Motorola MCX600E, Руководство по обслуживанию

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General
Theory of Operation 4-9
CONTROLLER BOARD AUDIO AND SIGNALLING CIRCUITS
3.0 General
3.1 Audio Signalling Filter IC (ASFIC)
The ASFIC (U0201) used in the controller has 4 functions;
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RX/TX audio shaping, i.e. filtering, amplification, attenuation
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RX/TX signalling, PL/DPL/HST/MDC/MPT
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Squelch detection
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Microprocessor clock signal generation (see Microprocessor Clock Synthesizer Description
Block).
The ASFIC is programmable through the SPI BUS (U0201-E3/F1/F2), normally receiving 21 bytes.
This programming sets up various paths within the ASFIC to route audio and/or signalling signals
through the appropriate filtering, gain and attenuator blocks. The ASFIC also has 6 General Control
Bits GCB0-5 which are CMOS level outputs and used for AUDIO PA ENABLE (GCB0) to switch the
audio PA on and off, EXTERNAL ALARM (GCB1) and B+ CONTROL (GCB2) to switch the voltage
regulators (and the radio) on and off. GCB3 controls output GPI/O (accessory connector J0400-12),
HIGH LOW BAND (GCB4) can be used to switch between band splits and GCB5 is available on the
option board connector J0102-3. The supply voltage for the ASFIC has additional filtering provided
by Q0200, D0200, R0200, L0200 and C0200. Diode D0200 increases the voltage at the base of
Q0200 about 0.6 volts above the 5 volt supply voltage to compensate the base - emitter voltage drop
of Q0200.
3.2 Audio Ground
VAG is the dc bias used as an audio ground for the op-amps that are external to the Audio Signalling
Filter IC (ASFIC). U0251-1 form this bias by dividing 9.3V with resistors R0251, R0252 and buffering
the 4.65V result with a voltage follower. VAG emerges at pin 1 of U0251-1. C0253 is a bypass
capacitor for VAG. The ASFIC generates its own 2.5V bias for its internal circuitry. C0221 is the
bypass for the ASFIC’s audio ground dc bias. Note that while there are ASFIC VAG, and BOARD
VAG (U0201-1) each of these are separate. They do not connect together.
4.0 Transmit Audio Circuits
Refer to Figure 4.1 for reference for the following sections.
4.1 Mic Input Path
The radio supports two distinct microphone paths known as internal (from Control Head) and
external mic (from accessory connector J0400-2) and an auxiliary path (FLAT TX AUDIO). The
microphones used for the radio require a DC biasing voltage provided by a resistive network.
These two microphone audio inputs are connected together through R0413; resistors R0414 and
R0415 are not placed. Following the internal mic path; the microphone is plugged into the radio
control head and is connected to the controller board via J101-16.
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