Heathkit SB-401 - Page 57

Page 132
Gg He4THxItT
T2
BANDPASS
COUPLER
8.395 -8. 895MHZ
C208
22PF TO HET.
MIXER
C209
TO LMO Bt TO HET.
MODE
SWITCH
Osc.
Figure 6-9
LMO
MIXER
(Figure
6-9)
The carrier generator signal (3,395 megahertz)
at the grid and the LMO signal (5,105 mega-
hertz) at the cathode of LMO mixer V4 are mixed
in the tube to produce the sum and difference
of these two frequencies, The LMO signal is
obtained from either the internal LMO (Trans-
mit operation) or from the external receiver
LMO (transceiver operation), depending on the
position of the Freq Control switch,
The output sideband signal from V4 is coupled
through
the
bandpass
coupler
to
heterodyne
mixer tube V5, The bandpass coupler trans-
former
is
tuned
to
pass
only
signals
with
a
frequency
between
8,395
and
8,895
megahertz,
Only
the
sum
of
the
carrier
generator
and
LMO
signal
frequencies
fall
within
this
frequency
range,
In
all
cases,
it
is
only
the
sum
of
the
frequencies
from
V4
that
is
passed
through
the
bandpass
filter,
as
the
different
frequencies
always
fall
outside
its
frequency
range,
HE
TERODYNE
OSCILLATOR
AND
HETERODYNE-OSCILLATOR
AMPLIFIER
(Figure
6-10,
fold-out
Page
133)
Heterodyne
oscillator
V8A
has
regulated
Bt
voltage
applied
only
when
the
Function
switch
is
in
the
Standby,
Transmit,
and
Spot
positions,
V8A operates as a tuned-plate crystal-controlled
oscillator,
Band
switch
wafer
1F
selects
the
proper
crystal
and
band
switch
wafer
2F
selects
the proper plate coil (L1 through L8) for the
desired frequency of operation, The output of
V8A
is
coupled
through
capacitors
C49
and
C209 to the grid of heterodyne mixer tube V5,
In
the
Transceive
mode,
B+
voltage
is
removed
from
V8A
and
applied
to
V8B,
the
heterodyne
oscillator
amplifier,
V8B
amplifies
a
hetero-
dyne
oscillator
signal
from
the
receiver,
and
uses
the
same
coils
(Li
through
L8)
as
did
V8A
for
proper
plate
tuning,
The
output
signal
is
coupled
through
capacitors
C49
and
C209
to
the
grid
of
heterodyne
mixer
V5,
HETERODYNE MIXER (Figure 6-11)
The sideband signal from the bandpass coupler
and the RF signal from the heterodyne oscillator
are coupled to the grid of heterodyne mixer V5,
These signals are mixed to produce their sum
and difference frequencies in tube V5, The
plate of V5 is tuned by capacitors C4A and C4B
with the proper tuned circuit (L9 through L14)
selected by the Band switch,
The plate frequency-selected circuits are such
that only the difference frequencies are per-
mitted to pass through, and all other frequencies