Technical data

VHF/AM RADIO FSG 2T
4 Recommended Set-up Sequence
50 FAV_D10048 June 2014
4.8 Ground checks with engine running (if applicable)
Make sure that the aircraft's electrical DC system voltage on the FSG 2T input terminals is
within the tolerances permitted for 14 Vdc systems (or 28 V with DC converter). Systems
supplied only by a battery may operate from 9 to 16.1 Vdc. Such testing requires normal
RPM rated at cruising speed.
This testing evaluates the audio / noise influence, and the RF radio range, at least from
10 to 30 km distance to the ground radio station.
At cruising RPM and properly adjusted microphone sensitivity, cabin noise background
content shall almost not exist. Instead, loud and clear communications shall be achieved.
In both Transmit and Intercom Mode the microphone shall be placed right at the lips in
order to obtain maximum voice level at lowest cabin noise content. Always speak loud
and clear. Verification of proper microphone sensitivity setting can be controlled visually
by observation of the TX/RX LED flickering YELLOW according to the voice modulation.
Adjustments are to be made once while on ground, prior to the first flight.
It is recommended to verify the FSG 2T communications quality as well as Radio Range /
Distance on both the low and high end of the VHF COM frequency band.
If noise (both with Squelch ON and OFF) occurs only with the engine running, and if its
frequency varies with the engine revolutions, this may be caused by an inadequate
suppressed ignition system or alternator / voltage regulator equipment, or by a poorly
stabilized on-board power supply.
In Receive Mode it is easier to determine between RF and AF (interference carried on
the lines) by temporary removing the antenna plug from the transceiver. RF interference,
which usually comes from the ignition or generator / regulator, or which are caused by
other onboard RF emitting sources, or even caused by RF interference emission sources
located in close proximity to the aircraft will then disappear.
According to Section 18 of the ED-14D / RTCA DO-160D environmental power supply
conditions requirements, the Category B limits for ripple voltages at 14 Vdc supplies are
0.28 V
rms
at 0.2 ... 1 kHz, or 0.7 V
rms
at 1 ... 15 kHz. Double these limits for 28 V systems.
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 like bad contacts,
wrong fuse, defective switches, inadequate wiring dimensions, ground loops (more than
one ground connection to the airframe), or from corroded power supply (fuselage
ground) contacts.
Often will bad contacts in the antenna system, or other vibration dependent metal parts
contacts also varying with the engine RPM, cause RF interfering distortion. A noise
attenuating headphone assists in typical acoustic failure characteristics identification.