12
Revision G
October 14, 1996
3.3 GENERAL INSTALLATION INSTRUCTIONS FOR AVIONICS (Continued)
Noise and/or interference is usually heard in the audio systems, although it may also
be detected as an interference to indicator operation. Unless a strong suspicion of the
exact source is suspected, it is best to begin a process of elimination, in the following
order:
1. Power Source: Check for low voltage when the avionics load is applied. A high
resistance battery cell in the A/C can cause numerous problems. View the avionics
power line at the avionics master and at the affected unit for noise on the power line.
If present, try to categorize the frequency (eg. alternator whine, which may be caused
by one or more bad alternator diodes, or if interference is present only when a
communication unit is transmitting, etc.). If the noise is present or worse at the
affected unit than at the avionics master, investigate the harness for noise coupling
between wires. If necessary, disconnect the affected unit power from the aircraft
power source and connect to an external power supply or battery.
2. Power Ground: View the power ground line at the avionics master and the affected
unit. If noise is discerned at the master source, ground strapping may be corroded or
partially broken. If only at unit, a larger wire size or wire re-routing may be required.
3. Interference: Both noise and interference may be either conducted or radiated, and in
some cases electromagnetically coupled between units. If it is determined that the
noise or interference is eliminated whenever another avionics unit is not transmitting,
first investigate the radiated alternative. Insure complete and proper bonding of
antennas to the aircraft surface, and check the coaxial cable and all shield
connections and connectors. Review the manufacturer’s recommendations for
antenna separations. Be aware that antenna radiation directly to conductors at the
rear of mounting trays or units may occur if shields are stripped too far back from the
connector or are improperly grounded. Disconnecting the interfering antenna and
substituting an external dummy load may assist diagnosis. Conducted interference
usually occurs through paths which are shared by the avionics equipment such as
power lines, ground points, audio equipment, or induced interference between
adjacent wires or harnesses. Review the manufacturer’s recommendations for
shielded wiring and ground points, and for separation of specific wires. Measure
ground points for a small but perceptible resistance to true ground and view power
lines with an oscilloscope, turning each unit on and off to detect changes. Recheck
common or adjacent connections to jacks, plugs, or shared equipment such as power
converters, breakers, or audio panels.
4. Compromise: In some cases noise or interference may be subdued but not
eliminated. With the inefficient and imperfect platform provided by the aircraft for
antennas and power source, etc, complete elimination of the problem may be very
expensive or impossible (eg. if there is simply not enough space to provide ground
plane or antenna separation as recommended). Or, the aircraft strobe noise is audible
but not objectionable, etc. These problems should be discussed early and thoroughly
with the customer.
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