Abstract:
This invention relates to a transmit and receive module for active phased array antenna system based upon a combination of hybrid microwave integrated circuit (MIC) as well as monolithic microwave integrated circuit (MMIC) technology. The module comprises a signal transmit chain having switching means ( 03 ) for switching the module to transmittance mode. Means are provided for applying pulsed RF signal to the said module from array manifold. A phase shifter ( 01 ) is connected to a digital attenuator ( 02 ) and the output of the attenuator ( 02 ) is connected to a power amplifier ( 04 ). The amplified signals from amplifier ( 04 ) are conveyed to a duplexer means ( 05 ) connected to said power amplifier ( 04 ) and for routing back the received signal through a receiver protector ( 06 ) and low noise amplifier means ( 07 ). Electronic means are connected to a power conditioner for controlling the operation of the device.

Description:
FIELD OF INVENTION  
         [0001]    This invention relates to a Transmit/Receive Module for Active Phased Array Antennas and more specifically to a Transmit/Receive module for L-Band Active Phased Array Antennas/Apertures which are employed in long range Active Phased Array Radars.  
         PRIOR ART  
         [0002]    A Radar based on Active Phased Array Antenna System basically includes a plurality of active radiating antenna elements each of which is driven by/drives an individual Transmit/Receive module located closely adjacent thereto. Active Phased Arrays or Active Aperture Arrays, are being utilised in modern day Radar Systems. The active Array architecture overcomes the major Passive Array problems viz, low reliability inherent with tube type Radar Transmitters and their attendant high voltage power supplies and modulation, and the losses presented by their reciprocal ferrite/PIN diode phase shifters with the associated Passive Array RF manifold. Active Phased Arrays use individual solid-state T/R microwave module element at each of its radiating element (antenna), thus avoiding the distribution and phase shifter losses encountered in the Passive Array design. For the same radiated power, Active Phased Array Systems have been found to be significantly efficient, smaller and lighter than the conventional Passive Array systems. Need to generate very large power to obtain large power aperture product for long-range surveillance can be satisfied only with Active Phased Array Systems utilising Active Aperture Array techniques.  
           [0003]    The performance of modern radar systems with Active Phased Array Antennas is mainly driven by the performance of the Transmit/Receive modules utilised in the system. As mentioned above, A Radar Systems with Active Phase Array Antenna may utilise a large number of Transmit/Receive modules, each connected to individual radiating elements (antenna) of the Active Array. In fact, the key element of the Active Phase Array is the Transmit/Receive microwave module whose performance decides the overall performance of the Radar. A long range Radar working in L-band (1.2-1.4 GHz) may typically employ 200 individual Transmit/Receive modules. The performance of Radar system with Active Phase Array Antenna is critically dependent on the availability of compact and minimum weight, low consumption and high reliability microwave Transmit/Receive modules. The major functions of a Transmit/Receive module are the generation of the transmit power, the low noise amplification of the received signals coupled to and received from the respective radiating element, the phase shift in the transmit and receive mode for beam steering, and the variable gain setting for aperture weighting during reception. The Transmit/Receive module architecture is closely related to the functionality required in the Active Apertures of the Array in which it is used.  
           [0004]    Parameters that determine T/R module architecture are: (1) the need for a high transmit power with maximized power added efficiency, (2) the need of maximize receive input 3 rd  order intercept with a low front-end noise figure, (3) the need for self-calibration and built-in test capability in the module, (4) the need for low array sidelobes on receive mode, (5) the need for a distributed beam steering computation, and (6) the need for an effective heat transfer with a low module weight and cost.  
           [0005]    The Transmit/receive modules utilised in Active Phased array Antennas are known in the art. However, these Transmit/Receive modules, known in the art suffer from following disadvantages.  
           [0006]    Primary disadvantage of Transmit/Receive modules, known in the art, is that these are realized through Microwave Integrated Circuit (MIC) architecture thereby making the size of the T/R module bulky.  
           [0007]    Another disadvantage of Transmit/Receive modules, known in the art, is that reliability of these T/R modules is less because of large number of interconnects therein.  
           [0008]    Yet another disadvantage of Transmit/Receive modules, known in the art, is that their repeatability characteristics for phase and amplitude over all the Transmit/Receive Modules is very low.  
