RECEIVING SSB SIGNALS
There are three types of signals presently used for communications in the
Citizens Band : AM, USB, LSB. When the MODE switch on your unit is
placed in the AM position, only standard double-sideband, full carrier signals
will be detected. An SSB signal may be recognized while in the AM mode by
its characteristic "Donald Duck" sound and the inability of the detector to
produce an intelligible output. The USB and LSB modes will detect upper
sideband and lower sideband respectively, and standard AM signals.
SSB reception differs from standard AM reception in that an SSB receiver
does not require a carrier or opposite sideband to produce an intelligible
signal. A single-sideband transmitted signal consists only of the upper or the
lower sideband and no carrier is transmitted. The elimination of the carrier
from the AM signal helps to eliminate the biggest cause of whistles and tones
heard on channels which make even moderately strong AM signals
unreadable. Also, SSB takes only half the space of an AM channel, therefore
two SSB conversations will fit into each channel, expanding the 40 AM
channels to 80 SSB channels. The reduction in channel space required also
helps in the receiver because only
half
of the noise and interference can be
received with 100% of the SSB signal.
An SSB signal may be received only when the listening receiver is
functioning in the same mode. In other words, an upper sideband signal
(USB) may be made intelligible
only
if the receiver is functioning in the USB
position.
If a lower side band (LSB) signal is heard when the receiver is in the USB
mode, no amount of tuning will make the signal intelligible. The reason for
this may be understood if you consider that when the modulation is applied
to the transmitter's microphone in the USB mode, the transmitter output
frequency is increased whereas in the LSB mode the transmitter's output
frequency is decreased.
The result in listening to the receiver is that when the MODE switch is in
the proper position (either USB or LSB), a true reproduction of a single tone
of modulation will result, and if the tone is increased in frequency (such as a
low-pitched whistle or a high-pitched whistle) you will hear the increase in
the output tone of the receiver. If the incorrect mode is selected, an increase
in tone of a whistle applied to the transmitter will cause a decrease in the
resultant tone from the receiver.
Thus when a voice is used in place of a whistle or tone, in the proper
listening mode the voice will be received correctly whereas in the incorrect
mode, the voice will be translated backwards and cannot be made intelligible
by the CLARIFIER control. When listening to an AM transmission, a correct
sideband is heard in either mode since both upper and lower sidebands are
received.
Once the desired SSB mode has been selected, frequency adjustment may
be necessary in order to make the incoming signal intelligible. The
CLARIFIER control allows the operator to vary frequency above or below
the exact frequency of the channel. If the sound of the incoming signal is
high or low pitched, adjust the operation of the CLARIFIER.
Consider it as performing the same function as a phonograph speed control.
When the speed is set too high, voices will be high-pitched and if set too low,
voice will be low-pitched. Also, there is only one correct speed that will
make a particular record produce the same sound that was recorded. If the
record is played on a turntable that is rotated in the wrong direction (opposite
sideband) no amount of speed control (CLARIFIER) will produce an
intelligible sound.
An AM signal received while listening in one of the SSB modes will
produce a steady tone (carrier) in addition to the intelligence, unless the SSB
receiver is tuned to exactly the same frequency by the CLARIFIER control.
For simplicity, it is recommended that the AM modes be used to listen to AM
signals.
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