3-
3 - 4 REFERENCE FREQUENCY GENERATOR CIRCUIT
Reference frequency generator IC203 consists of a crystal oscillator and a highspeed divider. X201 oscil-
lates
at
5.12MHz, which
is
divided by 2048. The 2.5KHz reference frequency
is
fed
to
phase
detector
IC202. This 2.5KHz reference frequency decides
the
variation step of the PLL
output
frequency.
3 - 3 - 5 PHASE DETECTOR
AND
LOOP
FILTER
CIRCUITS
Digital phase detector, IC202, detects
the
phase difference of
the
pulse signals
of
the
2.5KHz reference
frequency and
the
output
signal
of
the
programmable divider, and proportionately puts
out
pulse signals
at
Pin
3,
which becomes high impedance when
the
PLL
is
locked.
Pin 4
is
for detecting
the
lock failures and changes
to
ground level according
to
the
phase difference of
the
two
pulse signals. When
the
lock fails,
the
pulse signal from Pin 4
is
integrated by R202 and C215.
When
the
integrated voltage exceeds
the
junction voltage
of
0214's
base,
0214
is
turned
ON
and
then
0107
in
the
MAIN
boards
is
turned
ON.
The collector
of
0107
is
connected
to
the
base of
0108,
so
the
base voltage
of
0108
becomes ground
level, and
0108
and
0106
are
shut
off
to
prevent transmitting unwanted signals.
The
loop filter, consisting
of
R204, R205, R206, C213 and C214, converts
the
pulse signal from Pin 3
into a
DC
voltage and decides
the
response time
of
the
whole loop.
The
output
signals are fed
to
tuning diode D203
of
the
VCO circuit
as
the control voltage for
the
VCO
frequency set.
3-
3 - 6 VCO CIRCUIT
The VCO (Voltage-Controlled Oscilator)
is
a Colpitts circuit using
0201,
and oscillates
in
70 - 80MHz
range.
The
oscillator frequency
is
controlled by a
DC
voltage which
is
supplied from
the
loop filter
to
varactor
diode D203.
In
the
receive mode, R+6V
is
applied
to
D204's anode through L201, and D204
is
turned
ON
and shunts
C220. Thus
the
free-run frequency
of
the
VCO
is
lowered.
In
the
transmit mode, T +6V
is
applied
to
D204's
cathode
through D205 and L202, D204
is
turned
OFF,
and C220
is
inserted
in
the
oscillator circuit in series. Thus
the
free-run frequency of
the
VCO
is
increased.
In
the
same time,
the
VCO signal
is
frequency modulated by
the
audio signals from
the
microphone which are applied
to
the
gate
of
0201
and varies
0201
's mutual conductance.
3 - 4 OTHER CIRCUITRY
3 - 4 - 1
POWER
SUPPLY CIRCUIT
The
regulated 6V
is
supplied
to
the
main circuits, so
that
the
set operates under a stable condition with
as
low power voltage
as
possible.
The power supply voltage
is
fed
to
the
AF power amplifier through
the
squelch switching circuit and
to
the
6V regulator consisting
of
0117
-
0120
and zener diode D104. This regulated 6V
is
supplied
to
the
PLL circuit.
In
the
transmit mode,
the
base of
0123
is
grounded through R 155,
the
microphone and
the
PTT switch,
and
0123
is
turned
ON. Thus
0106
and
0108
are
turned
ON
and T +6V
is
actuated, and supplied
to
the
transmitter circuit.
At
the
same time, T +6V turns
0112
ON, and
the
power supply voltage
is
applied
to
the
MIC
amplifier circuit through
0112.
In
the
receive mode,
0123
is
turned
OFF
and
the
bias voltage
of
0109
ON. Thus
the
R+6V
is
actuated
and
supplied
to
the
PLL board
to
switch
the
local oscillator crystal and
the
driver transistors
of
the
transmitter
circuit.
At
the
same time, R+6V turns
ON
the
voltage boost circuit consisting
of
0110
and
0111,
and +6V
is
supplied
to
the
receiver circuit.
3-3
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