Receive Audio Circuits
5C.3-14 Theory of Operation
6.0 Receive Audio Circuits
Refer to Figure 3.3 for reference for the following sections.
Figure 3.3 Receive Audio Paths.
6.1 Squelch Detect
The radio’s RF circuits are constantly producing an output at the discriminator U5201-28 (UHF) /
U5201-28 (VHF) / U2201-28 (MB). 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 ACT. The state of CH ACT and SQ DET is high (logic 1) when carrier
is detected, otherwise low (logic 0).
FLT RX AUDIO
J0400
11
16
1
EXTERNAL
SPEAKER
INTERNAL
SPEAKER
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