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INTEROFFICE MEMORANDUM 5132-2004-165 Project: ANITA December 17, 2004 TO: Kim Leschley FROM: Tom Pierce SUBJECT: ANITA, Power to Receiver and Thermal Sensor Wiring Recommendations SUMMARY The ANITA.(ANtarctic Impulsive Transient Antenna) vehicle power and thermal sensor system grounding and wiring have been evaluated to optimize EMC properties. The ANITA system contains 40 wideband antennas and their respective digital-wideband receivers whose properties include a high sensitivity to ambient and onboard generated electromagnetic noise (EMI) that could potentially compromise the mission. ANITA system power distribution, thermal sensor wiring methods, and connector recommendations are presented in this memo. Grounding Strategy Ground circuits should carry no intended signal energy or power currents, their sole purpose being to provide reference point connectivity. Thus, in a good system design, all circuits should be complete unto themselves before adding ground wires or connections. A ground-loop is a condition where the powerline return current is shared by the structure ground and is a condition to be avoided. A single point grounding system is effective in minimizing the direct coupling effects that occur when sensitive low level analog signal returns share common conductors with inherently noisy logic or power returns. The single point refers to a point along the power circuit return and not a point on the vehicle structure. The point selected is usually at or close to the power source but technically it may positioned anywhere along the powerline. Receiver (RFCM units) Powerlines In the case of the RFCM units and their respective power supply, the power circuit ground reference termination point is limited to a point within each of the RFCM units. This requires that the RFCM powerline interface cables be braid shielded while the power source output terminals are DC isolated from structure ground. The powerline cable braids shall be structure grounded on both ends placing all the RFCM circuits as well as their power supply circuit and interface transmission lines within an electrically continuous Faraday cage. Kim Leschley -2- 5132-2004-165 Thermal Sensor Circuits In the case of the thermal sensors (Analog Devices AD-590), the sensors and their respective transmission lines require unusual wiring techniques in order to ambient field coupling properties to an optimally low value (see Figure 6-2). Because the thermal sensors represent unusually high interface cable termination impedance, the device requires that a small R-C network be added. Each sensor terminal pair shall be bridged with a 0.3 uF bypass capacitor and use a 1000 ohm resistor in series with each terminal. The transmission line shall be a twisted-shielded-pair having a controlled impedance of 100 ohms. The braid shall electrically float on the sensor end while structure terminated (grounded) using EMI connector backshells on the signal conditioner end. Like the sensor terminals, the signal conditioner terminals will use a pair of 1000 ohm series connected resistors. Signal conditioner network uses an Analog Devices AD524 signal conditioner. Connectors Coaxial cables shall use high reliability and dedicated coaxial connectors. Powerlines and most sensor lines may use a multi-pin connector and shall use adjacent connector pins for any given power or signal conductor pair. Cable/wire braids will not use connector pins but, instead, electrically terminate to the structure through connector EMI backshells (Glenair or Kern is recommended). Bundles of shielded cable braids should be electrically banded together inside the connector backshell and connected to the backshell through a short jumper. Kim Leschley -3- 5132-2004-165 Powerline Cables Powerline cables shall be either twisted-pair wires or shielded-twisted-pair (braided) wires. Powerline cables shall be routed as closely as possible to the structure. If a braid shield is used, it shall be structure terminated (grounded) on both ends using EMI connector backshells. If a powerline is included in a multiple pin connector, the powerline shall use adjacent connector pins. Power return current shall not be shared by the structure or by the braid (except for structure currents contributed by user input low-pass filters). If a fault-clearing wire (green wire) is included, it shall be carried as one wire of a powerline twisted-triplet and structure terminated at the power source. Signal Cables Signal cables include relay and discrete signal interface cables as well as low-level AC interface cable types not otherwise classified. Signals shall be carried on twisted-wire pairs; the signal return current shall be carried on a wire and not be carried by either the braid or the structure ground. Signal cables shall be treated as balanced transmission lines; coaxial cables or single-ended circuit techniques shall be avoided. Whether braid shielding is used shall be considered on a case-by-case basis but is generally recommended as a conservative design practice. If a signal pair is included in a multiple pin connector, the wire pair shall use adjacent connector pins. Cable shields may be bundle overbraids, twisted-pair braids, or both. If a braid shield is used it shall be structure terminated (grounded) on both ends using EMI connector backshells. Sensor Cables Sensor cables shall be carried as shielded-twisted-pair cables. Sensor cable braided shields are generally structure terminated on both ends but certain high impedance low level signal interface applications may use a twisted-pair braid that is grounded on one end only. Sensor cables shall be routed as closely as possible to the structure. Sensor cables shall be treated as balanced transmission lines; coaxial cables or single-ended circuit techniques shall be avoided. Sensor cable shields may use bundle overbraids, twisted-pair braids, or both. If a sensor cable is included in a multiple pin connector, the signal-pair shall use adjacent connector pins. A sensor wire pair may be ungrounded or a signal terminal ground referenced on one end but not grounded on both ends. Digital Data-bus Cables Digital data-bus interface cables shall be carried as impedance controlled, balanced, and shielded transmission lines. If a data-bus pair is included in a multiple pin connector, the wire pair shall use adjacent connector pins. Data-bus cables shall be treated as balanced and impedance-matched transmission lines; coaxial cables or singleended circuit techniques shall be avoided. If a data-bus pair is included in a multiple pin connector, the wire pair shall use adjacent connector pins. Wire pair braided shields and bundle overbraids shall be structure terminated (grounded) on both ends using EMI connector backshells. Kim Leschley -4- 5132-2004-165 RF Cables RF carrying cables are usually carried by coaxial cable whether an antenna cable or an RF interface cable that interconnects equipment. TRP/em cc: L. Compton J. Forgrave I. Jun D. Goldstein [email protected] P. Gorham [email protected] K. Liewer J. Link [email protected] P. Narvaez B. Ruff A. Whittlesey