Role of Circulator, Isolator and Power Divider in the C-Band Microwave Front End System C. Chandrasekharan, S. Raghavendran & Sumi Sunny VSSC/ISRO, Amal Jyothi College of Engineering E-mail : [email protected] dividers, attenuators and crystal detectors are Abstract - In space communications, the transponder aids in tracking the launch vehicle. Microwave Front End characterized and selected. System is designed for testing the onboard transponder. Those selected devices are inter-connected and Microwave front end unit acts as an interface to connect tested. Next step is the complete integration including the high power onboard transponder to the ground RF RF cables routing and device fixation to chassis. Then checkout system with suitable isolation between them. Microwave front end unit basically consists of microwave integrated level testing is performed and finally it is devices like circulator, isolator, attenuator, power divider deployed in the test bed. and crystal detector with RF cables and adaptors interconnecting them. In this paper, the devices like II. DESIGN AND OPERATION OF MICROWAVE circulator, isolator and power divider are characterized FRONT END SYSTEM based on the study of performance of each device in tandem with the theoretical specification of the devices in Based on the studies of characteristics and the designated frequency range and design procedure of performance analysis, various microwave devices are microwave front end unit is also explained. characterized and the required devices like circulators, Keywords - Circulator, Isolator, Microwave Front End Unit, isolators, power dividers and attenuators suitable for the Transponder. desired application are selected. Those selected devices are inter-connected and tested. Next step is the complete I. INTRODUCTION integration including RF cables routing and device Microwave front end unit acts as an interface to fixation to chassis. Then integrated level testing is connect the high power onboard transponder to the performed and finally it is deployed in the test bed. ground RF checkout system with suitable isolation The block diagram of microwave front end unit is between them. For a system like transponder, which is shown below. the system under test (SUT), a stimulus is required for triggering the system. Upon interrogation, the response signal from the SUT is obtained and characterized against the pre-defined test requirements. Here, the front end unit acts as the centralized unit for triggering the onboard as well as getting the response and further processing of the response signal from the SUT. Microwave front end unit basically consists of circulator, isolator, attenuator, power divider and crystal detector with RF cables & adaptors interconnecting them. Ground RF Checkout system consists of a Signal Generator as transmitter (trigger for SUT), Spectrum analyzer, Frequency Counter and Power meter for processing the response from SUT.In the first step for Fig.1 : Microwave Front End Unit the design of a Microwave Front End Unit, the microwave devices like circulators, Isolators, power ISSN (PRINT) : 2320 – 8945, Volume -1, Issue -2, 2013 21 ITSI Transactions on Electrical and Electronics Engineering (ITSI-TEEE) To test the launch vehicle transponder (SUT), a stimulus signal (trigger) is to be sent from the ground RF checkout (radar in real time) to SUT. The response signal from SUT is to be captured and processed by the ground RF checkout (radar in real time) through the Microwave front end unit. The uplink is from the ground to the onboard. The stimulus signal is synthesized in the RF signal generator. The signal is Fig: 2 : Energy flow diagram of circulators then taken via Isolator to prevent back reflections and then routed to port 1 of the circulator and taken out at port 2 and through a monostatic antenna, reaches the SUT. Circulator routes the stimulus signal between the ports with additional isolation in the reverse direction. The downlink is from the onboard to the ground. The response signal is received through the same antenna and enters port 2 of the circulator and taken out at port Fig.3 : Energy Flow Diagram of Isolators 3. The output signal is taken via an isolator to provide A piece of ferrite material is used to obtain a non isolation in addition to the circulator. Both fixed reciprocal effect. attenuator and variable attenuator attenuates the high power signal to the level suitable for the ground An RF isolator can be thought of as a diode for RF checkout system. The attenuated signal is distributed to energy. An isolator is simply a circulator with one of its the frequency counter, power meter, spectrum analyzer ports terminated with a matched 50Ω load. The device and crystal detector through a 4 port power divider. has only 2 ports, and as a result, has only one path for energy to flow without significant attenuation. Energy Frequency counter measures the frequency, pulse can only enter port 1 and travel to port 2. Any energy width and pulse period. that enters port 2 will be routed to the matched termination on port 3, and quickly dissipated as heat. The transponder power is measured using power This behavior heavily attenuates any signal entering port meter. 