Service Manual Part 2

CIRCUIT DESCRIPTION
4-13
September 2001
Part No. 001-5100-001
is then fed through the volume control potentiometer
to a second op amp buffer.
The buffer output is routed to a pair of audio
power amplifiers: one to drive the internal speaker and
another to drive the external speaker. Only one of
these audio power amplifiers can be active at a time.
The active power amplifier is selected by the OPT
SEL 1 line (J5-12). The MUTE line turns the active
power amplifier on or off by disconnecting the battery
voltage from the audio power amplifier IC using the
transistor Q4. Transistors Q8 and Q9 and their associ-
ated RC networks remove popping sounds from the
speaker audio by delaying the unmuting of the audio
amp compared to the unMUTE command.
In transmit mode, the audio for transmission can
be selected from either an internal or external micro-
phone, depending upon the presence of an external
microphone and which PTT is pressed. An analog
switch is used to route either the internal or external
microphone signal to the microphone amplifier. The
external microphone signal is buffered by an op amp.
The microphone amplifier has a gain of ten, and is
equipped with a pair of clipping diodes to prevent the
amplified microphone signal from over-driving the A/
D input on the ADSIC.
4.5.7 VOLTAGE REGULATION
The 5-Volt Digital Supply is produced by
switching DC-DC converter U12 which operates off
the Unswitched B+ Supply. The switching frequency
is around 160 kHz. A switching regulator is used to
improve efficiency since the 5-Volt Digital Supply
power consumption is a large percentage of the total
power consumption of the radio. The peak-to-peak
residual ripple on the 5-Volt Digital supply is approxi-
mately 50 mV.
The DC-DC converter is controlled by a wired
AND gate on the Shutdown pin of the device. The two
inputs of the wired AND gate are the SW_B+ and the
output PTC4 of the microcontroller U1. When either
input is high, the DC-DC converter is operating.
The DC-DC converter has a soft-start feature
(R98, C136) to prevent chattering of the output regu-
lated voltage due to “bouncing” of the on/off switch.
The converter has current limiting that limits output
current to 1.5 A. The under voltage protection turns
the converter off if the input (Unswitched B+) voltage
drops below 5.45 V.
The 5-Volt Analog Supply is produced by a linear
regulator running from the Unswitched B+ Supply.
The Unswitched B+ input to the regulator is switched
on and off by a FET that is turned on by the 5-Volt
Digital Supply. The peak-to-peak output ripple of the
5-Volt Analog regulator is less than 10 mV which is
appropriate for analog circuits.
4.6 TRANSMIT FREQUENCY
DETERMINATION
The operational frequency of the transmitter is
determined by the PLL (Phase-Locked-Loop)
consisting of synthesizer U204 and VCO circuit Q202/
U201. Reference oscillator U203 generates and
supplies a reference signal of 16.8 MHz to synthesizer.
The synthesizer contains a programmable reference
divider, programmable A and B dividers, a program-
mable prescaler counter (P), and a programmable frac-
tional N divider with two programmable values (N
numerator and N denominator).
All of these dividers are programmed through the
serial interface which connects the synthesizer to the
controller microprocessor. The 16.8 MHz reference
oscillator frequency is divided down to a synthesizer
reference frequency of 2.1, 2.4, or 2.225 MHz. This
signal is fed to the phase detector which generates the
steering voltage for the VCO. The output of the VCO
circuit is coupled back and divided by AP+B and then
divided by the fractional divider and fed into the
second input of the phase detector. The VCO buffer
has two outputs. One input goes to the input of Rx
mixer chip U2, and the other is applied to the input of
power amplifier module U105.
4.7 HARMONIC FILTER
The transmitter harmonic filter consists of C148-
C151, L126, L127, and L128.With VHF models only,
it also consists of C129 and C130. The circuit is essen-
tially a seven-pole low-pass filter. With VHF units
only, two additional poles are inserted by C129 and
C130 which are series resonant with L126 and L127.
KEYPAD BOARD (CONT’D)