Carrier (communications engineering)
Carrier signal , or shortly carrier (engl. Carrier ) is a periodically varying physical size (eg. As an AC voltage or a radio wave ) having constant characteristic parameters (eg. As frequency , amplitude , duty cycle , phase angle ). Carriers are used to transport a useful signal by means of a carrier frequency method; The useful signal is mapped ( modulated ) in one or more of the named parameters of the carrier signal . Example: With the carrier frequency method VHF broadcasting at 100 MHz, the useful signal (a single radio program) is displayed as a frequency change of the carrier in the range from 99.925 to 100.075 MHz. At the receiver, the carrier signal is the reference signal for demodulating a previously modulated carrier; with its help, the useful signal is separated from the carrier signal. In procedures in which the carrier itself is not transmitted, the carrier must be reconstructed at the recipient for this purpose.
The carrier signal can be the carrier or the carrier frequency itself or information from which the carrier can be reconstructed for demodulation if, for example, the carrier is not transmitted with certain types of modulation.
Carrier frequency
In transmission technology , the carrier is usually a sinusoidal oscillation of a certain frequency. In the case of radio stations , this is often the assigned frequency or nominal frequency of the radio wave emitted. According to the definition of the ITU VO radio , carrier frequencies belong to the characteristic frequencies .
In the case of infrared or laser transmission or other carrier frequency methods, however, other, different or several carrier frequencies can be used at the same time.
Symbol for the carrier frequency is or .
Various factors play a role when choosing the carrier frequency:
First of all, the transmission medium and the propagation behavior of the carrier signal should be mentioned here . The frequency range of the short wave enables radio connections with a very long range, as the radio waves are reflected on the ionospheric layers and the earth's surface. In wired communication, the transmission range depends on the quality and type of cable and the distance.
In order to concentrate both the transmitted wave energy on a desired receiver and to achieve a higher sensitivity in the direction of a desired transmitter, a directional effect of an antenna may be desired ( directional radio ). The directivity of an antenna depends on the wavelength used and thus on the carrier frequency. With a smaller wavelength and thus a higher frequency, the same directivity can be achieved with a smaller antenna.
Antennas with directional effect are used, for example, for terrestrial television reception (usually Yagi antennas ), directional radio links, communication with spacecraft, and satellite television (mostly parabolic antennas ).
More bandwidth is available at higher frequencies . This can be used to transmit more information (e.g. television compared to voice radio or radio ) or to transmit the same information in a higher quality (e.g. frequency modulation and amplitude modulation ).
In Germany, the frequency plan regulates which frequency ranges may be used under which conditions, i.e. who may use which carrier frequency for radio transmission, which degree and type of modulation may be used for this, and which maximum transmission power is permitted.
In the case of cable connections and so-called long - distance cables , carrier frequency systems are often used.
literature
- Rudolf Mäusl, Jürgen Göbel: Analog and digital modulation methods, baseband and carrier modulation . Hüthig, 2002, ISBN 3-7785-2886-6 .
Individual evidence
- ↑ VO Funk, 2012 edition, Article 1.149, definition: characteristic frequency