FSG 90 System
3 Installation
December 2014 FAV_D10024 31
If noise (both with Squelch On and Off) occurs only with the engine running, and if its
frequency varies with the engine revolutions, it may be caused by an inadequate
suppressed ignition system or alternator/voltage regulator equipment, or by poor
stabilized on-board supply.
According to ED-14C / RTCA DO-160C requirements, the limits for ripple voltages at
14 Vdc supplies are 0.79 V
pp
at 0.2 ... 1 kHz, or 1.98 V
pp
at 1 ... 15 kHz. Double these
limits for 28 V systems.
It is easier to determine in Receive Mode between RF and AF (interference carried on
the lines) by temporary removing the antenna plug at the transceiver. RF interference,
which usually comes from the ignition or generator/regulator will then disappear.
Ripple on the lines can be detected with an oscilloscope. Its source is usually the
generator equipment in conjunction with a poor battery, or poor wiring (bad contacts,
defective switches, inadequate wiring dimensions, ground loops (more than one ground
connection to the airframe).
RF interfering distortion is often caused by bad contacts in the antenna system, or other
vibration depending on metal part contacts varying with the engine RPM. A noise
attenuating headphone assists typical acoustic failure characteristics identification.
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