           [0009]    Still further disadvantage of Transmit/Receive modules, known in the art, is that their phase and amplitude setting accuracy is inferior.  
           [0010]    Yet another disadvantage of Transmit/Receive modules, known in the art, is that these are not cost effective.  
         OBJECTS OF THE INVENTION  
         [0011]    Primary object of the present invention is to provide a Transmit/Receive (T/R) module which is realized through hybrid architecture of Microwave Integrated Circuit (MIC) and Monolithic Microwave Integrated Circuit (MMIC) both thereby helping miniaturizing the complete T/R module.  
           [0012]    Another object of the present invention is to provide a Transmit/Receive module in which the transmit chain is realised through MIC architecture thus enabling it to handle high level of output power necessary for high range radars.  
           [0013]    Yet another object of the present invention is to provide a Transmit/Receive module in which the receive chain is realised through MMIC architecture thus helping in miniaturizing the receiver module.  
           [0014]    Yet further object of the present invention is to provide a Transmit/Receive Module in which the transmit chain can provide high peak and average power output thereby enhancing the range capability of the Radar.  
           [0015]    Still another object of the present invention is to provide a Transmit/Receive module in which high cooling efficiency is realised utilising cold plate with embedded microchannels underneath each of the power devices in Transmit Module.  
           [0016]    Still further object of the present invention is to provide a Transmit/Receive module, which is capable of operating in entire L-band Radar frequency.  
           [0017]    Still another object of the present invention is to provide a Transmit/Receive module, which has a low noise figure and a linear gain.  
           [0018]    Yet another object of the present invention is to provide a Transmit/Receive module which is highly reliable with high repeatable performance in the entire L-band.  
           [0019]    Still another object of the present invention is to provide a Transmit/Receive module, which has a very close phase and amplitude level match for all the individual T/R Modules.  
           [0020]    Still further object of the present invention is to provide a Transmit/Receive module, which is highly compact and cost effective.  
           [0021]    Still another object of the present invention is to provide a Transmit/Receive module which has a self-calibrating and built- in test facility.  
           [0022]    Yet another object of the present invention is to provide a Transmit/Receive module which has a distributed beam steering computation facility.  
           [0023]    Still another object of the present invention is to provide a Transmit/Receive module, which has a low front-end noise figure.  
           [0024]    Still further object of the present invention is to provide a Transmit/Receive module which has a capability of controlling transmit power output for realizing low side lobes for transmit radiation pattern.  
         STATEMENT OF INVENTION  
         [0025]    According to this invention there is provided a transmit and receive for active phased array antenna system based upon a combination of hybrid microwave integrated circuit (MIC) as well as monolithic microwave integrated circuit (MIC) technology comprising in combination:  
           [0026]    Signal transmit chain ( 10 ), comprising:  
           [0027]    Switching means ( 3 ) for switching the module to transmittance mode;  
           [0028]    Means for applying pulsed RF signal to the said module from array manifold;  
           [0029]    Phase shifter (l) connected to a digital attenuator ( 2 ), the output of said attenuated connected to a power amplifier ( 4 ); the amplified signal from amplifier ( 4 ) connected to a duplexer means ( 5 ) for routing back the received signal through a receiver protector ( 6 ) and low noise amplifier means ( 7 ), control electronics means ( 8 ), connected to a power conditioner ( 9 ).  
