Single Sideband Suppressed Carrier, SSBSC
- notes and details about single sideband suppressed carrier, SSBSC widely used for analogue communications, especially on the HF portion of the radio spectrum..
Single sideband suppressed carrier is the form of single sideband that is widely used for communications applications, particularly within the HF portion of the radio spectrum.
Single sideband suppressed carrier is particularly effective for communications because it makes effective use of the available transmitter power and the radio spectrum.
Single sideband suppressed carrier, SSBSC basics
As the name indicates, the defining parameter about single sideband suppressed carrier is that not only is one sideband removed, but so is the carrier. To regenerated the audio, the carrier needs to be re-inserted within the demodulation process.
Single sideband modulation demodulation scheme
In order that the audio is regenerated correctly, the carrier must be re-inserted on the same frequency as the original signal. In reality, as systems are not phase locked, except where there is a pilot carrier, it is not possible to re-insert the carrier on exactly the right frequency and some frequency offset will occur. This results in a offset in the pitch of the audio. Typically offsets up to 100 Hz are quite acceptable, although the audio will have a "communications" quality sound to it.
SSBSC power measurement
When the power for amplitude modulated signals is measured, a steady measurement is obtained regardless of the modulation - the power averaged over a second or less is constant. This is not the case for single sideband suppressed carrier. As no carrier is present, the sideband only appears when modulation is present, and its power is proportional to the audio at that instant.
As a result the power of the transmission is limited by the peak handling capability of the transmitter. Accordingly a measurement known as the peak envelope power, PEP is used.
The peak envelope power needs to be measured by a power meter with a short time constant - thermal ones do not respond fast enough. However today there are many power sensors and meters that will respond sufficiently quickly to enable peak envelope powers to be measured.
By Ian Poole
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