Sideband
Sidebands are the frequency bands that are generated by modulation in the immediate vicinity of a carrier frequency . Here, according to the addition theorems for trigonometric functions, sum and difference frequencies arise .
The frequency bands generated by the modulation are in frequency above and below the carrier frequency:
- the frequency band above the carrier frequency is referred to as upper side band (USB of English upper sideband )
- the frequency band below the carrier frequency is lower sideband called (LSB of English lower side band ).
Both contain the information of the useful signal .
In the amateur radio sector, the lower sideband (LSB) is used below 10 MHz transmission frequency, and the upper sideband (USB) for frequencies above 10 MHz.
The amplitude modulation of a radio transmitter with a carrier frequency of 1000 kHz and a 1 kHz useful signal (a 1 kHz tone) should serve as a simple example . The following signals are generated:
- a lower side frequency (difference frequency of 1000 kHz - 1 kHz = 999 kHz)
- an upper side frequency (sum frequency of 1000 kHz + 1 kHz = 1001 kHz)
If several useful signals are modulated at the same time, they occupy so-called bands .
In the case of radio or television , however, useful signals are now transmitted that cover a larger spectrum than a single frequency. If, for example, a voice recording is modulated onto a carrier, the sum and difference frequencies between these voice frequencies and the carrier frequency arise from all frequencies of the recording. The signal after the modulator comprises the carrier frequency, the upper sideband from the sum frequencies from the carrier and the voice band, the lower sideband from the difference frequencies from the carrier and voice band, as well as harmonics and a DC component.
The harmonics and their mixed products are filtered out with a bandpass before being broadcast . Depending on the type of modulation, other components of the signal can be filtered out before it is emitted, for example with single sideband modulation (SSB) only one of the sidebands is sent. The vestigial sideband modulation only partially suppresses one of the two sidebands. This results in a higher spectral efficiency . Demodulation is technically more complex in both cases, since the missing spectral components, such as the carrier frequency, have to be reconstructed in the receiver with the appropriate frequency and phase position.