Download INTEROFFICE MEMORANDUM 5132-2004

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
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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
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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