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.

How a sideband CB works.

SSB Transmission.
The sidebands are created by mixing the carrier and the sound in a circuit known as a Balanced Modulator. The output of this modulator will include both sidebands but, unlike a normal mixer, the carrier and audio are not present in that output.
The sidebands are then put through a filter, which will filter out the unwanted sideband.

SSB Reception.
To select which sideband you are tuned to, a filter is used. This filter will be more narrow than the one used for AM or FM.
In order to demodulate a signal, the carrier is required. As the transmitter did not send a carrier, the receiver has to generate it's own signal. For this purpose, the CB radio will use the carrier which it uses during transmission to create the sidebands.
As proper demodulation depends upon the carrier, it is extremely important that the transmitter and receiver are on exactly the same frequency and that the carrier frequencies are the same.
Any differences will distort the sound.

Selecting one sideband.
To allow a radio to operate all modes with just one filter, the radio shifts the carrier frequency.

First let us look at a filter being used for AM or FM. The carrier frequency is set to the middle of the filter bandwidth, where attenuation is equal above and below the carrier, and so for each sideband. The carrier, and both sidebands will pass through the filter.
	Now look at a filter being used for upper sideband. The carrier frequency set to the bottom edge of the filter passband. The lower sideband will be blocked by the filter, but the upper sideband will pass through. As the carrier oscillator has been shifted down, the VCO will need to be shifted by the same amount so that the output is still on the correct frequency.
Next, this same filter is being used for lower sideband. The carrier frequency is set to the top edge of the filter passband, allowing the filter to block the upper sideband signal. As the carrier oscillator has been shifted up, the VCOwill need to be shifted by the same amount so that the output is still on the correct frequency.

The filter used for SSB signals only needs to be as wide as the modulating bandwidth. For communications equipment, this is usually no more than 3KHZ.
As the bandwidth of AM and FM signals is much higher, a separate, wider filter needs to be used for these modes.