Transmit Audio Circuits; Mic Input Path; Flat Tx Audio Input Path; Ptt Sensing And Tx Audio Processing - Motorola M11URD6CB1_N Service Manual

Theory of Operation
Controller Audio & Signaling Circuits

Transmit Audio Circuits

Mic Input Path

The radio provides two microphone inputs, one on the
control head (internal mic audio) routed through J101-
16), and one on the accessory connector J400-2 (exter-
nal mic audio). The two inputs are connected in paral-
lel. The dc bias required by the microphone is provided
by resistor R205, derived from the 9V3 source and fil-
tered by R204 and C209. The microphone audio signal
is applied via R206 and C211, to the ASFIC at U201-B8.
Filter capacitor C210 provides low-pass filtering to
eliminate frequency components above 3 kHz, and
C211 serves as a DC blocking capacitor. The audio sig-
nal at U201-B8 should be approximately 80mV for 60%
of full system deviation.

Flat Tx Audio Input Path

The FLAT_TX_AUDIO signal from accessory connec-
tor J400-5 is buffered by op-amp U202-(1-3) and fed via
C205 to the ASFIC U201-D7.
CONTROL HEAD
CONNECTOR
J101
MIC
ACCESSORY
CONNECTOR
EXT MIC
J400
FLAT TX
AUDIO
8-10
16
MIC IN
B8
2
5
D7
AUX TX IN
Figure 8-3. Transmit Audio Paths
GTX LTR/Privacy Plus 800 MHz Mobile Service Manual

PTT Sensing and TX Audio Processing

MIC PTT is sensed by the µP. PTT can also be generated
by grounding pin 3 on the radio accessory connector,
J400. When microphone PTT is sensed, the µP will
always configure the ASFIC to enable the mic audio
path. PTT can be configured to enable the EXT_MIC
audio path when J400-2 is connected with an external
mic audio input. Data PTT is configured to enable the
flat transmit audio path (when J400-5 is configured for
FLAT_TX_AUDIO).
Inside the ASFIC, the mic audio is filtered to eliminate
components outside the 300-3000Hz voice band and
pre-emphasize. The capacitor, C231, between ASFIC
pre-emphasis out U201-C8 and ASFIC limiter in U201-
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
enabled. The signal is then limited to prevent the trans-
mitter 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 gen-
erated by the limiter. The audio is then routed to the
VCO 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 at U201-H8 MOD IN,
and is routed to the RF section as MOD IN.
ASFIC U201
FILTERS &
PREEMPHASIS
LS SUMMER
SPLATTER
FILTER
HS SUMMER
LIMITER
LIM IN
PRE EMP OUT
C8
6880905Z99-O
TO RF
SECTION
VCO
(SYNTHESIZER)
H8
ATN
MOD IN
ATTENUATOR
E8
June, 2000