Receive Audio Circuits; Squelch Detect - Motorola GM950E Service Manual

Receive Audio Circuits

6.0 Receive Audio Circuits
Refer to Figure 6.3-3 for reference for the following sections.
IF IC
DET AUDIO
SQ LIM OUT
SQ RECT IN
CSQ DET
FAST SQ
6.1

Squelch Detect

The IF IC controls the squelch characteristics of the radio. With a few external parts (R5222, C5229,
C5230, R5223) the squelch tail, hysteresis, attack and delay are optimized for the radio. To set the
squelch threshold the signal from IF IC pin 23 (line SQ ATT IN) is routed to the squelch attenuator
input of the AFIC (U0103-16). The attenuated signal (line SQ ATT OUT) from the AFIC (U0103-18)
enters the IF IC at pin 20 and is used to create a squelch indicator signal available at pin 15 (line
CSQ DET).
The microprocessor controlled ADAPT signal at pin 22 activates the fast squelch indicator signal at
IF IC pin 18 (FAST SQ). Both squelch indicator signals CSQ DET (pin 15) and FAST SQ (pin 18) are
combined, weighted by resistors R0111 / R0112 and fed to one of the microprocessor´s ADCs
(U0101-15) for interpretation. From the voltage weighted by the resistors the µP determines whether
CSQ DET, FAST SQ or both are active.
6.3-12
J0103-4
IN 2
OPTION
BOARD
J0103-2
OUT
IN 1
J0103-5
DET AUDIO
(DISCRIMINATOR
AUDIO)
28
23
20
15
18
Figure 6.3-3 Receive Audio Paths.
1
AUDIO
PA
U0401
9
SPKR+
ATTEN.
23
RX AUD OUT
UNAT
22
RX OUT
VOLUME
ATTEN.
AUX
6
RX IN
FILTER AND
AFIC
DEEMPHASIS
8 PL IN
U0103
7 RX IN
SQUELCH ATTENUATOR
SQ IN SQ OUT
16 18
15
ACCESSORY
CONNECTOR
11
FLT RX AUDIO
J0400
4
SPKR + 16
1
SPKR -
6
INT
SPKR-
INT
CONTROL
HEAD
CONNECTOR
2
1
J0101
14 HANDSET
AUDIO
MICRO CONTROLLER
U0101
CSQ DET
Theory of Operation
EXTERNAL
SPEAKER
INTERNAL
SPEAKER
GEPD 5429-1