Swann 400 - Signal Generation Process

I
MODEL 4OO
TRANSCEIVER
A. Circuit
Theoiy
(Cont)
this
signal is
leceived at a
level of 20 db
over 59. the
unwanted sideband
will be
pre6ent
at a
leve1 oI approximately
55.
The sahe
is true of carrie!
guPPression.
With carrier
suppression
of 50
db, and
a
signal
level of 50
db over 59,
carlier
will
be
present at a level
of
approxirnately
53
to 54.
In
the
Model 400
Transceiver,
the
single
sideband,
suppressed
carrie!
signal is
generated
by the crystal
lattice
filter
method.
Refer
to
the schematic
diagram'
and
to FiSule6
I and 2,
Block
Diagrams
SIGNAL
CENERATION
In
the TRANSMIT
position
(i.
e.,
when the
pu6h-to-talk switch on
the lnicroPhone
i3
pressed ot when the
Function Switch
is
moved to TRANSMIT),
the transmitter
portion of the
transceivel
is
activated,
and
generates a sinSle
sideband,
suppressed
ca!!ier
signal
in the follow_
ing manner:
Carrier
is
Senerated
by
Vl5,
Carrier
Oscillator,
which
is a Pierce
oscillator, with
the
crystal oPerating
in
parallel
resonance.
This stage
operates
in both th€
ttansoit
and leceive
rnodes.
When
transmitting,
the RF
outPut oJ the
oscillator
is injected
into
the control
grid
of the
Balanced
Modulator, VI4.
This
balanced
modulato!
is
a beam deflection
type, and
opeiates
5imila!
to a
cathode
ray
tube
in that
the elect!on bearn
frorn
the
cathode
i6 deflected
to one
outPut
plate or the
other by
the charge
aPPear_
ing
on the deflection
Plates.
The
RF
energy
fed to
the cootrol
Srid
of the
balanced
modulator appeals
on both
plates
oI the
output, in
the absence
of
signals
to the
deflection
plates.
The
two
output
plates feed
the carrier
to
Trans_
forrner
Zl40I
in
push
PuU,
and the
two
RF
signals
cancel
each other
out in the
output
ot the
transforn:rer.
The
defl€c
-
tion
plate reference voltages
are
adjusted
bv
means
of
the ca!rier
balance
control
60
that
with
no audio,
the
RF being
Ied
to
the output
plates will cancel
out' and
the
output lror'j.
Zl40l
will be
zero,
Audio
from
Microphone
Amplifier
Vl6
is superilnposed
on one deflection
Plate,
thereby
unbalancinS
lhe
modulator,
and
the
two sideba$ds
resulting
froh
the 3uln
and
diff€rence
frequencies
of the audio
and carrier
appear
as
a double
sideband,
suppressed
carrier
si8na1
in the
outPut
o{ Translormer
Zl40l.
The catrier
suppression
is aPproximately
50 db.
The
double
sideband,
suPPressed
carrier
siSnal is
then couPled
to
the crystal filter'
which
euppreeees
one sideband,
and
permit6 the
other sideband
to be
fed to
the
First IF
Amplifier,
V8.
The carrie!
frequency
crystal
alrd
the filte!
crystalg
are
selected
so that
in the LSB
poBition
on 4O and
8O hete!s,
the
sideband
siSnal
is
Senelated
with
a calrier
frequency
of
5l?2. 8
kc, and
this siSnal
\till
fall
within
the
bandpass
of the filt€r
6uch that the
lower
6ideband
will be attenuated
by at
least 40
db. See Figure
3.
On the USB
positron of 40 and
80 meiers,
the carrier
clystal
is 5I?6.8
kc, which
positiond
the
double
6ideband
signal
on the
other side
of
the response
culve
oJ the filter,
attenuating
the upper
sideband
by at
Ieast 40
db. In
lhe single
conversion
mixing
proces6, these
sidebands
become
inverted.
OnZ0, 15, and
l0 meters,
where
oPera-
tion
i6
generally on upPer
sideband,
the
siBnaI
i6
generated with
the same
carrier
crystal
used in
generating
the lower side-
band
on 40 and 80
meters.
The five
crystal
filter used
in the
tranaceive!
results
in an
improved
reaPonae
characteristic
on
the low frequency
end
of the bandpass,
and
advantage
is
taken
of this
eflect to
provide better sideband
suppression
on the lnost
used 6ideband
for
each
frequency
band,