Transmit Receive Module for S-Band Electronically Scanned Antenna With on Board Digital Control, Health Monitoring and Telemetry

Transmit Receive Module for S-Band Electronically Scanned Antenna With on Board Digital Control, Health Monitoring and Telemetry Paul J. Oleski Air Force Research Laboratory/IFGE Rome, NY 13441-4505 oleskip@rl.af.mil Sarjit S. Bharj Princeton Microwave Technology Princeton, NJ 08619 sarjit@princetonmicrowave.com Madan Thaduri Princeton Microwave Technology Princeton, NJ 08619 madan@princetonmicrowave.com ABSTRACT Satellites require timely tracking, telemetry, and command (TT&C) for payload operation. The ground antenna is one of the key elements that enable satellite control and payload operations. To support the operation of a large number of satellites at various orbits, operators need a network of antennas distributed around the globe, such as the Air Force Satellite Control Network (AFSCN), to contact satellites at a predetermined time and location. Currently, they use large mechanically steered parabolic dishes to provide hemispherical coverage and simultaneous transmit (Tx) and receive (Rx) capabilities in support of Department of Defense (DoD) satellite operations (SATOPS) Network designers used reflector antennas because of relatively low acquisition cost. The current reflector antennas used to support satellite operations are approximately 10 m in diameter and are susceptible to single point failure and long downtime for repair and maintenance. S-Band component technology provided by the cell phone industry will now allow an affordable electronically scanned antenna (ESA). Current SATOPS require a more efficient and flexible antenna system. The ESA can offer superior performance, operability, adaptability and maintainability for satellite operation. This paper will present the design of a TR module that can provide one Transmit (Tx) and two Receive (Rx) links to a satellite. The TR module will be part of a dome shaped antenna that could provide multiple simultaneous ground to satellite links. This geodome antenna will provide multiple simultaneous operations with pointing and acquisition taking seconds. One dome antenna can replace the capability of three AFSCN parabolic dishes. The next generation low cost TR module will be developed by the AFRL/ Information Grid Division (IFG) and Princeton Microwave Technology Inc. for the next generation of the AF Satellite Control Network (AFSCN). The TR module differs from previous modules in Ref [1..5] in that it consists of a single Tx channel capable of 33 dBm of output power and two Rx channels with a gain of 30 dB per channel. In addition, beam switching and on board digital control has been implemented where the Tx and Rx channels provide four-bit phase shift. In addition to the control functions, built-in test (BIT) circuits will monitor the health and status of the RF devices. This function utilizes a low-power micro-controller to output digital data for each of the power and low noise amplifiers, via A/D converters. The bandwidth of the TR module has been designed to cover both the Unified SBand (USB) and Satellite Ground Link Subsystem (SGLS). The TR functions are combined at the output via a ceramic resonator diplexer comprised of a band pass-band stop filter. The control of the TR module is conducted via a single programmable logic device (PLD) controller through a DAQ computer interface. The TR module has been designed to meet the cost objective for a dome antenna with approximately 47,000 TR modules. A 2nd generation 78 element triangular panel of TR modules is now planned to be developed, leveraging off lessons learned from generation one. This paper will describe the layout and design of the 2nd generation TR module. INTRODUCTION The development of a low cost TR module for the next generation of Phase Array Antennas for the Air Force Satellite Control Network (AFSCN) has been achieved. Low cost component design and implementation are critical in developing a practical phase array antenna. Combined RF, digital and monolithic circuits are important but not the only critical issue. This TR module differs from the previous modules, Ref.5, in that this 1 of 5 module has one transmit section and the transmitter has an output power capable of 2 Watts. Also the 90-deg hybrid has been removed from the