Heathkit SW-7800 - Page 32

Page
120
Heathkit
In
the
USB/CW
or
LSB
modes,
the
amplified
IF
sig-
nal
passes
through
a
buffer
to
the
product
detector.
The
BFO
(beat
frequency
oscillator)
is
activated
in
this
mode
and
produces
a
signal
that
is
also
applied
to
the
product
detector.
In
the
LSB
mode,
a
switch-
ing
transistor
adds
an
extra
capacitor
to
reduce
the
frequency
of
the
BFO.
Alter detection,
the
signal
is
processed
like
an
AM
narrow
signal.
‘A
mute
circuit
allows
you
to
disable
the
IF
amplifier
and final
section
of
the
active
filter
so
you
can
use
the
receiver
with
a
transmitter.
‘AC
power
passes
through
the Power
switch
and
a
fuse to
the
primary
of
the
power
transformer.
Recti-
fier
diodes change
the
AC
voltage
at
the
secondary
of
the
power
transformer
to
DC,
which
is
then
fil-
tered
and
applied
to
the
other
circuits
in
the
Re-
ceiver.
An
external
DC
voltage
passes
through
the
Power
switch
and
a
polarity-protection
diode
to
the
output
of
the
rectifiers.
CONTROLLER
CIRCUIT
BOARD
A
binary-coded
logic
signal
from
the
Band
switch
is
applied
to
decoders
on
this
circuit
board,
which
select
the
proper
input
filter
for
the
Receiver.
These
decoders
also
program
the
VCO
for
the
correct
fre-
quency
range,
and
the
synthesizer
dividers
for
prop-
er
synthesizer operation.
ADC
voltage
from
the
phase
detector,
on
the
synthe-
sizer
cicuit
board,
tunes
the
VCO.
The
decoders
con-
trol
two
switching
transistors
that
select
the
proper
frequency
range
for
the
VCO.
This
VCO
signal,
which
is
referred
to
as
the
LO
#1
signal, is
applied
to
the first
mixer
on
the
receiver
circuit
board
as
described
earlier.
‘The
main
tuning capacitor
tunes
the
VFO
circuit
The
output
of
this
circuit
is
applied
to
the
synthe-
sizer,
where
it
mixed
with
the
LO
#2
signal
to
prop-
erly
tune
the
receiver.
‘An
inverter
circuit
on
the
controller
circuit
board
converts
the
12-volt
DC
supply
to
15-volts
DC
for
use
by
other
synthesizer
circuits.
‘An
RF
detector
circuit
is
included
on
this
circuit
board
to
aid
in
alignment
and
troubleshooting,
This
circuit
allows
you
to
use
a
VOM
or
VIVM
to
mea-
sure
RF
signals.
SYNTHESIZER
CIRCUIT
BOARD
‘The
synthesizer
circuit
board contains
the
VCO
phase-locked
loop,
a
display
circuit,
and
a
display
driver/frequency
counter/reference
oscillator,
which
are
contained
in
a
single
integrated
circuit
(U201).
The
VCO
loop
is
phase
locked
to
the
10
MHz
crystal
oscillator
reference.
The 10
MHz
reference
signal
is
divided
by
100
to
produce
a
100
kHz
reference
signal
for
the
phase
detector
In
addition,
the
10
MHz
reference
signal
is
mullti-
plied
by
5
to
produce
a
50
MHz
reference
signal
that
is
mixed
with
the
VFO
signal.
The
difference
signal
is
then
mixed
with
the
VCO
signal
to
produce
a
second
difference
signal.
This
signal
is
then
di
vided
by
10
and
applied
to
the
programmable
divid-
ers
that
are
controlled
by
the
Band
switch.
‘The
phase
detector
compares
the
signal
coming
from
the
programmable
dividers
with
the 100
kHz
refer-
ence.
An
error
voltage
from
the
phase
detector
tunes
the
VCO
to
keep
it
phase
locked
to
the
reference
signal.
Whenever
the
VCO
is
not on
the
proper
frequency,
the
detector
produces
a
low
level
at
one
pin.
This
level
turns
on
the
on-board
LED
to
indicate
an
out
of-lock
condition:
The
display
circuitry
provides
the
50
MHz
reference
signal
for
the
second
receiver
mixer
(LO
#2)
and
the
VCO
loop,
and
mixes
it
witl
the
VCO signal
The
difference
signal
is
then
divided
by
10
before
it
is
applied
to
the
counter
circuit
for
the
display