Receive Audio Circuits; Squelch Detect - Motorola MCX600E Service Manual

6.0

Receive Audio Circuits

Refer to Figure 4.3 for reference for the following sections.
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).
Theory of Operation
J0103-4 H5
IN 2
J5
OPTION
BOARD
J0103-2
H6
OUT
IN 1
J0103-5
J6
J7
FROM
RF
DET AUDIO
SECTION
(DISCRIMINATOR AUDIO)
(IF IC)
Figure 4.3 Receive Audio Paths.
1
AUDIO
4
PA
U0401
6
9
INT
SPKR+
ATTEN.
J4
B2
UNIV IO
RX AUD OUT
UNAT RX OUT
VOLUME
EXP AUDIO IN
ATTEN.
RX IN
FILTER AND
DEEMPHASIS
AUX RX IN
PL IN PL FILTER
LIMITER
LIMITER, RECTIFIER
FILTER, COMPARATOR
SQ IN CH ACT
H2
H7
25
CONTROLLER
Receive Audio Circuits
ACCESSORY
CONNECTOR
11
FLT RX AUDIO
J0400
SPKR + 16
1
SPKR -
INT
SPKR-
CONTROL
HEAD
CONNECTOR
2
1
J0101
14
HANDSET
AUDIO
ASFIC
U0201
PL
A4
LIM
SQUELCH
CIRCUIT
SQ DET
H1
43
MICRO
10
U0101
GEPD 5428-1
EXTERNAL
SPEAKER
INTERNAL
SPEAKER
4-13