transmitter and receive section of the module. The hybrid is now mounted externally to the module. Also, the attenuator is removed from all the channels. The present TR module achieves a very low power control using a less complex digital controller without sacrificing any functionality. In addition, diplexers are utilized between each transmit and receive section. Both left-hand and right-hand polarization is used in a special polarization matrix not on the TR module, but between the TR and the radiating element interface. Affordable antenna arrays operating at microwave frequencies are envisioned to consist of active modules that employ microwave integrated circuits located at each radiating element of the aperture. The antenna system consists of combined transmit and receive patch radiators capable of rapid beam motion. Beam agility and highradiated power levels in association with the close spacing between the radiators drive the antenna design. The need for fast beam switching requires digital control circuits to calculate phase shift settings. A high RF radiated power level developed from closely spaced RF amplifiers generates large heat densities. This forces the transmit antenna to increase in area to where beam pointing accuracy limits the array size. The great number of elements in the array emphasizes the need to develop a practical method of distributing control signals throughout the array. A Geodesic Dome Phase Array Antenna (GDPAA) is considered for the AFSCN. Implicit in the system function array is the need to operate the array in full duplex operation. Additionally the array is capable of controlling fundamental radiation characteristics such as beam width, beam size, sidelobe levels and radiated power, in order to realize different antenna characteristics required by the various satellites. The array aperture consists of a large number of radiating elements that are spaced approximately half a wavelength at the upper end of the operational frequency band. The frequency response and excitation of each element in the aperture can be independently controlled. The aperture can be fully or partially utilized either to direct energy over a large volume or intentionally directed in a certain direction. The capability of the array to provide transmit and receive functions simultaneously and to rapidly alter the set of configurations is possible due to an active element control circuit. The active control circuits allow the phase array antenna to control its radiation characteristics. The aperture can be uniformly illuminated to achieve maximum gain or tapered to achieve low side lobes or a shaped beam. The phase shifter permits the antenna beam to be scanned in any direction. The filters specify the portion of the aperture used by a particular system. The phase shifter and the amplifier are components that have been developed in Microwave Monolithic Integrated Circuit technology (MMIC), in the last decade. The requirement of high isolation between transmit and receive channels focused the effort to investigate the exact performance that can be achieved from the low-cost ceramic diplexing filters. In addition, low cost MMIC based power amplifiers for the transmit channel have been used. Effort was also directed towards the design of a lowcost phase shifters. Due to the bandwidth of the transmit section a broadband phase shifter using low pass and high pass filter sections was designed. The receive band phase shifter was based on switched line methodology. Other important factors that were considered in the development of the TR module were: - One Transmit and Two Receive Channels - TR Module’s Interface with Beam former - Hot Replacement of TRs - Polarization Diversity - Low Cost with Justification - High Isolation between Transmit and Receive Channels - Digital Control on Board - Ruggedness and Reliability - Built in Test TRANSMIT AND RECEIVE (TR) MODULE RF PATH DESCRIPTION Each TR module consists of three channels comprising of one transmit and two receive channels. Figure 1 shows the block diagram of the TR module. The transmitter path is shown by a dotted line to distinguish it from the receiver path. The transmit frequency of operation is 1.75 – 2.1 GHz. The receiver frequency of operation is 2.2-2.3 GHz. The transmit path consists of an input at Tx and output from one of the two