Abstract:
In a system for receiving an input signal, for example, at radar frequencies, having polarization components and transmitting an output signal which is cross polarized with respect to an input signal, separate transmit and receive phase shift networks are utilized. Consequently, the phase shift circuitry utilized need not be reciprocal, in order to provide accurate cross polarization.

Description:
BACKGROUND OF THE INVENTION 
     The invention described herein was made in the course of or under a contract or subcontract thereunder with the Department of the Air Force. 
     This invention relates to reflected and/or otherwise return wave systems, and more particularly, to a system for transmitting an output signal which is cross polarized with respect to an input signal. 
     In the past, cross polarization has been accomplished by the use of a microwave nulling network using reciprocal electromechanical phase shift elements. This is a two-port network in which an input signal is resolved into two orthogonal components. The two outputs are used to derive error signals which are applied to control mechanical phase shifting devices until one of the output ports is nulled. Use of a two-port network in which an error signal is utilized to control a phase shifter to null an error is well-known in the art. Examples are found in, for example, monopulse radar tracking circuits. 
     A desirable improvement in such two-port networks would be the inclusion of solid-state phase shifters in place of mechanical phase shifters. Solid-state phase shifters have improved switching speed which improves system operation. However, an obstacle in the use of solid-state phase shifters is inadequate reciprocity. In other words, while coupling a signal in a first direction from antenna to receiver, a phase shift of a first value is provided, but a phase shift of a different value is provided when transmitting a signal in the opposite direction, from a transmitter to the antenna. 
     SUMMARY OF THE INVENTION 
     It is, therefore, a broad object of the present invention to provide a two-port network comprising a cross polarization repeater utilizing solid-state phase shifters. 
     It is a more particular object of the present invention to provide a two-port network of the type described utilizing a first set of phase shifters for signal transmission in a first direction, and a second set of phase shifters for signal transmission in a second direction, whereby precise cross-polarization may be provided without requiring the use of reciprocal phase shifters. 
     Briefly stated, in accordance with the present invention, there is provided a two-port network incorporating a polarimeter utilizing non-reciprocal phase shifters in which input signals from a dual polarized antenna are coupled to a first network for resolving signals transmitted in a first direction, and a second network coupled to the antenna, also utilizing non-reciprocal phase shifters provides a cross-polarized output signal. A test signal provided from the second network is cross-polarized and coupled to the first network in order to generate an error signal for setting phase shifters in the second network. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The means by which the foregoing objects and features of novelty are achieved are pointed out in the claims forming the concluding portion of the specification. The invention, both as to its organization and manner of operation, may be further understood by reference to the FIGURE which is a block diagrammatic representation of a two-port radio frequency network for cross-polarization constructed in accordance with the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the FIGURE, the two-port network may be broadly described as a first radio frequency network  1  in parallel with a second radio frequency network  2  and both coupled to a dual polarized antenna  3 . Feedback loops described below are provided for nulling of phase errors. The dual polarized antenna  3  provides a first output  9  and a second output  10 , respectively referred to as V and H outputs and which may correspond to outputs indicative of vertically and horizontically polarized components. The received input signal is coupled to the first radio frequency network  1 , from the outputs  9  and  10  by couplers  11  and  12  respectively, which comprise radio frequency coupling means. Couplers  13  and  14  are provided for coupling test inputs (described below) from the network  2  to the network  1 . A double pole double throw switch (or ganged radio frequency switches)  18  is provided having first and second terminals  19  and  20  connected to the network  1 . A 180 degree phase shifter  21  is connected in series with the coupler  14 . When the switch  18  is closed in a first position, the coupler  11  is coupled to the terminal  19  and the coupler  12  is coupled to the terminal  20  to couple the output of the antenna  3  to the network  1 . When the switch  18  is closed in a second position, the coupler  13  is coupled to the terminal  19  and the coupler  14  is coupled to the terminal  20 . The 180 degree phase shifter  21  in series with the coupler  14 , the coupler  13  and switch  18  comprise means  15  for cross-polarizing a test signal (described below) coupled from the network  2  to the network  1 . The network  1  includes cascaded 180 degree hybrids  25  and  26  connected between the switch  18  and a receiver  27 . The hybrids  25  and  26 , as well as the hybrids described below, are conventional 180 degree hybrids well-known in the art having A and B input ports, and C and D output ports. A variable phase shifter  30  is connected between the terminal  19  and the A input of the hybrid  25 ; the B input of the hybrid is connected to the terminal  20 . A variable phase shifter  31  is connected between the C output of the hybrid  25  and the A input of the hybrid  26 . Outputs of the receiver  27  are provided to a servo and controller  35  providing a first output  36  to control the position of the switch  18 , a second output  37  coupled to the phase shifter  30  for determining the value of phase shift provided thereof, and a third output  38  similarly coupled to the phase shifter  31 . 
