Southern Avionics Company SD100 Manual Do Utilizador

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The Audio PCB is an  OPTIONAL addition to the normal system which allows the user to broadcast AWOS 

announcements. The Option is implemented as a plug in piggyback board to the Digital I/O PCB and consists of four 
sections as follows:

Voice Amplifier
Automatic Gain Control
Chebychev Passband Filter 
Voice Detector

The Voice amplifier takes the nominal -17dBm signal level and boosts it to a level useable by the AGC 

section.
It is common mode balanced and its Gain is adjustable.

The AGC circuit is responsible for modulation limiting. It is adjusted to "capture" the level at -17dBm and  

output a constant modulation level to the Digital I/O PWM section up to levels exceeding +10 dBm. This section 
prevents inadvertent over modulation of the voice signal, and is necessary since the Controller / Monitor has no 
control over user input. 

The Chebychev Passband filter is designed for 300 to 3000 Hz. The output at 4000 Hz is down more than 45 

dB ensuring any out of band audio signals  from the USER will not cause adjacent channel interference.

The Voice Detector has two functions. It detects the presence of an announcement sequence and 

automatically attenuates the Keying tone level down to 20% modulation. The announcement level is summed with 
the Keying tone level and set to achieve 95% composite modulation. This action prevents overmodulation and allows 
the announcement to be heard over the beacon Morse code IDENT tones. It also signals the Controller /Monitor 
which in turn temporarily suspends Modulation Alarms since the IDENT tone level is dropped below the alarm 
threshold.