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
Wt
U1•
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.
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