Galaxy DX 98VHP Manuel du propriétaire - Page 6

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RECEIVING SSB SIGNALS

There are four types of signals presently used for communications: FM, AM, USB

and LSB. When the MODE switch on your unit is placed in the AM position, only

standard double-side band and in FM position, only frequency deviation, full carrier

signals will be detected. An SSB signal may be recognized while in the AM or FM

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 side band to produce an intelligible signal. A single-side

band transmitted signal consists only of the upper or the lower side band 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 that 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 side band 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 COARSE/FINE control.

When listening to an AM transmission, a correct side band is heard in either mode since

both upper and lower side bands are received.

Once the desired SSB mode has been selected, frequency adjustment may be

necessary in order to make the incoming signal intelligible. The COARSE/FINE 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 COARSE/FINE.

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 side band) no amount of speed control (COARSE/FINE)

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 COARSE/FINE control. For simplicity, it is recommended

that the AM modes be used to listen to AM signals.