Abstract:
Electronically scanned array antenna radiating elements include a phase comparator. The phase comparator connects the transmission circuitry to the receiving circuitry via a shunt switch to compare the intended phase with the actual phase. The phase of a signal received via the shunt switch is sensitive to the performance of power amplifiers in the transmission circuitry, transmitter/receiver switch, and load impedance. Distributed attenuators between the shunt switch and a phase comparator reduce crosstalk. Phase comparison circuitry is useful for detecting faults in the ESA radiating element.

Description:
BACKGROUND 
     Electronically scanned array (ESA) systems rely on precise phase disparity between radiating elements to produce a directional beam. Especially in aviation and military applications, precise phase and amplitude control of each radiating element is critical to meet point angle, beam width, and sidelobe requirements. 
     As the power amplifier is driven into the compression region and the output power of the power amplifier is saturated, phase distortion of the power output (AM-PM distortion) becomes significant. The phase of radio frequency (RF) output at the 1 dB compression point is about 22 degree deviated from the phase in the linear region for a 2 W amplifier design. Power output in saturation is less sensitive to input power; however, the output phase becomes much more sensitive to input power. For amplitude taper, the phase of RF output from elements at different power levels in the compression region including saturation may be significantly different as compared to that from elements at much lower power levels in the linear region. The phase imbalance among the elements must be minimized in order to meet high performance system requirements. 
     Calibration methodology based on a table of calibration values (open loop control) cannot accurately control phase to meet high performance requirements when the amplifier is in the compression region. In the compression region, one dB input power variation could cause a phase shift of about 3-4 degrees. In order to achieve 0.5 degree phase accuracy of a high performance ESA system, the calibration method must have 0.25 dB accuracy to control the RF input power. When the amplifier enters saturation, phase shift per dB accelerates substantially. In practice accurately controlling the phase and amplitude at each element is difficult. 
     SUMMARY 
     Accordingly, embodiments of the inventive concepts disclosed herein are directed to a novel method and apparatus for adjusting phase modulation of radiating elements in an ESA. 
     In one aspect, embodiments of the inventive concepts disclosed herein are directed to an ESA radiating element which includes a phase comparator feedback loop. The feedback loop connects the transmission circuitry to the receiving circuitry via a shunt switch to compare the intended phase with the actual phase. A signal received via the shunt switch is sensitive to the performance of the power amplifier, transmitter/receiver switch, and load impedance. 
     In some embodiments, distributed attenuators between the shunt switch and a phase comparator reduce crosstalk. 
     In some embodiments, phase comparison circuitry is useful for detecting faults in the ESA radiating element. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and should not restrict the scope of the claims. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate exemplary embodiments of the inventive concepts disclosed herein and together with the general description, serve to explain the principles. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The numerous advantages of the embodiments of the inventive concepts disclosed herein may be better understood by those skilled in the art by reference to the accompanying figures in which: 
         FIG. 1  shows a block diagram of a communication system according to one embodiment of the inventive concepts disclosed herein; 
         FIG. 2  shows an environmental view of one embodiment of a communication system according to the inventive concepts disclosed herein; 
         FIG. 3  shows an ESA according to one embodiment of the inventive concepts disclosed herein; 
         FIG. 4  shows a diagram of a circuit according to one embodiment of to the inventive concepts disclosed herein; and 
         FIG. 5  shows a diagram of another circuit according to one embodiment of to the inventive concepts disclosed herein. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to the subject matter disclosed, which is illustrated in the accompanying drawings. The scope of the inventive concepts disclosed herein is limited only by the claims; numerous alternatives, modifications and equivalents are encompassed. For the purpose of clarity, technical material that is known in the technical fields related to the embodiments has not been described in detail to avoid unnecessarily obscuring the description. 
     Referring to  FIG. 1 , a block diagram of a communication system  100  according to one embodiment of the inventive concepts disclosed herein is shown. The communication system  100  comprises a processor  102  connected to a plurality of radiating elements  112  and  114  in an ESA  110  via radiating element circuitry  108  comprising transmitter, receiver, and sampling circuitry associated with each radiating element. The processor  102  is further connected to a memory element  104  for storing processor executable code. 
