Receive Audio Circuits - Motorola 6864115B62-C Detailed Service Manual

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2.4

Receive Audio Circuits

Refer to Figure5-5 for reference for the following sections.
SECTION
2.4.1 Squelch Detect
The radio's RF circuits are constantly producing an output at the discriminator (IF IC). This signal
(DISC AUDIO) is routed to the ASFIC CMP's squelch detect circuitry input DISC (U0221-2). All of
the squelch detect circuitry is contained within the ASFIC CMP. Therefore from a user's point of
view, DISC AUDIO enters the ASFIC CMP, and the ASFIC CMP produces two CMOS logic outputs
based on the result. They are CH ACT (U0221-16) and SQ DET (U0221-17).
The squelch signal entering the ASFIC CMP 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 (U0221-17) from CH ACT. The state of CH ACT and SQ DET is high (logic 1) when
carrier is detected, otherwise low (logic 0).
CH ACT is routed to the µP pin 84 while SQ DET is routed to the µP pin 83.
SQ DET is used to determine all audio mute / unmute decisions except for Conventional Scan. In
this case CH ACT is a pre-indicator as it occurs slightly faster than SQ DET.
35 28
IN IN
34 43
OUT
OPTION
BOARD
J0551
IN
7
DISC
FROM
AUDIO
RF
(IF IC)
17
J0451
EXPANSION
BOARD
Figure 5-1 Receive Audio Paths
1
AUDIO
PA
U0271
9
10 39 41
AUDIO
URX OUT
U IO
VOLUME
AUX RX
ATTEN.
FILTER AND
DEEMPHASIS
2 DISC
LIMITER, RECTIFIER
FILTER, COMPARATOR
CH ACT
THEORY OF OPERATION
ACCESSORY
CONNECTOR
11
J0501
4 16
SPKR +
1
SPKR -
6
INT
INT
SPKR+
SPKR-
CONTROLHEAD
CONNECTOR
3
2
J0401
7
HANDSET
AUDIO
ASFIC_CMP
U0221
18
LS IO
PL FILTER
LIMITER
SQUELCH
CIRCUIT
SQ DET
16
17
84 83
MICRO
80
CONTROLLER
U0101
85
FLT/FLAT RX AUDIO
EXTERNAL
SPEAKER
INTERNAL
SPEAKER