Heathkit SW-7800 - Page 39

Heathkit
Page
127
vco
The
VCO
consists
of
an
FET
hartley
oscillator
(Q316)
and
buffers
Q317
and
Q318.
Switch
transistors
Q319
and
Q321
add
or
remove
additional
trimmer
capacitors
C314
and
C316,
respectively,
depending
upon
the
tuning
range
that
is
selected.
Varactor
diodes
D323
and
D304
tune the
VCO
through
the
proper
range
by
changing
value
as
the
phase detector
output
voltage,
coming
from
the
synthesizer,
changes.
Diode
D305
reduces
the
harmonic
content
of
the
VCO
signal.
As
described
in
the
decoder
description,
Q303
and
Q306
perform
the
VCO
range
switching.
When
logic
point
H
is
low,
Q303
conducts.
This
allows
the
base
of
Q319
to
go
high
and
turns
it
on.
D306
also
couples
this
high
to
the
base
of
Q321
to
turn
it
on.
Since
both
transistors
are
on,
C314
and
C316
are
effec-
tively
grounded.
The
added
capacitance
lowers
the
VCO
frequency
to
its
lowest
range
of
50.605
to
58.455
MHz,
When
the
bandswitch
is
above
7,
Q303
is
tuned
off.
Between
bands
8
and
15,
however,
Q306
is
on.
This
keeps
Q321
turned
on
through
D307,
while
D306
prevents
Q319
from
turning
on.
The
result
is
to
re-
move
only
C314
from
the
oscillator
circuit
so
the
VCO
frequency
range
will
increase
to
58.455
to
65.455
MHz.
When
the
bandswitch
is
above
15,
both
Q303 and
Q306
are
turned
off,
which
removes
both
capacitors
from
the
circuit
and
changes
the
VCO
fr
quency
range
to
65.455
to
80.455
MHz.
The
VCO
signal
is
used
as
LO
#1
in
the
receiver
to
determine
the
first
IF.
It
also
mixes
with
the
50.0 MHz
reference
in
the
display
circuitry
to
provide
the
proper
fre-
quency
readout
VFO
‘The
VFO
circuit
consists
of
an
FET
colpitts
oscillator
(Q324)
and
buffer
Q325.
Main
tuning
capacitor
C3
and
coil
L306
tune
this
oscillator.
Diode
D311
re-
duces
the
harmonic
level
of
the
VFO
signal.
The
VFO
is
centered
around
5.545 to
6.545
MHz,
but
will
typically tune
5.4
to
6.7
MHz.
This
signal con-
trols
the
VCO
frequency
within
the
desired
tuning
range
Inverter
Power
Supply
An
inverter
circuit
converts
the
13.8-volt
DC
power
supply
voltage
to
15-volts
DC
for
use
by
the
control.
ler
and
synthesizer
circuits.
It
consists
of
an
oscil-
lator,
voltage
amplifiers,
a
voltage
doubler,
and
a
voltage
regulator.
U304
is a
timer,
which
forms
an
astable
multivib-
rator.
A
30
kHz
square
wave
from
U304
is
fed
to
Q322
and
Q323,
amplified,
and
rectified
by
a
voltage
doubler.
D308,
D309,
C325,
and
C326 form
the
dou-
bler.
U305
regulates
the
rectified
DC
to
15
volts.
328 and
L304
filter
any
30
kHz
signal
from
the
15-volt
line,
while
C324
and
L303
do
the
same
for
the
13.8-voll
line.
RF
Detector
‘The
RF
detector
consists
of
a
voltage
doubler
formed
by
D313,
D314,
and
C338.
C337
couples
an
RF
sam-
ple
to
the
diodes
where
it
is
rectified.
Any
DC
VOM
or
VIVM
can
then
measure
the
RF
signal
without
the
necessity
of
a
separate
RF
probe.