           [0030]    In accordance with the present invention, the improved Transmit/Receive module for Active Phased Array Antenna elements operating in L-band is realized through hybrid architecture employing both Microwave Integrated Circuit (MIC) as well as Monolithic Microwave Integrated Circuit (MMC). The use of MIC components in transmit chain of the module helped in generating high power output necessary for long ranging while incorporation of MMIC technology in receives chain of the module helps in miniaturizing the same thus reducing the size of the complete Transmit/Receive module. The proposed Transmit/Receive module can operate in entire L-band providing high peak and average power output with a very high degree of reliability and repeatability. The module is able to provide very close amplitude and phase level matching and tracking for the Transmit/Receive Modules. The Transmit Chain of the module is designed to generate a high peak power output, with a large pulse width and duty over the large RF bandwidth, using Silicon (Si) bipolar transistors operating in efficient class ‘C’ mode. Low Noise Amplifier (LNA), Digital Attenuator and Shared Phase Shifter with T/R switches in the Receive Chain of the T/R module, use GaAs (Gallium Arsenide) MMICs for a reliable cost effective solution. Si PIN diodes having high breakdown voltage are used for realizing Receiver Protector Circuitry. The module has an integral on-mounted driver/control circuitry using a microcontroller and miniature hybrid packaging employing SMDs (Surface Mount Devices). The Transmit and Receive Chains are configured using microstrip circuitry on two soft ceramic microwave laminates, which are stacked compactly in a signal T/R module housing. The transmit circuit laminate is screwed on to the integrated liquid cooled cold plate of the module housing, which provides the best cooling efficiency by utilising microchannel cooling underneath each of the power devices of the Transmit Chain. The overall module size is compact and fits in a triangular array grid.  
           [0031]    Any further characteristics, advantages and applications of the invention will become evident from the detailed description of the preferred embodiment which has been described and illustrated with the help of following drawings wherein,  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0032]    [0032]FIG. 1 is an electrical block diagram illustrative of the Transmit/Receive module.  
         [0033]    [0033]FIG. 2 is a detailed electrical block diagram illustrative of the preferred embodiment of the present invention.  
         [0034]    [0034]FIG. 3 is a diagram illustrative of the two stacked layers of the T/R module with the details on the microstrip and digital circuit layouts.  
         [0035]    [0035]FIG. 4 is an exploded perspective view of the T/R module components, viz., Module Housing, the Transmit and Receive Chain substrates and the interconnections between them and with the Module connectors.  
         [0036]    [0036]FIG. 5 is a perspective view generally illustrative of the T/R Unit consisting of 8 T/R modules and associates circuitry, being plugged into the back of a planar array of an Active Phased Array Antenna System.  
         [0037]    [0037]FIG. 6 is an exploded view of one T/R Unit drawn out of the Array unit of FIG. 5, illustrative of the T/R modules and associated circuitry of the particular T/R Unit. 
     
    
     DESCRIPTION OF THE INVENTION  
       [0038]    Referring to FIG. 1, T/R switch  03  is shown in the transmit mode of operation. During the transmit mode of Radar, a pulsed RF signal (Radar Exciter Output) is supplied to the module from the array manifold. This signal is phase Shifted in a digital phase shifter  01  and amplitude adjusted in digital attenuator  02  at each of the module site to produce the desired radiation beam. The signal is then amplified by transmits driver and final amplifiers  04 , and routed through the circulator-duplexer  05  to the radiating element. In the receive mode, Radar return signals are routed back through the duplexer  05 , the receiver protector  06  and the low noise amplifier  07  which largely establishes the system noise figure. The amplified return signal is amplitude adjusted and phase shifted in the same digital attenuator  02  and phase shifter  01  respectively and routed to the array manifold.  
         [0039]    The amplitude weighting (through the digitally controlled attenuator  02 ) on the transmit and receive mode is used for synthesizing the low sidelobe pattern of the array both during transmit and receive modes. Thus, during transmit, the receive amplifier  07  output is turned off and during receive, the transmit amplifier input is turned off by the SPDT T/R switch  03 . The Radar dead time is utilized for changing the phase and attenuator values and for switching channel select T/R switch  03 . The control electronics  08  serves to interferface the module to the array controllers, providing beam steering and timing information needed by the module. The power-conditioning block  09  provides the necessary sequential biases and switching commands for the respective module components.  
         [0040]    Referring to FIG. 2, the Transmit Power Amplifier Chain of the T/R module operates in saturated mode using multistage transmit amplifier  12 - 18  based on four stage class ‘C’ amplification  13 - 18  with a GaAs MMIC Medium Power Amplifier (MPA) as the input driver  12 . Silicon Bipolar transistors are used in the class ‘C’ Transmit Chain. The Final Power Amplifier Chain utilizes balanced stage  17 - 18  with wire-line hybrid divider  19 /combiner  20 , driven by power output generated by the Driver Amplifier Chain  12 - 16 . The Final Power Amplifier develops an output power of +57 dBm (peak) minimum, at 10% duty with a transmit pulse width capability of 100 usec over the complete L-band Radar frequency. The DC power required is derived from on-mounted adjustable voltage regulators from the Power Conditioner  38  and would be used for transmit power control, for amplitude weighting of the array in the transmit mode to realise low sidelobe patterns. A smooth amplitude taper across the array could be realised by employing proper control/adjustment of the DC supplies from the power conditioner, to different stages of the Transmit Chain.  