antenna ports (A1/A2). The transmitter signal passes through a four-bit phase shifter (? shift of 22.50, 450, 90 0, 1800), a SPDT switch to open/close the RF path, a preamplifier and then through the embedded power combiner. The output from the power divider passes through complementary 2W power amplifiers. The overall gain of the transmitter channel is 20 dB. For the downlink, the input signal is fed to a high rejection band pass ceramic filter using A1/A2 port. The input signal passes through a series of amplifiers, phase shifters and SPDT switches before reaching the receiver ports (Rx1/Rx2). The total gain across the receiver band is 30 dB. 2 of 5 Rx2 Amp, G = 16 dB SPDT Phase Shifter - Micro controller Unit (MCU) Amplifier G=15dB DIPLEXERS Low Noise Amp SPDT A1 Diplexer 2w Power Amp 3-dB div A2 Diplexer 2w Power Amp 3-dB div Tx Phase Shifter SPDT Power Div 3-dB div 3-dB div Drive Amp Low Noise Amp Rx1 Amp, G = 16 dB SPDT Phase Shifter Amplifier G=15dB Figure 1. Block diagram of the TR module The specifications for the transmit and receive sections of the TR module are detailed in Table I and Table II. Table I. Transmit channel specification PARAMETER Frequency Gain Power output per channel Phase shift Control Retrofit Efficiency Spurious Levels SPECIFICATIONS 1.75-2.1GHz 20dB Two diplexers are required to maintain optimum performance. The transmit side of the diplexer filter, inserted after the transmit amplifier, prevents wideband noise from entering the receiver, and degrading performance. The receive section of the diplexer, prevents the coupled transmit signal from degrading the linearity of the receive Low Noise Amplifier (LNA). The diplexer filters are made of high Q ceramic resonators. Two types of diplexers were investigated. The first consisted of a band pass type of response to provide at least 60 dB of rejection at the crossover point between the bands. This filter provided a loss of 1 dB in the transmit pass band and 1.5 dB in the receive path. We then investigate a diplexer with a bandstop bandpass type of diplexer. This filter that produces an insertion loss of 1 dB in the receive band consists of ten resonators in a coaxial structure. The transmit section loss was 0.5 dB with a rejection of 65dB at the crossover frequency. The bandstop filter consisted of three sections of notch filtering using ceramic technology. Table III. Transmit filter specification 33dBm 360° Electronics Change in condition > 40 % <-85 PARAMETER Frequency Insertion loss Return loss Rejection at 2.15 GHz Hot Frequency Gain Noise Figure Phase shift 1.75-2.1 GHz 1.0 dB Max <-15 dB <-50 Table IV. Receive filter specification Table II. Receive channel specification PARAMETER SPECIFICATIONS PARAMETER Frequency Insertion loss Return loss Rejection at 2.15 GHz SPECIFICATIONS 2.2-2.3 GHz 30dB 1.2dB 360° SPECIFICATIONS 2.2-2.3 GHz 1.0 dB Max <-15 dB <-50 LOW NOISE MMIC AMPLIFIER The main components of the Transmit-Receive (TR) module are: - Ceramic diplexer with high rejection - Low noise MMIC amplifiers - High Power MMIC driver and Power MMIC Amplifiers - Quadrature and in phase hybrids - 4-Bit Transmit and Receive digital Phase Shifter - Polarization selection A low noise MMIC amplifier developed for the satellite and the radio market for the frequency of 2.2 to 2.3 GHz has been used. The device provides a gain of 18 dB with an associated noise figure of 1 dB. It is based on E-D MESFET process and consumes very low current. 3 of 5 4 BIT TRANSMIT AND RECEIVE PHASE SHIFTERS DIGITAL CONTROLLER A phase shifter design based on the MMIC switch incorporating a single pole double throw was procured from Marconi. The design of the Transmit and Receive channel phase shifters was detailed in Ref 1. The insertion loss of the phase shifter was measured at 8 dB with a total change in insertion loss of 0.4 dB in all phase states. The transmit phase shifters were designed based on low pass, high-pass filter sections switched between paths. The phase shifter provided 22.5, 45, 90 and 180 -degree phase shifts with an error of 10 degree for the 180 degree bit. Total amplitude change for the phase shifter was less than 1 dB for all phase states. The receive phase shifters were based on the switch line approach and exhibited an insertion loss of 6 dB. Again the total amplitude change was less than 1 dB for all phase states. The