Heathkit SW-7800 - Page 37

Heathkit
Page
125
ables
U303B
whenever
any
other
bands
are
selected.
Their
inputs
are
logic
H,
the
carry
output
from
adder
U301,
and
bandswitch
line
2
U303A
pin
7
selects
band
4
(4—8
MHz).
Inputs
from
bandswitch
line
2,
for
input
A
(pin
2),
and
logic
point
G
(U301
pin
13),
for
input
B
(pin
3),
cause
the
decoder
to
select
band
4
at
the
proper
time.
Log:
point
G
represents
the
bit
4
sum
of
U301
A
logical
NOR
circuit,
formed
by
R306, R307,
and
Q304,
selects
band
5.
The
inputs
to
the
NOR
are
logic
point
G
and
bandswitch line
4.
Band
filter
5
is
only
selected
when
both
lines
are
at
a
logic
0.
Bandswitch
line
4
selects
band
6.
Integrated
circuits
U301
and
U302
are
4-bit
binary
adders.
For
the
synthesizer
to
properly
tune
the
VCO,
a
binary
7
(0111)
must
be
added
to
the
bandswitch
logic.
U302
performs
this
addition, with
its
carry
line
connected
to
U301.
This effectivel:
makes
U302 and
U301
an
8-bit adder.
A
summary
of
the
inputs
to
both
adders
is
shown
below
Whenever
the bandswitch
value
exceeds
7,
switch
ine
B3
or
B4
becomes
high,
which
causes
an
auto-
matic
carry
to
U301
bit
4
sum.
This
causes
logic
point
G
to
go
low
and
remain
low
for
bands
0
through
7.
This,
in
turn,
insures
that
logic point
H
remains
high
for
bands
8
through
29,
since
H
is
the
carry
line
for
U301
As
we
mentioned
earlier,
the
band
5
filter
is
only
selected
when
bandswitch
line
4
and
point
G
are
both
low.
Bandswitch
line
4
is
low
for
all
bands
above
7
(7 MHz|
in
the
8.
These
logic
levels
are
equal
o:
15
range,
which
is
the
tuning
range
of
filter
Point
H
controls
the
VCO
range
by
303
off
or on.
A
logic
low
must
be
present
on
the
base
of
Q303
to
turn
it
on
and
this
or
0—7.
As
you
will
see
e
band
select
voltage
from
Q306,
for
band
5
(B—15
MHz),
also
the
VCO
tuning
range.
ents
on
bands
changes
us02
‘Synthesizer
Divider
Logic
301
‘Adder
inputs
4}3
2
Input
A
1[ea}a
Input
8
o}ea}.
Sum
Carry
Bt
(logichf
Bit.
sum
(logic
6)
Preset
data
=
0or1
Bw
bandswitch
logic
line
#