         [0041]    A microstrip coupler  22  with a detector  24  cum matching circuitry at the transmit amplifier output provides a power monitor. A drop-in circulator  21  used at the power amplifier output acts as a high power T/R duplexer for a good input VSWR and non-reciprocal characteristics, handling peak power in excess of +58 dBm. Also, there is a provision of reflected power monitoring for diagnostics, through an asymmetric coupler  23  and detector  25  in addition to the transmit power sample through SPST switch  26  for any on-line testing to be performed. The complete Transmit Chain is configured using microstrip circuitry on thin soft ceramic microwave laminate with aluminium back-up for ease of circuit fabrication, machining, as well as, connector-less drop-in packaging FIG. 2 illustrates the Transmit Chain Substrate  10 .  
         [0042]    The Receive Chain of the T/R module employs MMIC technology. The GaAs MMICs used are packaged surface mount type. Two front-end Low Noise Amplifiers (LNAs)  31  and  33 , each with 1.8 dB Noise Figure, 25 dB Gain and P out  (1 dB) of +14 dBm, are employed in cascade prior to a digitally controlled attenuator  34 . An adjustable attenuator pad  32  is placed in between the two LNAs to control the overall receive gain of the T/R module and also to optimize the saturation level of the overall front-end low noise amplification. The 6-Bit MMIC attenuator  34  provides maximum of 31.5-dB attenuation with a resolution of 0.5 dB. The shared MMIC Phase Shifter  36  uses a 6-Bit control with an LSB of 5.625°. The T/R channel select switch  35  is also based on the MMIC technology, offering a minimum of 40 dB isolation.  
         [0043]    The receiver protector function in the T/R module is realised by a high power switch  28  and limiter  29  combination. Another drop-in circulator  27  configured as an isolator, at the input of the high power switch  28 , forms a part of the receiver protector. This also offers a good match for the Transmit Power Amplifier output during transmit period, by making the high power reflecting type of switch  28  to act as an absorptive one. The high power switch  28  employs shunt mounted high voltage PIN diodes and operates on T/R switching command during transmit period and is designed to handle the required high peak and average power experienced when the antenna port of T/R module is, by mistake, disconnected from the antenna array element. Hence, the high power switch  28  and the isolator  27  are mounted on the Transmit Chain substrate  10  itself for proper heat transfer and cooling. The high power limiter  29  also utilises high breakdown voltage PIN diodes and meets with identical high power handling requirements, so that, in case of non-operation of the high power switch  28 , LNAs  31  and  33  are protected from any high power output reflection from the antenna port by limiting the reflected power to a limited threshold with a good spike suppression.  
         [0044]    RF pre-selection filtering at the front-end of the Receive Chain of the T/R module is realised by a low loss MIC dropin bandpass filter  30 . This filter is realised on a temperature stable ceramic substrate and offers a very low insertion loss over the RF pass band with a good skirt response. Similar to the Transmit Chain, the Receive chain circuitry is also laid out on another thin soft ceramic microwave laminate with aluminium backup.  