     In one embodiment, the radiating element circuitry  108  comprises a phase comparator to create a feedback loop that modifies the phase of a signal as more fully described herein. In some embodiment, the processor  102  is configured to receive a leakage RF power signal from the radiating element circuitry  108  while in a transmit mode and modify signals received by the radiating element circuitry  108  accordingly. 
     In one embodiment, the processor  102  may also be connected to a data storage element  106  for storing base line reference values for a radiating element calibration process. 
     Referring to  FIG. 2 , an environmental view of one embodiment of a communication system  100  incorporated into an aircraft  202  according to the inventive concepts disclosed herein is shown. The aircraft  202  includes a communication system  100  comprising processing elements connected to one or more ESAs  110 . 
     In one embodiment, the communication system  100  is configured, either via software or specialized hardware, to receive signals through radiating element receiver circuitry corresponding to signals generated by radiating element transmitter circuitry, and compare the phase of the received signals to the projected phase of the signals sent to the transmitter circuitry. In some embodiments, each radiating element in at least one of the one or more ESAs  110  comprises phase comparator circuitry connecting the transmitter circuitry to the receiver circuitry in a feedback loop such that the phase comparator circuitry determines a phase disparity and alters the transmitter circuitry accordingly. 
     Referring to  FIG. 3 , an ESA  300  according to one embodiment of the inventive concepts disclosed herein is shown. The ESA  300  comprises a plurality of radiating elements  302 , each radiating element  302  connected to radiating element circuitry  304  comprising transmitter circuitry  306 , receiver circuitry  310 , and calibration circuitry  308  for periodically or continuously adjusting the phase of signals sent via the transmitter circuitry  306  to adjust the directionality of the signal sent by the ESA  300 . 
     Referring to  FIG. 4 , a diagram of a circuit  400  according to one embodiment of to the inventive concepts disclosed herein is shown. The circuit comprises a transmitter branch  402  and a receiver branch  404 . The transmitter branch  402  receives signals to transmit via a transmitter contact pad  406 ; such signal received via the transmitter contact pad  406  is amplified by one or more transmitter amplifiers  408 ,  410 , and  412 . The amplified signal is delivered to a radiating element contact pad  414 , and thereby applied to a connected radiating element  436  to produce a desired radiation pattern in concert with other radiating elements in an ESA. The receiver branch  404  comprises one or more receiver amplifiers  416  to receive a signal from the radiating element contact pad  414  and deliver an amplified signal to a receiver contact pad  418 . 
     Transmitter/receiver switches  420  and  422  control the input and output of signals going to and coming from the radiating element contact pad  414 . In at least one embodiment, a shunt switch  424  allows leakage RF power from the receiver switch  422  to pass through, and to a ground via a shunt resistance  426 , while the circuit  400  is in a transmit mode (the transmitter switch  420  in an on state while the receiver switch is in an off state). The signal from the shunt switch  424  may pass through one or more attenuators  428  and  430  to a phase comparator  432 ; the attenuators  428  and  430  are configured to reduce crosstalk between the shunt switch  424  and the transmitter branch  402 . The phase comparator  432  also receives a corresponding signal from the transmitter contact pad  406  via a current filtering capacitor  434 . The phase comparator  432  identifies any disparity in the phase of a signal actually produced at the radiating element contact pad  414  as compared to the signal applied to the transmitter contact pad  406 . The phase comparator  432  may then apply a corrective signal to one of the one or more transmitter amplifiers  408 ,  410 , and  412  to negate the disparity. The corrective signal may comprise a signal to adjust the gate bias of one or more of the transmitter amplifiers  408 ,  410 , and  412 . In some embodiments, the phase comparator  432  may be embodied in a phase-locked loop (PLL). 
     In one embodiment, the one or more attenuators  428  and  430  are separated from the transmitter branch  402  by an isolation distance  432  sufficient to prevent signals in the transmitter branch  402  from interfering in the phase comparison. The isolation distance may be 30 dB or greater. 
