Motorola Commercial Series Service Information - Page 12

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Motorola Commercial Series Service Information
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3.3

Audio Circuitry

3.3.1
Transmit and Receive Low-Level Audio Circuitry
The majority of RX and TX audio processing is performed by U451, the Audio Filter IC
(ASFIC_CMP), which provides the following functions:
Tone PL/Digital PL encoding and decode filtering
Tone PL/Digital PL rejection filter in RX audio path
TX pre-emphasis amplifier
TX audio modulation limiter
Post-limiter (splatter) filter
TX deviation adjust (digitally-controlled attenuators)
Programmable microphone gain attenuator
RX audio volume control (digitally controlled attenuator)
Carrier squelch adjustment (digitally controlled attenuator)
Microprocessor output port expansion
2.5 volt dc reference source
Microprocessor clock generation (from the 16.8 MHz reference oscillator input)
The parameters of U451 that are programmable are selected by the microprocessor via the CLOCK
(U451 pin 21), DATA (U451 pin 22) and chip enable (U451 pin 20) lines.
RX audio buffer U510 amplifies the audio level from the DEMOD output of the IFIC before being
applied to the audio filter IC input (DISC, U451 pin 2). The buffer is DC coupled to avoid corruption
of low-frequency data waveforms such as DPL. Because such waveforms are polarity sensitive, this
buffer is configured as a single-stage inverting amplifier (U510-1 only) for VHF models where high-
side first injection is used, or is configured as a two-stage non-inverting amplifier (U510-1 and -2) for
UHF models using low-side first injection. The gain of the buffer is 1.5 times or 3.5 dB.
U480 and associated components are not used in CP040. Stage U480-1 is bypassed by jumper
R487.
Volume adjustment is performed by a digital attenuator within U451. The volume control (10KO, part
of S444) is connected to D_3.3V and ground via R506 and R507. When the volume control is
rotated, it varies the dc voltage applied to microprocessor A/D input port PE1 (U401 pin 66) between
approximately 0 volts dc at minimum volume to 3.3 volts dc at maximum volume. Depending on this
voltage, the appropriate setting of the digital volume attenuator is selected. This technique is less
susceptible to noise than a conventional analog volume control.
THEORY OF OPERATION