         [0045]    The driver and control/logic circuitry  37  is also mounted on the same substrate forming part of the receive circuitry. This employs a microcontroller for computation and providing the necessary 6-bit beam steering commands and amplitude excitation respectively to the phase shifter  36  and attenuator  34 , as required for the respective radiating element connected to the module, based on the phase and amplitude gradients in X and Y-direction, and address identity required on a serial link from the main Beam Steering Controller of the array. The microcontroller also stores the phase and amplitude errors generated by calibaration and applies them to realise respective element excitations for synthesising low sidelobe patterns through the array scan angles. The T/R switch commands for the two switches  35  and  28  are also generated in the microcontroller  37 . The on-line diagnostics of module power supplies and forward/reverse sample power outputs obtained from the detectors  24 / 25 , is also performed in the microcontroller card  37 , providing ultimately as a ‘status out’ from the module. In addition, the Receive Chain circuitry also houses a bias sequencer-modulator circuit  39  for proper sequencing the gate and drain supplies to the MMICs  31 ,  33 ,  34 - 36  and MPA  12  and providing the drain pulse drive required for the MPA  12  during the T/R transmit period, so as to conserve the average power drawn/dissipated by the MPA  12 . This circuit  39  employs opto-couplers for fast switching and controlled delays, with current drivers used for the MMIC drain supplies. An additional MOSFET switching circuit is employed to generate drain pulse drive required for the MPA  12 . Both the microcontroller card  37  and the bias sequencer-modulator circuit  39  are realised using all surface mount custom silicon Ics and components mounted on miniature size multilayer PCBs.  
         [0046]    The Transmit Chain houses two Tx/Rx interface PCBs  40  and  41  for interconnecting to the module DC/Signal input connectors  50 /JI  42  and the Receive Chain circuitry on the top layer, through the two functional PCBs in the Receive Chain, vis., microcontroller card  37  and bias sequencer-modulator card  39 . The final transmit power output to the antenna element and the transmit sample power for monitoring/calibration are taken out through the RF connectors J 2   43  and J 3   44  respectively.  
         [0047]    Referring to FIG. 3, the Transmit and Receive Chains  10  and  11  of the T/R module are realised on two different soft ceramic (high dielectric constant) microwave laminates. Three short low loss RF cable assemblies (j 1   42 -J 4   45 , J 5   46 -J 6   47  and J 7   48 -J 8   49 ) connect the RF ports between the Transmit and Receive Substrates and to the Tx In/Rx Out connector J 1   42 .  
         [0048]    Referring to FIG. 4, the size of the T/R module housing is made compact with lateral dimensions exactly fitting the triangular array grid in L-band Radar frequency. Transmit and Receive channel circuitries are laid out in the stacked two-layer configuration in a compact T/R module housing  51  with the top cover  52 . The transmit circuit laminate  10  is the lower one and thus is made to have a good thermal contact with the floor of the housing. The bottom of the housing is made as an integrated liquid cooled cold plate with water inlet  53  and outlet  54  entries lengthwise on either side of the module housing. The transmit circuitry is screwed on to this integrated cold plate floor. When the module operates in its full duty of 10%. The Transmit Chain of the module is required to dissipate around 120W of heat and the cold plate design caters for this by use of microchannels embedded underneath each of the high power devices of the Transmit Chain.  
         [0049]    Referring to FIG. 5, the triangular array lattice of the radiating elements  55 ,  56  mounted on the array back-up plate  57 , dictate the lateral size of the T/R module. The array back-up structure  58  with LRUs (Line Replaceable Units)  59  for mounting of T/R modules and associated circuitry, is generally illustrative of a High Power Active Phased Array architecture. The T/R modules, eight in number, are shown housed in each of the LRUs  59  (also called as T/R units) with their associated component circuitry of Tx/Rx RF manifold  60 , control circuitry based on microcontroller  61  in the T/R Unit level, and power supplies  62 .  
         [0050]    Referring to FIG. 6, T/R Unit level cooling is through a common cold plate used for cooling of power supply units, and Coolant manifolds (In and Out) for the eight T/R modules  51  and  52  stacked four on either side of the LRU. The coolant inlet/outlet in the T/R unit level are through the snap-on connection ports  63  and  64  and the DC/Signal connections are through the connector  65 . RF signal interface to the higher level of the array (viz., a group of T/R Units) will be through the RF connection ports  66 . The RF cabling in the T/R unit is implemented and brought out on the array side in a coaxial snap-on connections, so that, the T/R Units could be installed or removed as LRUs for any testing or repair maintenance.  
         [0051]    The present embodiment of the invention, which has been set forth above, was for the purpose of illustration and is not intended to limit the scope of the invention. It is to be understood that various changes, adaptations and modifications can be made in the invention described above by those skilled in the art without departing from the scope of the invention which has been defined by following claims: