December 6, 2004 68P81083C20-D
7-18 Controller Section Theory of Operation: Audio and Signalling Circuits
.
Figure 7-2. Transmit Audio Paths
Inside the ASFIC, the mic audio is filtered to eliminate components outside the 300-3000 Hz voice
band, and pre-emphasized if pre-emphasis is enabled. The capacitor between ASFIC pre-emphasis
out U0200-C8 and ASFIC limiter in U0200-E8 AC couples the signal between ASFIC blocks and
prevents the DC bias at the ASFIC output U0200-H8 from shifting when the ASFIC transmit circuits
are powered up. The signal is then limited to prevent the transmitter from over deviating. The limited
mic audio is then routed through a summer, 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 two attenuators, which are tuned in the factory or the field to set the proper amount of
FM deviation. The TX audio emerges from the ASFIC at U0200-H8 AUDIO MOD, at which point it is
routed to the RF section.
Auxiliary TX Audio Paths
There are three auxiliary transmit audio inputs that are routed to the auxiliary transmit path in the
ASFIC. These are AUX TX IN1, AUX TX IN2, and AUX TX IN3. The 3 paths, only one of which can
be active at a time, are buffered by U0202.
HEARCLEAR - I
IC
U0250
ASFIC
U0200
COMP BUF IN
COMP OUT
HPF
MIC IN
TX IN
MIC AMP OUT
EXT MIC IN
AUX TX IN
PRE EMP OUT
LIM IN
VCO ATN
AUDIO MOD
TO
RF
SECTION
(VCO)
C7A6
D3
E8C8
H8
J408
INTERNAL OPTION
CONNECTOR
J401
INTERNAL OPTION
CONNECTOR
J403
ACCESSORY
CONNECTOR
J405
CONTROL HEAD
CONNECTOR
MIC
EXT MIC
AUX TX
D7
F3
AUX TX
AUX TX
14
14
24
A7
13
B8
9
23
FILTERS AND
PREEMPHASIS
SPLATTER
FILTER
LS SUMMER
HS SUMMER
LIMITER
ATTENUATOR
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