     In one embodiment, a circuit  400  is useful for identifying various failure states. There are many potential failure mechanisms that may cause elements of ESA to fail. ESAs and ESA radiating elements  436  can fail due to discontinuity between the transmitter amplifiers  408 ,  410 , and  412  and the transmitter switch  420 ; discontinuity between the receiver switch  422  and the transmitter switch  420 ; discontinuity between the radiating element contact pad  414  and the remainder of the circuit  400 ; failure of circuitry (not shown) driving the transmitter switch  420  and receiver switch  422 ; and load impedance at the radiating element contact pad  414  due to changing operational conditions or degradation of the circuit board. 
     By comparing signals sampled from the shunt switch  424  to the transmitter contact pad  406 , the circuit  400  may accurately detect any of these failures. A deviation in amplitude between the shunt switch  424  signal as compared to the transmitter contact pad  406  signal can identify discontinuities between the transmitter amplifiers  408 ,  410 , and  412  and the transmitter switch  420 ; discontinuities between the receiver switch  422  and the transmitter switch  420 ; and discontinuities between the radiating element contact pad  414  and the remainder of the circuit  400 . Regarding failures of the transmitter switch  420  and receiver switch  422  driving circuitry, a deviation in amplitude may only be a marginal difference of 2.5 dB-4.5 dB; however such a failure may produce a deviation in phase of between 8 degrees and 17 degrees, which is clearly detectable by the phase comparator  432 . 
     Over the lifetime of the circuit  400 , load impedance experienced by the transmitter amplifiers  408 ,  410 , and  412  may change due to degradation. Load impedance changes can be detected over time by tracking the phase and amplitude of sampled signals at the shunt switch  424  and the transmitter contact pad  406 . A circuit according to at least one embodiment may thereby determine whether the radiating element  436  and the contact pads  406 ,  414 , and  418  are still functional within specified parameters. 
     Referring to  FIG. 5 , a diagram of another circuit  500  according to one embodiment of to the inventive concepts disclosed herein is shown. The circuit comprises a transmitter branch  502  and a receiver branch  504 . The transmitter branch  502  receives signals to transmit via a transmitter contact pad  506 ; such signal received via the transmitter contact pad  506  is amplified by one or more transmitter amplifiers  508 ,  510 , and  512 . The amplified signal is delivered to a radiating element contact pad  514 , and thereby applied to a connected radiating element to produce a desired radiation pattern in concert with other radiating elements in an ESA. The receiver branch  504  comprises one or more receiver amplifiers  516  to receive a signal from the radiating element contact pad  514  and deliver an amplified signal to a receiver contact pad  518 . 
     Transmitter/receiver switches  520  and  522  control the input and output of signals going to and coming from the radiating element contact pad  514 . In at least one embodiment, a shunt switch  524  allows leakage current from the receiver switch  522  to pass through, and to a ground via a shunt resistance  526 , while the circuit  500  is in a transmit mode (the transmitter switch  520  in an on state while the receiver switch  522  is in an off state). The signal from the shunt switch  524  may pass through one or more attenuators  528  and  530  to a sampling switch  532 ; the attenuators  528  and  530  configured to reduce crosstalk between the shunt switch  524  and the transmitter branch  502 . The signal sampling switch  532  allows a processing element connected to the transmitter contact pad  506  and receiver contact pad  518  to analyze any disparity in the phase of a signal actually produced at the radiating element contact pad  514  as compared to the signal applied to the transmitter contact pad  506  and apply a correction to signals applied to the transmitter contact pad  506  to negate the disparity. 
     In one embodiment, the one or more attenuators  528  and  530  are separated from the transmitter branch  502  by an isolation distance  534  sufficient to prevent signals in the transmitter branch  502  from interfering in the phase comparison. 
     Embodiments according to the inventive concepts disclosed herein allow sampling of transmitter/receiver power without degrading either transmitter or receiver performance. Some embodiments provide very high sensitivity with no false negative results. Furthermore, while the exemplary embodiments described herein specifically apply to ESA systems, the principles are also applicable to any system utilizing signal phase interferometry. 
     It is believed that the inventive concepts disclosed herein and many of their attendant advantages will be understood by the foregoing description of embodiments of the inventive concepts disclosed, and it will be apparent that various changes may be made in the form, construction, and arrangement of the components thereof without departing from the broad scope of the inventive concepts disclosed herein or without sacrificing all of their material advantages. The form herein before described being merely an explanatory embodiment thereof, it is the intention of the following claims to encompass and include such changes.