Swann 400 - Circuit Theory of the Swan Model 400

PART I MODEL 4OO
TRANSCEIVER
CIRCUlT
THEORY
GENERAL
DISCUSSION
The Sr
ran
400 Transceiver
plovides siagle
sideband,
Buppressed
ca!rier
tran€ceive
operation; and
generates the Bingle
side-
band eignal
by mean6
of a crystal
lattice
filte!.
To
permlt
a
logical diEcussion
of
thi6 lnode
of
operation, certain
definitions
ate
nece€5aly,
In a
normal AM signal
(double
dideband
with
carrier), a
radio
frequency
i6 modulated with
an audio Jre-
quency
signal. This
is considered
by
many
to be merely
a ca8e of varying
the
amplitude
of the carlie!
at an audi.o
rate.
In fact,
howeve!,
there are actual.ly
side-
band
frequencies
generated
which
ale the
!eBults
of mlxing
the RI. and A!' 6ignal3.
These
sideband3 are
the suh ot, and
the
difference
between
the two hetelodyned
6ignals. For
detection by means
of
conventional
diode detectols,
the two
sidebands are
mixed with
the carlier to
detect
aDd to
demodulate
the
audio
intelligence.
This inef{icient hean€
of
transrnis
sion
p€rrnits
only approximately
25
per cent
of the full tlanahitted
power
to
b€ used
to tras6lnit intelligence.
There
are
other attendant
drawbacks,
also.
The
bandwidth
of the transmitted
6igna1 is
on the order
of 6 kc, while
the
actual
dehodulated audio
is 1e66 than
3 kc. The
lesult i. very
limited use
of
the band, and
over half
of the allotted
frequency
range iB unuaable
becauge oI
heterodyne6, ioterference,
and congestion.
In
the single sideband,
suppressed carlier
rrode of transhioeion,
only ono sideband
of the Rr. and AF heterodyned
signal is
tranBmitted,
the other sideband and
the
carrier being Buppressed
to a level which
effectively
permits using only the audio
intelligence bandwidth.
This results in
increasing the transrnission
efficiency
many times
ove!,
and
pellnits
an effec
-
tive doubling
of the use of the allocated
frequenciea.
It
rnust be
remerrlbered that in the single
aideband,
suppresned calrier mode
of
transhission, both
the unn,anted side-
band and
the calrier ale
oaly
Buppressed,
not entile1y eliminated. Thu6, with a
transmitted sigDal frorn a
transmittei
with
40 db sideband
6uppression, the
other,
or unwanted sideband is
pleBent,
and it i6 transrnitted,
but its leve1 is
40 db below the'wanted
sideband.
When
3