Audio Power Amplifier; Internal Microphone Audio Voice Path; Ptt Circuits - Motorola EP450 Service Manual

Controller Theory of Operation: Controller
• 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.
Volume adjustment is performed by a digital attenuator within U451. The volume control (10KO, part
of S444) is connected to D_3.3 V 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.

3.1.2.2 Audio Power Amplifier

The audio power amplifier IC U490 amplifies receiver audio from U451 pin 41 to a level sufficient to
drive a loudspeaker. U490 is a bridge amplifier delivering 3.46 volts rms between pins 5 and 8
without distortion, which is sufficient to develop 500 milliwatts of audio power into the internal 24 ohm
speaker or an external 24 ohm load. The audio power amplifier is muted whenever speaker audio is
not required to reduce current drain. The audio amp is muted when U451 pin 14 is low. When U451
pin 14 is high, U490 pin 1 is pulled low by Q490, enabling the audio amplifier.
Because the power amplifier is a bridge-type, neither speaker terminal is grounded. Care should be
taken that any test equipment used to measure the speaker audio voltage does not ground either
speaker output terminal, otherwise damage to the audio power amplifier IC may result. When a 24-
ohm load resistor is used it should be connected between the tip and the sleeve of accessory jack
J471 (3.5mm port), never to ground. External SPKR plug insertion mechanically disconnects the
internal speaker. Voltage measurements using test equipment that is not isolated from ground may
be made from one side of the speaker or load resistor (either the tip or the sleeve of J471) to chassis
ground, in which case the voltage indicated will be one half of the voltage applied to the speaker or
load resistor. The Motorola RLN4460 Portable Test Set and AAPMKN4004 Programming Test Cable
provide the proper interface between the radio's ungrounded audio output and ground-referenced
test equipment.

3.1.2.3 Internal Microphone Audio Voice Path

Microphone audio from internal microphone is routed from J470-1 via C475, L471, and C470 to the
ASFIC_CMP mic audio input (MICINT, U451 pin 46). During transmit, Q470 is turned on by a low at
U451 pin 35, providing dc bias for the internal MIC via R478. External MIC plug insertion
mechanically disconnects the internal microphone. External MIC audio is coupled through L471 and
C470 to the mic audio input. An input level of 10 mV at J471 pin 4 produces 200 mV at the output of
U451 pin 40, which corresponds to 60% deviation.

3.1.2.4 PTT Circuits

The internal side-mounted PTT switch (S441) is sensed directly by microprocessor port PJ0 (U401
pin 71). External mic PTT is sensed by measuring the current drawn through the accessory
connector (J471-4) by the mic cartridge (which is in series with the accessory PTT switch). This
current is drawn through the base (pin 5) and emitter (pin 4) of a transistor in Q470, causing its
collector (pin 3) to supply a logic-high to microprocessor port PJ1 (pin 72).
HKLN4216A
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November 24, 2003