Receive Audio Circuits
3.1-14 Introduction/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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