7-18
Theory of Operation
Receive Audio Circuits
Refer to the following sections.
Figure 7-5 Receive Audio Paths
Squelch Detect
The radio’s RF circuits are constantly producing an output at the discriminator
(U5201-28). 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
J0400
11
16
1
FLAT RX AUDIO
EXTERNAL
INTERNAL
ACCESSORY CONNECTOR
CONTROL HEAD CONNECTOR
HANDSET
14
1
2
J0101
INT
SPKR+
SPKR-
1
9
ATTEN.
H6
J7
J6
DET AUDIO
H7
J4
B2
INT
4
6
RX IN
PL IN
AUX RX IN
SQ IN
ASFIC U0201
AUDIO
VOLUME
FILTER &
H1
MICRO
10
FROM
LIMITER, RECTIFIER
SQ DET
UNAT RX OUT
SQUELCH
H2
PL FILTER
CH ACT
EXP AUDIO IN
J5
A4
PL
H5
UNIV IO
RX AUD OUT
25
68
SPEAKER
SPEAKER
AUDIO
DEEMPHASIS
ATTEN.
RF SECTION
(IFIC)
(DISCRIMINATOR AUDIO)
FILTER, COMPARATOR
CIRCUIT
CONTROLLER
U0101
LIMITER
SPKR+
SPKR-
PA
U0401
LIM