Realistic PRO-2021 - Principles of Operation

PRINCIPLES OF OPERATION
The PRO-2021 is a PLL (Phase Locked Loop) Synthesized VHF/UHF AM/FM Receiver, controlled by a
CPU (Central Processing Unit) via the keyboard.
The VHF Low Band (30 — 54 MHz) or Mid Band (68 ~ 88 MHz) and High Band (138 ~ 174 MHz) are
received in 5 kHz increments. Aircraft Band (108 — 136 MHz) is set up for 25 kHz increments, and the
UHF Band (380 ~ 512 MHz) for 12.5 kHz increments.
Receiving freguency range, freguency determination, scanning speed, delay time, etc., are all functions
controlled by the CPU. The CPU is able to do only the assigned functions, and no modification of the CPU
is feasible.
The following paragraphs explain the operation of the circuit in terms of the functional blocks:
А varactor (variable capacitance diode) tuning (“Automatic Tuning System”) is employed on all Bands.
Field-effect
transistors
(FET)
are
used
in
the
RF/MIX
circuits
of
Low
(Mid),
High
and
Aircraft
Bands,
to
achieve
optimum
mix-modulation
and mutuai-modulation
characteristics.
Q14 amplifies the 10.7 MHz IF.
A 10.7 MHz monolithic Crystal Filter is incorporated for better selectivity.
ІС-2
contains
the
Local
Oscillator,
Mixer,
IF
Amplifier,
Quadrature
FM
Detector,
Noise
Amplifier
and
DC
amplifier.
A
Crystal
Oscillator
produces
10.245
MHz,
which
mixed
with
10.7
MHz,
resulting
in
455
kHz
IF.
A
455
kHz
Ceramic
filter
is
provided
to
increase
selectivity.
The
455
kHz
IF
is
amplified
in
the
IF
Amp
stage,
and
a
Quadrature
FM
Detector
detects
it
to
an
audio
signal.
A
portion
of
the
455
kHz
Ceramic
Filter
output
is
picked
up,
amplified
and
detected
by
the
AM
IF
Amp,
which
consists
of
Q15
and
Q16
and is then
applied to
the
next
stage
via
the
ANL
circuit.
Detected
output
of
АМ
and
FM
is
applied
to
ІС-9.
ІС-9
amplifies
audio
signals
and
feeds
them
to
the
speaker.
The
zeromatic
circuit
consists
of
1C-5
and
ІС-6
and
020,
21
and
functions
in
the
search
mode.
IC-11
is
the
CPU.
The
CPU
does
data
processing,
calculation,
etc.
Any
unstable
supply
voltage
(VDD)
to
the
CPU
can
produce
CPU
malfunctions,
such
as
wrong
data
processing,
wrong
data
transfer,
etc.
To
overcome
this
C149
and
R146
“initialize”
the
CPU.
Initialization
is
done
when
RESET
switch
is
pushed.
Figure
A
shows
initializing
waveform.
The
CPU
contains
two
clocks.
One
produces
the
waveform
shown
in
Fig.
B
at
Pins
49
and
50,
which
is
used
for
program
functioning.
The
other
waveform,
which
appears
at
Pins
3
and
4,
is
derived
from
X-3
(32.768
kHz)
which
is
used
to
determine
Delay
time,
LCD
display,
etc.
(Shown
in
Fig.
C).
IC7
is
the
PLL
frequency
synthesizer.
The
VCO
output
is
1/64
or
1/65
divided
by
Pre-scaler
ІСТ.
The
output
of
Pin
15
and
16
of
IC7
is
applied
to
the
VCO
control
via
the
Low
Pass
Filter
(022,
23,
24).