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NOTES
ON
THE
DESIGN OF
THE SSB, JR.
RIG
Because the SSB Jr.
rig design
is
made possible
by a new
type
of phase -shift network, and a new
style modulator, it seemed desirable to have the
designer, W2KUJ, explain these units
in
further
detail
for
the benefit of the technically minded
readers of Ham
News.-Lighthouse
Larry
The SSB
Jr.
is
a
superbly simple rig. Such things
just don't
happen by accident, however. Throughout
the
design
many
new ideas were employed
to save
space and reduce complication while not sacrificing
performance
in
any
way.
Easy adjustment
for
optimum performance
was
a
foremost point of design.
The phase -shift network
is
an example of simplifica-
tion of this sort. Literally hundreds of laborious
cal-
culations
were made along
the
way to the final solu-
tion. The result
is a
better
performing network
that
has only eight
parts
and
is
really very easy to
adjust
properly. Two methods of
adjustment
are possible.
The first (and preferred
one) has
already
been
ex-
plained
in
detail. The other
one
is
obvious. Merely
put
in
accurately measured values and
call
the job
done.
The problem here
is
to obtain the accuracy
needed (absolute accuracy)
since
standards
of
re-
sistance and capacity are obviously of
a
different
nature.
By making
adjustments
which involve
both
resistance and capacitance
values simultaneously
in
conjunction with
a
single reference
frequency, almost
all sources of
error are eliminated. And
that
is
why
the preferred method
is
preferred.
All
this accuracy
is
wasted, however,
if
the components
used are not
stable enough to
hold
their values after selection. This
is
why precision resistors are specified, and why only
a
small range of
adjustment
is
provided by the trim-
mer capacitors, since the trimmers are the most likely
circuit elements to change. In this way
good
stability
is
obtained.
A
word
about operating conditions necessary
for
the phase -shift networks. The outputs must
feed
very
high impedance circuits. The effective source imped-
ance should be low, and the voltage supplied to A,E
must
be
minus
0.2857
times the voltage supplied to
D.
Incidentally, the voltage output
of each section
is
equal to the voltage at A,E from zero frequency to
a
matter
of megacycles.
The
design center frequency
for
the
two networks (yes,
there
are
actually two)
is
800
CPS. The differential phase -shift versus frequency
curve
is
symmetrical about this point and holds to
within
1.3
degrees from
225
CPS to
2750
CPS,
as
indicated
in
Fig
12. A
slight error
in
setting the refer
-
USEFUL
~WC
200
300
500
E00
1250
2000 3000
rnEOUENCT
M O.P.S.
Fig.
12.
Audio
phase
-shift network performance
10
ence frequency (3960 CPS)
will
result only
in
shifting
this band up
or down by
the
same percentage. The
operating band
is
adequate-even desirable-for
voice
communication.
One need
not fear reports of
poor
quality
when using
this
rig.
Another simplification which deserves comment
is
the balanced modulator used
in
SSB
Jr. Let's take
a
few
moments to consider what takes
place
in
the
cir-
cuit. Fig.
13
shows
just
one
modulator consisting
of
two
germanium diodes,
G1
and
G2
with associated
cir-
cuits.
First,
suppose
a
high frequency signal of
a few
volts
is
applied
at point
R. On
the positive crest of
signal,
current
passes
through
G2
into the center
tapped resonant circuit
and
tends
to pull point
S
in
the
same direction. Point T
naturally tends to
go
negative
because of the phase inverting properties of
the
res-
onant circuit, but,
of course, no
current
flows
through
G1.
One
half
cycle
later current
passes
through
G, from
the
source,
tending to
pull
point T
in
the negative
direction. But at this time point T would
be
at
a
positive potential because
of
the
"inertia"
of
the
resonant circuit. The net result of the battle between
G, and
G2
to cause current to
flow in
the resonant
cir-
cuit
is
a
draw. No net voltage appears across this
circuit at the source frequency and energy
is
dissipated
in
the balancing resistor and
in
G1
and
G2.
Thus
far,
we
have currents
in
the resonant circuit, but none at
its
operating frequency. This
seems like
a
long way to
go
to
get
nothing, but wait.
Now, let us imagine
a
bias applied at U.
If the
voltage at
U
is
positive,
G2
will
pass more
current into
the resonant circuit, and
G1
will
pass
less
current.
This,
in effect,
unbalances the circuit and
a
radio
frequency voltage
will
appear
across the resonant
cir-
cuit, with point
S
in
phase with the voltage
at
R.
If
the bias voltage at
U
is
negative,
G1
passes more
current than
G2,
and the circuit
is
unbalanced
in
the
other direction. Under this condition the voltage at
T
will be in
phase with
that
at
R. Obviously, if
the
voltage at
U is an
audio frequency voltage, the
cir-
cuit
is
unbalanced
in
one
direction
or
the other (at an
audio frequency rate) and the resulting radio
fre-
quency voltage across the resonant circuit
is
actually
two sets of sidebands with no carrier. When
another
pair of diodes (such
as
G3
and G, of Fig.
2)
is
connected
to
feed
currents into the resonant circuit
from
related
audio frequency and radio frequency sources respec-
tively
90°
out of phase with the first, sideband
cur-
rents
caused by these signals
flow
through the
res-
onant circuit
in
such
a
manner
as
to reinforce
one set
of sidebands and to cancel the other set. The result
is
a
single-sideband suppressed carrier signal. In the
case of SSB
Jr.,
it
is
a
really high grade
one.
The function
of
the balancing resistors (R,; and
1217
of
Fig.
2) is
to
equalize minor differences
in
the
characteristics
of
the diodes and to balance out stray
couplings. Thus, any one balanced
modulator
is
not
necessarily perfectly balanced, but the action of two
such
modulators
fed
with polyphase signals allows
a
complete composite balance.
What about operating
SSB
Jr.
in
other amateur
bands
or
at other
frequencies,
in
general?
As
described,
the radio frequency circuit
design
is
for
the
75
meter
(Continued
on page 12)
GI
Fig.
13.
SSB
Jr.
modulator circuit
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