Receive Audio Circuits; Squelch Detect - Motorola CM200 Service Manual

Receive Audio Circuits

9.0 Receive Audio Circuits
9.1

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 (U504-pin 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 (U504-16) and SQ DET (U504-17).
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 (U504-17) 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.
37
GCB4
43
AUX RX
DISC
FROM
AUDIO
2
RF
SECTION
(IF IC)
Figure 2-9 Receive Audio Paths
1
AUDIO
4
PA
U502
6
9
INT
SPKR+
MUTE
U509
U505
10 39 41 14
U IO
URX OUT AUDIO GCB1
VOLUME
ASFIC_CMP
ATTEN.
U504
FILTER AND
DEEMPHASIS
DISC
PL FILTER
LIMITER
LIMITER, RECTIFIER SQUELCH
FILTER, COMPARATOR
CIRCUIT
CH ACT
16
84
MICRO
CONTROLLER
U403
ACCESSORY
CONNECTOR
11
FLT RX AUDIO
P1
16
SPKR +
EXTERNAL
1
SPKR -
SPEAKER
INT
SPKR-
CONTROL HEAD
CONNECTOR
19
INTERNAL
20
SPEAKER
J2
18
HANDSET
AUDIO
18
LS IO
SQ DET
17
83
80
85
2-19