Receive Audio Circuits - Motorola MCS 2000 Service Instructions Manual

Controller Section Theory of Operation: Audio and Signalling Circuits
MDC Data
The MDC signal follows exactly the same path as the DTMF high group tone. MDC data utilizes MSK
modulation, in which a logic zero is represented by one cycle of a 1200 Hz, and a logic one by 1.5
cycles of an 1800 Hz. To generate the data, the microcontroller first programs the ASFIC (U0200)
with TXSG1 and TXSG0 control lines to the proper filter and gain settings. It then begins strobing
U0200/Trunking Clock In with a pulse every time there should be a transition in the MDC waveform.
The output waveform from U0200's 5-3-2 State Encoder is then fed to the post-limiter summer block
and then the splatter filter. From that point it is routed through the modulation attenuators and then
out of the ASFIC to the RF board. Microphone audio is muted during MDC signalling.

Receive Audio Circuits

(Refer to Table 7-4 for reference for the following sections)
Squelch Detect
The radio's RF circuits are constantly producing an output at the discriminator. In addition to the raw
discriminator signal DISC (IF1-3-2-1), the RF board's Zero IF IC also provides a separate buffered
version of the discriminator signal that is dedicated to the ASFIC's squelch detect circuitry
SQUELCH (IF1-2-17-1). This signal enters the controller board and is routed to the ASFIC on
U0200-H7. All of the squelch detect circuitry is contained within the ASFIC. Therefore from a user's
point of view, SQUELCH enters the ASFIC, and the ASFIC produces two CMOS logic outputs based
on the result. They are CH ACT (U0200-H1) and SQ DET (U0200-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 (U0200-H1) from CH ACT. The state of CH ACT and SQ DET is high (logic 1) when carrier
is detected, otherwise low (logic 0). Both CH ACT and SQ DET are routed to the SLIC (U0104-F4/
H1).
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.
Audio Processing and Digital Volume Control
The signal enters the controller section from the ZIF on DISC (IF1-3-2-1) and passes through RC
filter R0245 and C0236 which filters out ZIF sampling noise. The signal is AC coupled by C0202 and
enters the ASFIC via the PL IN pin U0200-J7. Inside the IC, the signal goes through 2 paths in
parallel.
68P81083C20-D
7-21
December 6, 2004