Sideband radio

You may have already realized that all of the information that is sent by an AM radio transmitter is contained in the sidebands. The carrier is unchanged and therefore contains no information. When an AM transmitter is at maximum modulation (100% modulation), 50% of the power output is in the carrier. Each sideband contains 25% of the power. If all the information is in the sidebands, and 50% of the power is wasted in the carrier, why transmit the carrier at all? Why not just transmit the sidebands?

The problem is that the carrier is necessary to reconstruct the original signal. carrier and the sidebands must be mixed together in the receiver. Without the carrier, you will only hear garbled noise at the receiver.

The sound of sideband radio on an AM receiver and on a sideband receiver

The carrier is just a sine wave at a particular frequency. Why not remove the carrier at the transmitter and replace it using a sine wave generator at the receiver? In fact, a product receiver has just such a generator, the Beat Frequency Oscillator (BFO). All you have to do is tune the BFO to precisely the same frequency as the original carrier. In reality, it is even easier than that. In a Superheterodyne receiver, the carrier is converted from its original frequency to 455 kHz (or some other intermediate frequency). If we remove the carrier at the transmitter, all the receiver ends up with are sidebands centered on the missing carrier, which would be 455 kHz if it weren't missing. To properly receive such a suppressed carrier transmission, a Superheterodyne receiver needs only a fixed-tuned BFO operating at 455 kHz.

There is one drawback to suppressed carrier (sideband) transmission. A Superheterodyne receiver may not tune the original signal to precisely 455 kHz. This is no problem with full-carrier AM. If the receiver's tuning is off a bit, the carrier and sidebands are all off by the same amount. The tuning doesn't have to be perfect. If you suppress the carrier and replace it with a 455 kHz oscillator, and the tuning is imprecise, the sidebands will not match the carrier. The result is that voices sound higher or lower pitched than normal. With a manually-tuned receiver, you must adjust the tuning until the pitch sounds right. Fixed tuned receivers (such as crystal-controlled receivers) usually have an adjustable BFO (often called a clarifier) to adjust the pitch.  Some operators deliberately off-tune sideband receivers slightly to produce a higher-pitched output. They say the higher-pitched voices are easier to understand.^[1]

Advantages

Where sideband transmission is allowed by law, transmitters are allowed to effectively double their power output when using sideband transmission. This is because the carrier contains 50% of the power. If the carrier is removed from the signal, the power of the sidebands can be doubled to make up the difference.

A double-sideband transmission is produced with a circuit called a balanced modulator. This essentially modulates two copies of the carrier that are 180 degrees out-of-phase with each other with two copies of the modulating signal that are in-phase with each other. The result is that the carrier gets canceled and only the sidebands are produced.

Single sideband

Each sideband contains all the information needed to recreate the original signal when mixed with the correct carrier frequency. Therefore, it is even more efficient to transmit only one sideband than to send both. Either the upper sideband or the lower work equally well. This has the advantage of requiring only 25% of the power that a full-carrier transmission requires. When single sideband transmission is allowed by law, transmitters are allowed to effectively quadruple their power. It also reduces the bandwidth requirement. Where a 10 kHz bandwidth is required for full-carrier AM or double-sideband, single sideband would require only 5 kHz. Two transmitters can actually operate on the same frequency (same carrier frequency) without interference, one using the upper sideband and the other using the lower sideband. This was a common practice in the heyday of Citizens Band radio in the U. S..

Where single-sideband is used for professional communication, such as ship-to-shore radiotelephone, a small amount of the carrier is sent with the transmission. The BFO in the receiver is able to lock on to this attenuated carrier to produce a frequency that is precisely the same as the original carrier. This results in a high-quality output without manual tuning of the BFO.

A single side-band transmission is produced by a balanced modulator just as double-sideband is. After modulation, the unwanted sideband is removed with a filter.