Audio And Signalling Circuits; Audio Signalling Filter Ic (Asfic); Audio Ground; Transmit Audio Circuits - Motorola MCS 2000 Service Instructions Manual

Audio and
Signalling
Circuits
Audio Signalling
Filter IC (ASFIC)

Audio Ground

(Refer to schematic
page 10-25 for
reference)
Transmit Audio
Circuits
(Refer to schematic
page 10-23 for
reference)
(Refer to ASFIC schematic page 10-23 for reference)
The ASFIC has 4 functions;
1. RX/TX audio shaping, i.e. filtering, amplification, attenuation
2. RX/TX signalling, PL/DPL/HST/MDC/MPT
3. Squelch detection
4. Microprocessor clock signal generation (see Microprocessor Clock
Synthesizer Description Block).
The ASFIC is programmable through the SPI BUS (U0200-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. In this radio all, except GCB2 used for AUX TX
IN2 (see Aux TX), are used to control the Hear Clear IC. (See Hear Clear
Description Block for details).
VAG is the dc bias used as an audio ground for the op-amps that are external
to the Audio Signalling Filter IC (ASFIC). U0201 forms this bias by dividing
9.3 V with R0206 and R0207 and buffering the 4.65 V result with a voltage
follower. VAG emerges at pin 1 of U0201. C0235 is a bypass capacitor for VAG.
The ASFIC generates its own 2.5 V bias for its internal circuitry. C0210 is the
bypass for the ASFIC's audio ground dc bias. Note that while there are ASFIC
VAG, BOARD VAG (U0201), and Hear Clear VAG, each of these are separate.
They do not connect together.
(Refer to Figure 7-2 for reference for the following sections)
The radio supports 2 distinct microphone paths known as internal and
external mic. The microphones used for the radio require a DC biasing voltage
provided by R0222 and R0223 for internal, and R0231 and R0230 for external.
These two microphone audio input paths enter the ASFIC at U0200-A7
(external mic) and U0200-B8 (internal mic). Following the internal mic path;
the microphone is plugged into the radio control head and is connected to the
controller board via J0405-9.
From here the signal is routed to R0224. R0222 and R0223 provide the
9.3 VDC bias and R0224 provides input protection for the CMOS amplifier
input. R0223 and C0219 provide a 560 ohm AC path to ground that sets the
input impedance for the microphone and determines the gain based on the
emitter resistor in the microphone's amplifier circuit.
Filter capacitor C0224 provides lowpass filtering to eliminate frequency
components above 3 kHz, and C0218 serves as a DC blocking capacitor. C0427
bypasses RF interference which may couple onto the line to ground. The audio
signal at U0200-B8 should be approximately 80 mV to achieve 60% of
maximum system deviation.
The ASFIC has an internal AGC that can control the gain in the mic audio
path. The AGC can be disabled/enabled by the µP. Another feature that can be
Controller Section Theory of Operation 7-17