconnect can be used to fulfil the function of interconnect  75 . For example, if a single circuit board is employed then a circuit trace could be used. However, the interconnect should be injection matched to the interface on the two printed circuit boards, i.e. it should behave like a transmission line. 
     It is noted that mutually phase injecting two oscillators can cause instability. The oscillator phase and frequency are determined by the phase locked loop elements. Having an oscillator try to injection lock and satisfy PLL loop equation simultaneously is only possible if the two phases are very similar. For this purpose, the phase adjust block  18  is provided to allow an adjustment of the phase between the two references to ensure that the phase locked loop phase and injection lock phases agree. Alternatively, the lengths of all the various lines, traces and connections could be carefully designed such that no phase inconsistency results. 
     FIG. 2 is a circuit diagram for a coupling block according to another embodiment of the invention. Components of the coupling block in FIG. 2 which are in common with the coupling block  18  of FIG. 1 are similarly numbered. In this embodiment, a first circuit trace  100  couples a signal on output trace  54  through a first interconnect  104  to the input of buffer amplifier  62 . Similarly, a second circuit trace  102  couples a signal output trace  56  through a second interconnect  106  to the input of buffer amplifier  66 . The difference between this embodiment and the previous embodiment is that rather than having a single interconnect  75  to bi-directionally couple between the circuit traces on the two printed circuit boards  71 , 73  of FIG. 1, two separate unidirectional interconnect wires  104 , 106  are used. 
     Numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practised otherwise than as specifically described herein. 
     In the preferred embodiment of the invention, the above discussed coupled oscillator system is applied in a wideband receiver using maximum ratio combining. However, more generally it could be used in any application, system or method requiring the phase tracking of two or more independent sources. 
     In the illustrated embodiments, directional coupling has been achieved by the proximal adjacent location of coupling portions  74 , 82  which are portions of larger circuit traces, and the output traces  54 , 56 . It is to be understood that other approaches to obtaining a coupled portion of the output signals may be employed. For example, one can couple multiple traces by cascading the coupled lines to reduce interference among the coupled signals. 
     All of the illustrated embodiments have included two phase lock loops which are to be coupled together. More generally, a larger number of phase lock loops may be coupled together in a similar manner, by providing a coupling between the signals produced by each phase locked loop with the signal produced by each other phase locked loop. Wilkinson splitters might be used for example to split coupling signals such that they may be coupled to multiple outputs. For example, in the case that there are three receivers, each output would have a directional coupling to produce a coupled signal. Each coupled signal would then be passed through a Wilkinson splitter (or other appropriate splitting device) to produce two copies of the coupled signal. Then each of the coupled signals would be connected to the output of the remaining two receivers.