Single Sideband, SSB Modulation
- notes, details and essential facts about single sideband modulation use for radio communications systems, especially on the HF portion of the radio spectrum.
Single sideband modulation is a form of amplitude modulation. As the name implies, single sideband, SSB uses only one sideband for a given audio path to provide the final signal.
Single sideband modulation, SSB, provides a considerably more efficient form of communication when compared to ordinary amplitude modulation. It is far more efficient in terms of the radio spectrum used, and also the power used to transmit the signal.
In view of its advantages single sideband modulation has been widely used for many years, providing effective communications, as well as forms being used for some analogue television signals, and some other applications.
Single sideband modulation basics
Single sideband modulation can be viewed as an amplitude modulation signal with elements removed or reduced. In order to see how single sideband is created, it is necessary to use an amplitude modulated signal as the starting point.
An amplitude modulated carrier
showing sidebands either side of the carrier
From this it can be seen that the signal has two sidebands, each the mirror of the other, and the carrier. To improve the efficient of the signal, both in terms of the power and spectrum usage, it is possible to remove the carrier, or at least reduce it, and remove one sideband - one is the mirror image of the other.
A single sideband signal therefore consists of a single sideband, and often no carrier, although the various variants of single sideband are detailed below.
Single sideband modulation
showing upper and lower sideband signals
It can be seen that either the upper sideband or lower sideband can be used. There is no advantage between using either the upper or lower sideband. The main criterion is to use the same sideband as used by other users for the given frequency band and application. The upper sideband is more commonly used for professional applications.
Forms of SSB, single sideband modulation
There is a number of different formats of single sideband modulation that are used:
- Single sideband suppressed carrier, SSBSC : SSBSC is the form of single sideband modulation that is most widely used for communications applications on the HF portion of the radio spectrum. There is only one sideband - the other sideband and the carrier are both removed. Having no carrier, this needs to be re-inserted within the receiver. Any slight differences in carrier re-insertion frequency give rise to changes in pitch of the audio. However it gives the most efficient spectrum and power usage of any single sideband modulation format.
- Single sideband reduced carrier : This form of single sideband modulation removes one sideband, but retains a small amount of carrier. The reduced carrier element can then be used to lock a local oscillator for carrier re-insertion and the demodulation of the sideband with the correct audio pitch. Naturally the power efficiency of this form of single sideband modulation is not as high as single sideband suppressed carrier.
- Single sideband full carrier : This form of single sideband modulation is normally used where receivers may not have the ability to re-insert the carrier - it can be demodulated using a simple diode detector. However, having one sideband only, it occupies only half the bandwidth. Power efficiency is very poor, because the full carrier is required, but only one sideband is present to carry the modulation.
- Single sideband vestigial carrier : Vestigial sideband is a form of single sideband modulation where one sideband is present, but the other has been only partly cut off or suppressed. Vestigial sideband is used for analogue AM television transmission and is used to reduce the overall bandwidth while still keeping one sideband with the lower frequency information.
- Independent sideband, ISB: This form of single sideband is not strictly "single" sideband because it has two sidebands. However each sideband carries different modulation, and therefore provides a doubling in the spectrum efficiency.
These different formats of single sideband modulation can be found being radiated in various portions of the radio spectrum for their different applications.
By Ian Poole
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