What is Sideband?
Sidebands are the two sets of frequencies created when a carrier is modulated.
These sidebands exist for
all modulation types, including
AM
and
FM. The difference
with a Sideband radio is that
only the sidebands are used - the carrier
is removed.
Where are these Sidebands?
The two bands of frequencies lie just above (
USB) and just below the carrier frequency (
LSB).
For a carrier being amplitude modulated by a single frequency, there will be
just two side frequencies... Carrier + modulating frequency, and carrier -
modulating frequency.
For
FM, the set of
frequencies generated is more complicated, but the bands are still just above
and below the carrier.
Why use Sideband?
As a single sideband signal uses no carrier and just one of the sidebands,
this mode is very efficient and greater distances can be achieved with the
same power output. As the required bandwidth is equal to the bandwidth of the
modulating signal (1/2 that of
AM), this is also efficient on band space.
What frequency is a sideband CB on?
This is an issue which causes confusion among many
CB users.
A sideband radio uses the same channels as
AM or
FM. If you are using
AM, and switch to
SSB, you are still on the same
frequency.
|
|
|
|---|
When switched to AM,
the radio will transmit the carrier and both sidebands. FM also transmits carrier and sidebands, but
the side frequencies generated are not as simple as for AM.
|
|
|
The same radio, when switched to upper sideband, will now suppress the
carrier and lower sideband.
As shown on the left, the radio now only transmits the upper sideband.
|
Now, when switched to lower sideband, the radio will suppress the carrier
and upper sideband.
As shown on the right, the radio now only transmits the lower sideband.
|
|
As should be seen from the above, the difference between
AM and
SSB is which parts of the signal are transmitted.
All the signals, carrier and sidebands, are generated in all modes. The
carrier is needed to generate the upper or lower sideband, but the unwanted
signals are filtered out before they get to the transmitter output stages.
