1-14 THEORY OF OPERATION
2.4 Receive Audio Circuits
Refer to Figure5-5 for reference for the following sections.
Figure 5-1 Receive Audio Paths
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.
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.
FLT/FLAT RX AUDIO
J0501
11
16
1
EXTERNAL
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