     For purposes of description, the A and C ports of each 180 degree hybrid are referred to as the upper ports, and the B and D ports are referred to as the lower ports. 
     The servo and controller  35  includes conventional circuitry for providing control signals to solid state phase shifters and timing circuitry for controlling the sequence of operation. The servo and controller  35  is coupled to provide control signals to a transmitter-power unit  40  having an output coupled to an upper port, the C port of a hybrid  42  and providing power to a transmitter-adjust unit  43  providing an input to a lower port, the D port of the hybrid  42 . The transmitter-power unit  40  may, for example, include a traveling wave tube, and the transmitter adjust unit  43  may be a low power oscillator. The hybrid  42  is coupled to another hybrid  45 . A phase shifter  47  is coupled between the upper ports of the hybrids  42  and  45 . The lower, B port of the hybrid  45  is connected to the antenna terminal  10 , and a variable phase shifter  49  is connected between the upper output port of the hybrid  45  and the antenna terminal  10 . Outputs  52  and  53  are respectively connected to the phase shifters  47  and  49  respectively for controlling the value of phase shift provided by each. For maximum speed and frequency response, the phase sifters  30 ,  31 ,  47  and  49  are solid state and may include, for example, conventional voltage-controlled ferrite phase-shifting elements. The phase shifter  30  provides a phase shift having a value a, similarly the phase shifter  31  provides a phase shift, the phase shifter  47  provides a phase shift  6 , and the phase shifter  49  provides a phase shift p. 
     Operation of the Circuit 
     In operation, a signal is received from the antenna  3  in the network  1 , the switch  18  is put in the first position by the servo and controller  35 . The delta channel of the hybrid  26 , the C port, is used to “set” the phase shifters  30  and  31 . This setting is a conventional operation in which the voltage-signal outputs  37  and  38  of the servo and controller  35  operate on the solid-state phase shift elements  30  and  31  until the signal at the C output port of the hybrid  26  nulls, i.e., reduces to zero. The radio frequency network  1 , in conjunction with the servo and controller  35 , may thus be viewed as a polarimeter which measures the polarization of the input signal. 
     The timing circuitry in the servo and controller  35  next closes the switch  18  in its second position and enables the transmitter-adjust unit  43 . A test signal is transmitted into the D port of the hybrid  42 . The test signal emanating from the network  2  has approximately the same polarization components as the input signal from antenna  3  since the shifters  47  and  49  retain the setting given them during the preceding cycle of operation (the assumption being that the polarization of the input changes slowly in comparison to the repetition rate of the test cycles). 
     It is described to match the signal thus coupled to the network  1  to the signal which was coupled from the antenna  3 . The phase shifters  30  and  31  retain their setting. The servo and controller  35  operates to adjust the phase shifters  47  and  49  by varying the voltages on lines  52  and  53  until the signal at the D port of hybrid  26  nulls. The D port is used as the reference port in this instance rather than the C port due to the cross-polarization effect of the network  15 . When the D port output nulls, the phase sifters  47  and  49  are sent such that ρ=α and δ=β. Under these conditions the polarization components of the signal from network  2  exactly match those which had been coupled to network  1  from the antenna  3  just before the reversal of switch  18 . 
     The servo and controller  35  next enables the transmitter power unit  40 . The radio frequency network  2  may be considered to be a polarimeter which modifies the polarization between its input, the C and D ports of the hybrid  42  and its output. The output of the polarimeter comprising the radio frequency network  2  appears at the B port of the hybrid  45  and the terminal of the phase shifter  49  remote from the A port of the hybrid  45 . By virtue of the above operation, the signal emitted from antenna  3  in response to the output of the radio frequency network  2  is cross-polarized with respect to the input signal. (The servo and controller  35  may be programmed by well-known means to disable the receiver  27  during this operation). 
     Mathematically, this operation is demonstrated as follows: 
     First, consider an input signal such that
 
