RCA SSB-1, Instruction Book

Page: 13 / 79

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CIRCUIT ARRANGEMENT
1. G E N E R A L.
When a single-sideband signal is gen­
erated at a low frequency, frequency multi­
plying circuits cannot be used to raise the
signal to the desired frequency of transmis­
sion; they would not preserve the original
modulation. Heterodyning, or frequency mix­
ing, methods are used instead .
When two frequencies are mixed together,
the resultant output contains a frequency
component which is the sum of the original
two frequencies (upper sideband) and a com­
ponent which is the difference of the original
two frequencies (lower sideband).
Either
the sum or the difference component can be
extracted from the composite signal by
using suitable filters.
The SSB-1 uses three crystal oscillators
to heterodyne the original modulating signal
up to the transmitter output frequency. The
same three oscillators operate with the
receiving circuits to heterodyne the re­
ceived r-f down to the original modulating
signal. By the use of conventional balanced
modulators in the heterodyning process, the
crystal oscillator frequencies, and hence, the
carrier frequency also, are suppressed.
2. S U P P R E S S E
D C
A R R I E R
T
R A N S MI S S I
0
N.
The intelligence to be transmitted may
be either a voice or telegraph signal. A
voice signal would be applied from the
microphone to the microphone amplifier;
a telegraph signal is applied by keying the
tone oscillator which feeds the microphone
amplifier.
In the discussion which follows, it will
be assumed that the modulating signal is
a 1 kc tone from the tone oscillator, with
the understanding that the discussion is
equally valid for a voice signal. Refer to
the block diagram, figure 5.
E!:.· The tone oscillator, V106A, (one half
of a type 12AT7 dual triode) is a phase­
shift oscillator operating at 1000 cps. One
contact of keying relay KlO 1, in the cathode
circuit of V106A, keys the oscillator. When
9
K101 is energized by pressing the telegraph
key, another contact of the relay energizes
the transmit-receive relay, K201, which
connects the antenna to the transmitter .
Relay K20 1 is held energized (transmit
position) by
a
resistance-capacitance delay
circuit bridging the "key-off" time as long
as normal keying is continued. This keying
system provides "break-in" type switching
between receiver and transmitter on telegraph
operation, and thus requires no manual
switching.
�· Audio output from the keyed tone oscil­
lator or voice current from the microphone
of handset HS101 (with "push-to-talk" button
depressed) is fed to the microphone amplifier
which consists of two stages, an AF voltage
amplifier, V106B, (one half of a 12AT7 dual
triode) and a cathode follower, V107A, (also
one half of a 12AT7 tube).
c. Assume that a 1 kc test tone is applied
either through the microphone of handset
HS101 or by the tone oscillator through the
microphone amplifier to the speech clipper.
The speech clipper is a plug-in unit con­
sisting of an AF amplifier, V401A, (the
pentode section of a 6U8 triode-pentode)
a clipper tube, V402, (a 6AL5 dual-diode)
and a cathode follower, V401B, (the triode
section of the 6U8).
The speech clipper
limits the peaks of a varying amplitude
voice signal so that the average intelligence
signal level can be kept high. When a tone
is applied the speech clipper functions only
as a buffer amplifier.
d. The 1 kc tone is applied to the grids
of
V
207A and V 207B ( a 12AT7 dual-triode),
the first balanced modulator stage in op­
posite phase through AF transformer T202
(figure 29) and TRANSMITTER GAIN control
R233 ( a dual potentiometer with sections in
series and center grounded). At the same
time a 250 kc signal is fed from crystal
oscillator V210 to both V207 grids in phase.
These phase relationships result in the
250 kc signal cancelling out in the series­
connected output of the circuit and the
generation of the sum and difference fre­
quencies, 251 kc and 249 kc. Potentiometer