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Garmin GTN 650 Manual

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190-01007-A3 GTN 6XX/7XX AML STC Installation Manual
Rev. 4 Page 2-37
2.4.12 Audio Electrical Noise
Because the audio panel is a point in the aircraft where signals from many pieces of equipment are
brought together, take care to minimize effects from coupled interference and ground loops. Coupled
interference can creep into audio system interconnecting cables when they are routed near large AC
electric fields, AC voltage sources and pulse equipment (strobes, spark plugs, magnetos, EL displays,
CRTs, etc). Interference can also couple into audio system interconnecting cables by magnetic induction
when they are routed near large AC current-carrying conductors or switched DC equipment (heaters,
solenoids, fans, autopilot servos, etc).
Ground loops are created when there is more than one path in which return currents flow or when signal
returns share the same path as large currents from other equipment. These large currents create
differences in ground potential between the various equipment operating in the aircraft. These differences
in potential can produce an additive effect on audio panel input signals. The audio panel may “hear” the
desired input signal plus an unwanted component injected by ground differentials, a common cause of
alternator-related noise. This is the main reason why all audio jacks should be isolated from ground.
Terminating audio shields at one end eliminates a potential ground loop injection point. The audio shields
should only be grounded at one end to prevent ground loops. The wiring diagrams and accompanying
notes in this manual should be followed closely to minimize noise effect
s.
2.4.13 Transmit interlock and Split COM Operation
In small aircraft, COM and NAV receiver interference is affected by both the distance between antennas
and the tuned frequency separation. With transmit interlock activated in the COM transceivers, split COM
operation between a flight crew of more than one pilot is affected.
In aircraft that have a transmit interlock feature, when either transmitter is keyed, all other receivers are
muted so that they won’t pick up interference from the active COM transmitter. This is the preferred
option for single pilot operation.
For aircraft with two flight crew members, transmit-interlock would likely interfere with communications.
When the pilot or the copilot transmit, no audio is heard on any other receiver. This means that if the pilot
is communicating with ATC while the copilot transmits on another radio, all pilot reception is cut off
during the time of copilot transmission.
If the installation does not have transmit-interlock activated, all the receivers are listening all the time
whether any radio is transmitting or not. Split COM performance varies significantly across installations.
If the transceivers interfere with each other, transmission by one radio produces static or squeal, thus loss
of communication from any other radio.
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