purpose of the digital control board is to accept asynchronous serial data and latch these commands once the modules address matches the inherent command address. Each modules address is hardwired using mechanical jumpers external to the module. The health status of the module is obtained and sent back to the antenna control computer upon request. TR MODULE LAYOUT The mechanical assembly of the TR module with the associated control circuitry is shown in Figure 3. The TR module length is reduced from 9.25” to 6.5”. The different parts of the housing are shown in Figure 4. The combined thickness including housing has also reduced from 1’ to 0.7” and the width is 2.9” POLARIZATION SWITCHING In the latest S-Band antenna panel design, a 90–deg hybrid polarization switching scheme will be designed to lie between and the TR module and the radiating element interface in order to induce left hand circular polarization (LHCP) and right hand circular polarization (RHCP) in the signal. The Rx polarization layout is shown in Figure 2. SPDT Figure 3. Mechanical housing of the TR module POL = 0 SIGNAL INPUT OUTPUT 3-dB div POL = 1 SPDT SIGNAL INPUT 3-dB div POL = 0 OUTPUT POL = 1 Figure 4. Open view of the T/R housing Figure 2. Receiver polarization switching schematic The new design places the 90–deg hybrid outside the TR module in order to reduce very difficult phase an amplitude matching requirements on the TR module channels. Trade offs will be made between single and multiple stage hybrids in order to generate an optimal geometry from a loss, return loss and axial ratio perspective. DESIGN TO COST The design to cost of the TR module has been conducted from project initiation. From onset, the cost associated with the components without compromise in the performance has been the guiding rule. The availability of the active devices for the PCS market has greatly influenced the cost of amplifiers and phase shifters. Added with novel design and layout, the design to cost goal is near reality. 4 of 5 REFERENCES MEASURED DATA Measured data for the TR module diplexer performance over transmit and receive band is shown in Figure 5 below. Ref [1] S.S.Bharj, P.J.Oleski, A.Merzhevskiy, B. Tomasic, S. Liu, “Affordable Antenna Array For Multiple Satellites Links”, 2000 Antenna Applications Symposium, Robert Allerton Park, P 401. Ref [2] S.S.Bharj, P.J.Oleski, A. Merzhevskiy B. Tomasic, S. Liu, John Turtle, N. Patel, “Low cost Transmit/Receive module for space ground link subsystem, 2002 Antenna Applications Symposium, P 1. Ref [3] S.S. Bharj, P.J.Oleski, A.Merzhevskiy B. Tomasic, S. Liu, J. Turtle, “Multi-Beam Transmit Receive Module for USB and SGLS Band Satellite Links’, 2003 Antenna Applications Symposium P 1. Ref [4] S. S. Bharj, M. Thaduri, P. J. Oleski, Lt. R. Patton, B. Tomasic, J. Turtle, S. Liu, “Daisy Chain Controlled Multi-Beam T/R Module for AFSCN”, 2004 Antenna Applications Symposium Proceedings, P 308. Figure 5. Diplexer performance CONCLUSIONS An S Band low power Transmit and Receive (TR) module has been developed for Telemetry, Tracking and Commanding (TT&C) and communications at Unified SBand (USB) and Space Ground Link System (SGLS) frequencies to meet the operational needs of the AF Satellite Control Network (AFSCN). A design to cost exercise was conducted to ensure a low cost product. A 1st generation TR module with a transmit power output of 33 dBm, 4-BIT phase shifter, and polarization switching has been achieved. An overall TX gain of 20 dB was achieved. In the receive channel a noise figure of 2 dB was measured with an overall gain of 30 dB. This was demonstrated in a six panel antenna in Ref 5. A 2nd generation subarray of 78 elements (one panel) will be designed and fabricated next. The TR module and beamformer is being redesigned to accommodate one Tx channel and two Rx channels because satellite and GDPAA architecture analysis has shown that one TX beam per TR module is adequate to meet mission needs given the multiple aperture capability of the GDPAA. This also has reduced the complexity, size, weight and cost of the TR modules. Ref [5] P.J.Oleski, Lt.R.Patton, S.S. Bharj, M. Thaduri, “Transmit Receive Module for Space Ground Link Subsystem (SGLS) and Unified S-Band (USB) Satellite Telemetry, Tracking and Commanding (TT&C) and Communications, MILCOM 2004 Proceedings, paper 449. 5 of 5