Amplitude shift keying

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Amplitude shift keying (English amplitude-shift keying , abbreviated as ASK ) is a digital modulation . The amplitude of the carriers is changed in order to transmit different values. The simplest form of amplitude keying is so-called On-Off Keying ( OOK ), in which the carrier is switched on or off in order to transmit a 1 or a 0.

Instead of just two different amplitude values, several gradations can also be used. This means that several bits can be coded per symbol step . If you put z. B. four different amplitudes in a signal, these amplitudes represent two bits (symbols: 00, 01, 10 and 11).


Example for the amplitude-modulated transmission power of DCF77 as a function of time

An example of amplitude shift keying is the German DCF77 signal from the radio clock , also known as the atomic clock . Other countries have implemented this idea on other frequencies. Here, a long wave carrier of 77.5 kHz is weakened in its amplitude every second to 25% of the nominal power. The beginning of the attenuation is used for precise time synchronization, and the duration of the attenuation (100 ms or 200 ms) represents one bit. Within a minute you have collected enough bits to display the date, time, leap second , normal time / summer time and possibly other information.

Another example are radio beacons for location determination. The carrier is first modulated with a tone in the audio range (e.g. 1 kHz). This subcarrier, in turn, is modulated (keyed) with a Morse code as the identifier of the radio beacon.