Receive Audio Circuits; Squelch Detect; Audio Processing And Digital Volume Control; Figure 2-5. Receive Audio Paths - Motorola CDM1550 LS+ Detailed Service Manual

Theory of Operation 2-13
2.3.5
Receive Audio Circuits
Refer to Figure Figure 2-5 for the descriptions that follow.
OPTION
BOARD
J0551
1 ~1
7
FROM
A~~7o
SE~~ON ----'1-t------"-1 ~
r D ::;; ISC ;:___--+-+---1
(IFIC)
EXPANSION
BOARD
ACCESSORY
r.c:•«• .:f()n
1'111
INT
WtU1• o<Pl<rt
ASFIC CMP
U0221
LSIO 1S
16
17
1l4 63
MICRO
CONTROLLER
U0101
ao
Figure 2-5. Receive Audio Paths
FLTRX AUDIO
EXTERNAL
SPEAKER
INTERNAL
SPEAKER
2.3.5.1 Squelch Detect
The squelch detect circuits are all contained within the ASFIC CMP as shown in Figure 2-5. The
radio's RF circuits are constantly producing an output (DISC AUDIO) at the discriminator IF IC. The
output signal is applied to the ASFIC CMP's squelch detect circuits DISC input (U0221, pin 2). 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 (CH ACT). The squelch circuit produces the SQ
DET signal at U0221, pin 17 from the CH ACT input signal. The state of CH ACT and SQ DET go
from a low (logic 0) to a high (logic 1) when an RF carrier is detected. The CH ACT and SQ DET
signals from the squelch circuit are applied to the 1-1P pins 84 and 83 respectively.
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.
2.3.5.2 Audio Processing and Digital Volume Control
The receiver audio signal (DISC AUDIO) enters the controller section from the IF IC where it is AC
coupled by C0227 before entering the ASFIC CMP via the DISC input at U0221, pin 2. The signal is
then applied to both the audio and the PL/DPL paths.