Receive Audio Circuits; Squelch Detect - Motorola GTX/LCS 2000 Service Manual

Receive Audio Circuits

H5
J5
H6
J6
J7
FROM
RF SECTION
(IFIC)
DET AUDIO
(DISCRIMINATOR AUDIO)

Squelch Detect

7-18 Theory of Operation
Refer to the following sections.
1
9
ATTEN.
B2 J4
UNIV IO RX AUD OUT
UNAT RX OUT
VOLUME
EXP AUDIO IN
ATTEN.
RX IN
FILTER &
DEEMPHASIS
AUX RX IN
PL IN
LIMITER, RECTIFIER
FILTER, COMPARATOR
SQ IN
CH ACT
H7
Figure 7-5 Receive Audio Paths
The radio's RF circuits are constantly producing an output at the discriminator
(U5201-28). This signal (DET AUDIO) is routed to the ASFIC's squelch detect
circuitry input SQ IN (U0201-H7). All of the squelch detect circuitry is contained
within the ASFIC. Therefore from a user's point of view, DET AUDIO enters the
ASFIC, and the ASFIC produces two CMOS logic outputs based on the result.
They are CH ACT (U0201-H2) and SQ DET (U0201-H1).
The squelch signal entering the ASFIC is amplified, filtered, attenuated, and
rectified. It is then sent to a comparator to produce an active high signal on CH
ACT. A squelch tail circuit is used to produce SQ DET (U0201-H1) from CH
ACCESSORY CONNECTOR
J0400
SPKR+
4
AUDIO
PA
SPKR-
U0401
6
INT
INT
SPKR-
SPKR+
CONTROL HEAD CONNECTOR
J0101
ASFIC U0201
PL
LIM A4
PL FILTER
LIMITER
SQUELCH
CIRCUIT
SQ DET
H2 H1
25 68
MICRO
10
CONTROLLER
U0101
11
FLAT RX AUDIO
16
EXTERNAL
1
SPEAKER
2
INTERNAL
1
SPEAKER
14
HANDSET
AUDIO