2 before it reaches port 1, yet allows almost all of a signal entering port 1 to reach port 2. Spectrum analyzer measures the downlink spectrum. When selecting an isolator and/or circulator, it is important to understand some common electrical Crystal detector detects the downlink pulse. specifications that tells how well it is capable of performing the application. The three basic III. CHARACTERIZATION OF CIRCULATORS AND specifications for isolators and circulators are insertion ISOLATORS loss, VSWR, and isolation. Ferrite isolators and circulators play a fundamental Insertion loss describes how much energy is lost and valuable role in RF systems. They are passive, during the process of transferring a signal from one port ferrite devices that act as traffic conductors for RF of an isolator/circulator to another. It is essentially a energy in a system, routing signals wherever a system measure of how much energy it costs a designer to use an isolator/circulator in their system. As stated above, designer needs them to go. Their ability to behave non- isolators and circulators are passive components, so a reciprocally (non reversible, allowing energy to pass in signal traveling through them has to do so using its own only one direction through the device) when RF energy energy. As in any real system, there will be some is applied to them is very important for a number of attenuation to the signal as it travels through the device. applications This attenuation is called insertion loss, and it is measured in decibels (dB). The higher the insertion loss, The circulator is a passive device with 3 or 4 ports, the more energy it costs to use the isolator or circulator. where power is transferred from one port to the other in This energy is converted into heat on its way through a prescribed manner. That means for a 3-port circulator, the device. However, insertion loss specifications are power entering port 1 leaves port 2, port 3 is decoupled; relatively small, so the benefits a system receives from power entering port 2 leaves port 3, port 1 is decoupled; the use of an isolator/circulator are usually worth the and power entering port 3 leaves port 1, port 2 is energy cost of implementing them. Typical insertion decoupled. loss specifications are on the order of 0.4 dB for octave ISSN (PRINT) : 2320 – 8945, Volume -1, Issue -2, 2013 22 ITSI Transactions on Electrical and Electronics Engineering (ITSI-TEEE) bandwidth units, however the specification can be as circular waveguide and a transition to rectangular small as 0.15 dB for narrowband units, and as high as waveguide is located at each end. 1.7 dB for certain broadband units. If all the ports of a circulator are matched, then a signal applied to one port, for instance to port 1, will emerge from port 2 with a loss called insertion loss. The insertion loss is expressed in dB according to the following formula. IL (dB) =10 log 10 P out2 /Pin 1 (1) A small part of the input signal also emerges from port 3.and the ratio of this signal to the input signal is called isolation. Isolation is expressed in decibels according to the equation I (dB) = 10 log 10 Pout 3/Pin1 (2) Fig.4 : The Faraday Effect The next parameter which characterizes the reflective property of each port of a circulator is the magnitude of the reflection coefficient Г or VSWR.VSWR stands for voltage standing wave ratio. It is a ratio of the maximum voltage to the minimum voltage of a standing wave created by an imperfect impedance match where two boundaries meet (typically where a source meets a load). This standing wave is produced by energy reflecting off of the boundary, and traveling back the way it came. In the case of isolators and circulators, VSWR is the measure of how much of the signal that you want to send through the isolator will reflect back towards the transmitter that sent it. A low VSWR spec is always desirable. Lower VSWRs mean that there is less energy reflecting off of port 1 of the isolator/circulator and going where you don’t want it to go. A. Theory and Operation Fig.5: The Faraday Rotation Circulator The rectangular guide at port 2 is rotated through 45 Michael Faraday discovered in 1845 that a piece of degrees with respect to the guide at port 1.Two other glass become optically active when placed in a strong waveguides emerge radially from the circular magnetic field. The Faraday effect occurs as shown in waveguide at ports 3 and 4.The axis of these two guides fig.4.