Receive Audio Circuits - Motorola CDM Series Service Manual

Controller Board Audio and Signaling Circuits
2.4

Receive Audio Circuits

Refer to Figure 3-5 for reference for the following sections.
FROM
RF
SECTION
(IF IC)
J0451
EXPANSION
BOARD
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.
35 28
IN
IN
34 43 AUX RX
OUT
OPTION
BOARD
J0551
IN
7
DISC
AUDIO
2 DISC
17
Figure 3-5 Receive Audio Paths
1
AUDIO
4
PA
U0271
6
9
INT
SPKR+
10 39 41
URX OUT AUDIO
U IO
VOLUME
ASFIC_CMP
ATTEN.
U0221
FILTER AND
DEEMPHASIS
PL FILTER
LIMITER
LIMITER, RECTIFIER
SQUELCH
FILTER, COMPARATOR
CIRCUIT
CH ACT SQ DET
16
84
MICRO
CONTROLLER
U0101
Chapter 3: 1-15
ACCESSORY
CONNECTOR
11
FLT/FLAT RX AUDIO
J0501
16
SPKR +
EXTERNAL
1
SPKR -
SPEAKER
INT
SPKR-
CONTROLHEAD
CONNECTOR
3
INTERNAL
2
SPEAKER
J0401
7
HANDSET
AUDIO
LS IO 18
17
83
80
85
6881091C63-A