Motorola MCX600E, Service Manual, Page: 51 / 105

Receive Audio Circuits
Theory of Operation 4-13
6.0 Receive Audio Circuits
Refer to Figure 4.3 for reference for the following sections.
Figure 4.3 Receive Audio Paths.
6.1 Squelch Detect
The radio’s RF circuits are constantly producing an output at the discriminator U5201-28 (UHF) /
U5201-28 (VHF) / U8201-28 (300MHz-R1). 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 ACT. The state of CH ACT and SQ DET is high (logic 1) when carrier
is detected, otherwise low (logic 0).
FLT RX AUDIO
J0400
11
16
1
EXTERNAL
SPEAKER
INTERNAL
SPEAKER
ACCESSORY
CONNECTOR
CONTROL
HEAD
CONNECTOR
HANDSET
AUDIO
14
1
2
J0101
INT
SPKR-
SPKR +
SPKR -
1
9
ATTEN.
J0103-2
H6
J7
J6
J0103-4
J0103-5
DET AUDIO
(DISCRIMINATOR AUDIO)
GEPD 5428-1
H7
J4
B2
INT
SPKR+
4
6
RX IN
PL IN
AUX RX IN
SQ IN
ASFIC
U0201
AUDIO
PA
U0401
IN 2
OPTION
BOARD
IN 1
OUT
VOLUME
ATTEN.
FILTER AND
DEEMPHASIS
H1
MICRO
CONTROLLER
U0101
10
FROM
RF
SECTION
(IF IC)
LIMITER, RECTIFIER
FILTER, COMPARATOR
SQ DET
UNAT RX OUT
SQUELCH
CIRCUIT
H2
PL FILTER
LIMITER
CH ACT
EXP AUDIO IN
J5
A4
PL
LIM
H5
UNIV IO RX AUD OUT
25 43
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