 V =sin              e   jθ   (1)
 
 H =cos            (2)

     Now, if the switch  18  is in the first position, and the shifters are set to provide a delta port, or C port of the hybrid  26 , null, then the phase shifter  30  provides a phase shift of
 
α=π/2−θ  (3)
 
and the phase shifter  31  provides a phase shift of
 
β=π+2             (4)

     It is necessary to set the phase shifters  47  and  49  such that
 
ρ=α  (5)
 
δ=β  (6)
 
     Then a transmitted signal from the network  2  would be cross-polarized with respect to the input signal as described previously. 
     Assume that
 
ρ=α=π/2−θ  (7)
 
δ=β=π+2             (8)

     The signals at each port of the hybrids  25 ,  26 ,  42  and  45  are defined in the well-known manner as follows: A=(C+D)/√{square root over (2)}, B=(D−C)/√{square root over (2)}, C=(A−B)/√{square root over (2)}, D=(A+B)/√{square root over (2)}. 
     The signals at the various points of the circuit are as follows (the convention is used of denoting a terminal by a letter corresponding to a port followed by a number denoting the hybrid including that port): 
     C42=O 
     D42=A 
     A42=A/√{square root over (2)} 
     B42=A/√{square root over (2)} 
     C45=A/√{square root over (2)}e j(π+2               )  
     D45=A/√{square root over (2)} 
     A45=A/2e j(π+2               )+A/2  
     B45=A/2−A/2e j(π+2               ) 
               Signal at coupler 14     =         A   /   2     ⁢     ⅇ     j   (         3   ⁢   π     2     +     2   ⁢   ⁢   ⁢     -   ϕ     )         +       A   /   2     ⁢     ⅇ     j   ⁡     (       π   2     -   ϕ     )                   
Signal at coupler  13 =A/2−A/2 e j(π−2               )  
 
which reduces to
 
Signal at coupler  14 =A sin          e j(             −θ)  
 
Signal at coupler  13 =A cos          e j           
 
With the switch  18  set in the second position,
 
Signal at  19 =A cos          e j           
 
Signal at  20 =A sin          e j(π+             −θ) 

               A   ⁢           ⁢   25     =     A   ⁢           ⁢   cos   ⁢           ⁢   ⁢           ⁢     ⅇ     j   (       π   2     +     ⁢     -   ϕ     )               
B25=A sin            e j(π+             −θ)  
 
Removing the common factors for convenience only,
 
A25=cos          e jo  
 
B25=sin          e j π/2 

     
       
         
           
             
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               ⁢ 
               
                   
               
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               25 
             
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                 1 
                 
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                     ( 
                     
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               D 
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               25 
             
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                 1 
                 
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               A 
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               26 
             
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               B   ⁢           ⁢   26     =       1     2       ⁢     ⅇ     j   ⁡     (   )                 
C26=1 e j(π+               )  
 
D26=0
 
Note that in this case the sigma or D port of the hybrid  26  has been nulled. Therefore, the outputs  52  and  53  can be used to set the phase shifters  47  and  49  to the correct values.

     Many departures may be made from the exact circuitry of the FIGURE. For example, with the proper alteration of the networks  1  and  2  the transmitter-adjust unit  43  and transmitter-power unit  40  may both be coupled to either the C or D port of the hybrid  42 , or their connections as described above may be reversed. Also, a single transmit function may be used to produce a test signal and a transmitted signal. The dual function of the preferred embodiment is utilized to minimize the test signal. Circuit modifications may then be made in accordance with the above teachings. For example, modification in the means  15  for the cross-polarization may be required. To cross polarize a signal coupled from the network  2  to the network  1 , it is necessary to reverse its polarity and add a 180 degree phase shift in series with a coupler  13  or  14 . In order to produce the desired phase shifts, it may be necessary to null a different one of the C and D ports of the hybrid  26  during different portions of the operating cycle described above. Which of the ports is the correct one to null for a given configuration can be determined by using the form of analysis set forth above. 
     As described above, one test signal is coupled from the network  2  to the network  1  for each input pulse received from the antenna  3 . However, the servo and controller  35  may be preset by well-known means to provide a plurality of test signals and adjustments, e.g. three, for each input pulse. The number of iterations performed in setting the values of p and  6  for the phase shifters  47  and  49  respectively for each input pulse is a matter of choice based on servo loop analysis, primarily taking into account gain considerations. 
     It should also be pointed out that the system depicted is digital. The various signals, i.e. the input pulse and test signal(s) are produced only during small percentage of the interpulse period of the pulse train of which the input pulse discussed above is a part. The phase shifters  30 ,  31 ,  47  and  49  are solid state and have fast response times. Consequently, the cross-polarization repeater of the present invention is capable of responding to pulses within a plurality of interlaced pulse trains. 
     It is noted that a cross-polarization repeater has been built in accordance with the present invention utilizing presently available, non-reciprocal solid state phase shifters providing an accuracy of better than one degree, even in response to a dynamic polarization input pulse train. In a prior circuit utilizing similar phase shifters in a reciprocal mode, errors of at least five degrees would be provided. In typical applications, an output having this error is not useable. 
     What is thus provided is a cross-polarization repeater providing substantially exact cross-polarization without the requirement for reciprocal phase shifters. Consequently, solid state phase shifters may be utilized which are faster in response and have better frequency response than mechanical phase shifters. Those skilled in the art should be able to construct a circuit in accordance with the present invention differing in specific details from the preferred embodiment in accordance with the above teachings.