Vol. 9 No.3 June 2001
T/R Module Technology for L-B & Active Phased Array
Active phased arrays or active aperture arrays have now become practical propositions for modern day radar systems. Their 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. They use individual solid-state transmit/receive (T/R) microwave module element at each of their raDIATing element, 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. Furthermore, the active aperture array techniques can help satisfy the need to generate very large power to obtain large power aperture product for long-range surveillance and tracking radar systems. Moreover, low sidelobe level control on the radar receive mode and adaptive null placement to minimise jamming are possible only through the active array solutions.
DRDO has indigenously developed and realised all the constituent subsystems of the small size active array, viz., microwave T/R module key elements with architecture based on new hybrid MIC/MMIC (microwave integrated circuit/monolithic microwave integrated circuit) technology, raDIATing T/R Module. Technology elements, RF feed network, distributed beam steering unit with control/data distribution network, distributed low profile high efficiency PSUs (power supply units), and mechanical and thermal aspects. The basic T/R unit building blocks of the active array have also been conceptualised and developed, to be integrated in an array unit frame, in an LRU (line replaceable unit) fashion, with array raDIATing elements mounted on an integrated array back-up panel. The complete active aperture array system has been fully assembled, integrated and tested for its subsystem as well as system level performance.
The performance of modern radar systems with active phased array antennas is mainly driven by the performance of the microwave T/R modules. The concept of active aperture array is critically dependent on the availability of compact and minimum weight, low consumption and high reliability T/R modules. The large number of individual T/R modules integrated with the respective raDIATing elements of the active array ensures a great degree of redundancy in case of failure of elements (graceful degradation). Due to the close connection of the T /R modules to the raDIATing elements, the losses in both cases, transmit and receive, are low, compared to passive array systems. This leads to a low receive noise figure and high transmit efficiency.
The major functions of a T /R module are: (i) generation of transmit power, (ii) low noise amplification of received signals coupled to and received from the respective raDIATing element, (iii) phase shift in transmit and receive modes for beam steering, and (iv) variable gain setting for aperture weighing during reception. The T/R module architecture is closely related to the functionality required in the active apertures of the array in which it is used. Parameters that determine T/R module architecture are: (i) need for high transmit power with maximised power added efficiency, (ii) need to maximise receive input third order intercept with a low front-end noise figure, (iii) need for self-calibration and built-in test capability in the module, (iv) need for low array sidelobes on receive mode, (v) need for a distributed beam steering computation, and (vi) need for an effective heat transfer with a low module weight and cost. All the foregoing objects required for a T/R module have been achieved by critical design, indigenous development and production of the T/R module key elements for the small size L-band active aperture array.
The T/R modules are configured based on a new hybrid MIC/MMIC architecture. 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 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 MMICs for a reliable cost-effective solution. Silicon PIN diodes having high breakdown voltage are used for realising 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 single 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 utilizing 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. A close module-to- module phase and amplitude tracking over temperature, a large operating bandwidth, an adjustable transmit power control and a compact packaging with an effective integrated cooling arrangement have been achieved.
A production base has been established in BEL, Bangalore, for productionisation of gain and phase matched L-band T /R modules.
Active Array Architecture
The T/R module configuration has been evolved based on its functionality in an active array architecture as illustrated in the full-fledged active array unit developed. The triangular array lattice of the raDIATing elements mounted on the array back-up plate, dictate the lateral size of the T/R module. The array back-up structure with LRUs for mounting of T/R modules and associated circuitry is generally employed for a high power active phased array architecture. The T /R modules, eight in number, are shown housed in each of the LRUs, also called as T /R units, with their associated component circuitry of Tx/Rx RF manifold, control circuitry based on microcontroller in the T/R unit level, and power supplies. T/R unit level cooling is through a common cold plate for power supply units, and coolant manifolds (in and out) for the eight T /R modules 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 and the DC/signal connections are through the circular miniature connectors. RF signal interface to the higher level of the array (viz. , a group of T/R units) is through the RF connection ports. 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.
The complete active array unit, housing the eight T/R units and 64 array raDIATing elements mounted on the integrated array back-up panel, has been assembled, integrated and tested for performance capabilities of each of the constituent subsystems as well as system level performance in terms of beam steering commands, a very low sidelobe level control, and online status monitoring, over the large frequency band of operation in L-band.