2-10
Theory of Operation
Multi switch U0251 controlled by ASFIC CMP port GCB4 selects either the external microphone
input
signal
or
the voice storage playback signal for entering the ASFIC CMP at pin 48. The audio
signal at U0221-48 (TP0222) is approximately 14mVrms for 1.5kHz deviation with 12.5kHz channel
spacing.
The FLAT TX
AUDIO signal from accessory connector J0501-5 is fed to the ASFIC CMP (U0221,
pin
42) through C0541 and line FLAT TX RTN.
The ASFIC has an internal AGC that controls the gain
in
the microphone audio path. The AGC can
be disabled/enabled by the
IJP.
Another feature that can be enabled/disabled
in
the ASFIC
is
the
VOX.
This circuit, along with the capacitor at U0221, pin
7,
provides a
de
voltage allows the
IJP
to
detect microphone audio. The ASFIC can also be programmed to route the microphone audio to a
speaker for public address operation.
2.3.3.1 PTT Sensing and
TX
Audio Processing
The microphone PTT signal coming from the control head
is
sent via the SBEP bus to the
IJP.
An
external PTT can be generated by grounding pin 3 on the accessory connector if this input
is
programmed for PTT by the CPS. When microphone PTT
is
sensed, the
IJP
always configures the
ASFIC CMP for the "internal" microphone audio path, and external PTT results
in
the external
microphone audio path being selected.
Inside the ASFIC
CMP,
the microphone audio is filtered to eliminate frequency components outside
the 300-3000Hz voice band, and pre-emphasized if pre-emphasis is enabled. The signal
is
then
limited to prevent the transmitter from over deviating. The limited microphone audio
is
then routed
through a summer, which is used to add
in
signalling data, and then to a splatter filter to eliminate
high frequency spectral components that could be generated by the limiter. The audio is then routed
to
an
attenuator, which is tuned
in
the factory or the field to set the proper amount
of
FM deviation.
The TX audio emerges from the ASFIC CMP at U0221-40 MOD
IN,
at which point it
is
routed to the
RF
section.
2.3.3.2 TX Secure Audio (optional)
The audio follows the normal transmit audio processing until it emerges from the ASFIC CMP TX
SND pin (U0221-44), which
is
fed to the Secure board residing at option connector J0551-33. The
Secure board contains circuits to amplify, encrypt, and filter the audio. The encrypted signal
is
then
fed
back from J0551-32 to the ASFIC CMP TX RTN input (U0221-36). The signal level at this pin
should be about 65mVrms. The signal is then routed through the
TX
path
in
the ASFIC CMP and
emerges at MOD
IN
pin 40.
2.3.3.3 Option Board Transmit Audio
The audio follows the normal transmit audio processing until it emerges from the ASFIC CMP TX
SND pin (U0221-44), which
is
fed to the option board residing at option connector J0551-33. The
option board contains circuits to process the audio. The processed signal
is
then fed back from
J0551-32 to the ASFIC CMP TX
RTN
input (U0221-36). The signal level at this pin is approximately
65mVrms. The signal
is
then routed through the TX path
in
the ASFIC CMP and out at MOD
IN,
pin
40.
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