External Mic Path; Ptt Sensing And Tx Audio Processing - Motorola GTX/LCS 2000 Service Manual

J0101
16
MIC
CONTROL HEAD
CONNECTOR
2
EXT MIC
J0400
5
FLAT TX
AUDIO
ACCESSORY
CONNECTOR

External Mic Path

PTT Sensing and TX Audio
Processing
A6
MIC AMP OUT
MIC IN
B8
MIC
EXT IN
MIC IN
A7
D7
AUX TX IN
PRE EMP OUT
C8
Figure 7-3 Transmit Audio Paths
Filter capacitor C0210 provides low-pass filtering to eliminate frequency
components above 3 kHz, and C0211 serves as a DC blocking capacitor. The
audio signal at U0201-B8 should be approximately 80mV for 3kHz of deviation
with 25 kHz channel spacing.
The FLAT TX AUDIO signal from accessory connector J0400-5 is buffered by
op-amp U0202-1 and fed to the ASFIC U0201-D7 through C0205.
The external microphone signal enters the radio on accessory connector J0400 pin
2 and connects to the standard microphone input through R0413. The signal is
routed to the ASFIC (U0201-A7) through resistor R0414 and capacitors C0413
and C0414, with DC bias provided by R0415 / R0416.
Mic PTT is sensed by the µP. An external PTT can be generated by grounding pin
3 on the radio accessory connector. When microphone PTT is sensed, the µP will
always configure the ASFIC for the "internal" mic audio path, and external PTT
will result in the external mic audio path being selected for models with separated
MIC and EXT MIC signals.
Inside the ASFIC, the mic audio is filtered to eliminate components outside the
300-3000Hz voice band, pre-emphasized if pre-emphasis is enabled. The
capacitor between ASFIC pre-emphasis out U0201-C8 and ASFIC limiter in
U0201-E8 AC couples the signal between ASFIC blocks and prevents the DC bias
at the ASFIC output U0201-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 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
C7
TX IN
FILTERS &
PREEMPHASIS
ASFIC U0201
LS SUMMER
SPLATTER
FILTER
VCO
ATN
HS SUMMER H8
LIMITER
ATTENUATOR
LIM IN
E8
Theory of Operation
MOD IN
TO RF
SECTION
(SYNTHESIZER)
7-15