both magnitude and
direction,
such as force
or alternating voltage, are
called vector
quantities
and
are
usually represented by vector
diagrams.
In
vector
diagrams, the length of the vector
indicates its magnitude, and the vector's
angular
displacement from some
reference
point
indi-
cates its direction.
In relerring to vector
diagrams
of alternating voltage, remember that
time
is frozen for the instant of
observation
since, when
dealing with cyclic waveforms, the vector,
in
actuality, would
be rotating at
a rate equal
to one full 3600
revolution
per
cyclic
period.
UPPER
SIDEBAND
LOWER
SIDEBAND
CARRIER
OF
REFERENCE
PHASE
UPPER
SI
DEBAN
D
LOWER
SI DEBAN D
UPPER
SIDEBAND
CARRIER
Figure 2
Referring to
FigUre
2,
suppose
a carrier
of
reference
phase
OA is fed to two
"black
boxes"
where it is modulated and operated
upon until
the output of
"black
box
A"
can
be represented
by the entire vectordiagramA,
and
the output
of
"black
box
B"can
be
represented by the
entire
vector
diagram B. Diagram
A represents
the
reference carrier,
modulated with
a reference
tone,
plus
another carrier
component
equal
in magnitude and
1800 out of
phas^e.
Diagram
B
represents acarrier equal
in magnitude
to the
reference carrierbut
displaced
90",
plus
a-nother
carrier component equal
in magfritude
and
l80o out of
phase,^
The
diagram further
indicates
that the displiced cariier
is mdulated
by a
tone
displaced 90o from the
reference
modulating
tone. By simple vector addition
of both
diagrams,
it
can be
seen in
Diagram C that a"II
carrier
voltage is cancelled out along
with
the lower
sideband, while
the upper
sideband
reinforces
to
become twice
its
original
magnitude.
Thus,
the
phasing
method of
SSB sigral
generation
re-
quires
circuitry to
provide
90o carrier
phase
shift,
90o
audio
phase
shift, and
carrier
suppres-
sion.
In
actual
practice,
each
"black
box"
referred
to above
contains a balanced
modulator
and a 45o
carrier
phase
shi-ft
network, while
the
90o
audio
phase
shift
is
accomplished
externally
before
application.
Referring to
Figure 3 on
page
6, the
incoming RF is split
into two components
by
the two R1 Cl
passive phase
shift
networks,
one leading the
incoming
signal by 45o'
the
other
lagging ty
+So. The
net result is the
required
phase
difference
in
the
RF input to the
two
bal-
anced
modulators V1 V2 and V3
V4 of
90o.
The
audio signa.l is
fed
to
an RC network
whose two
outputs have a
differtnc-e in
pha"se
6f
90o otter
the
frequency r:rnge of
speech.
By
applying
these
two outputs to
T1 and T2,
two modutating
voltages
l80o
out
of
phase
with each
other can be
applied to balanced
modulator V1
V2 and
another_two
180o out-of-phase
modulating voltages
can
be applied to
balanced modulators
VSV4.
The
l80u out-of-phase
voltages arerequiredfor
proper
modulation of abalanced modulatorl
However,
the
net
phase
differences appliedto
each balanced
modulator
is
only 90o.
Page 5
D
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B
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