Amplitude Shift Keying (ASK) is the digital modulation technique. In amplitude shift keying, the amplitude of the carrier signal is varied to create signal elements. Both frequency and phase remain constant while the amplitude changes. In ASK, the amplitude of the carrier assumes one of the two amplitudes dependent on the logic states of the input bit stream. This modulated signal can be expressed as:
Note that the modulated signal is still an on-off signal.
Amplitude shift keying (ASK) in the context of digital signal communications is a modulation process, which imparts to a sinusoid two or more discrete amplitude levels. These are related to the number of levels adopted by the digital message. For a binary message sequence there are two levels, one of which is typically zero. Thus the modulated waveform consists of bursts of a sinusoid. Figure 1 illustrates a binary ASK signal (lower), together with the binary sequence which initiated it (upper). Neither signal has been band limited.
Fig: an ASK signal (below) and the message (above)
There are sharp discontinuities shown at the transition points. These result in the signal having an unnecessarily wide bandwidth. Band limiting is generally introduced before transmission, in which case these discontinuities would be ‘rounded off’. The band limiting may be applied to the digital message, or the modulated signal itself. The data rate is often made a sub-multiple of the carrier frequency. This has been done in the waveform of Fig.
One of the disadvantages of ASK, compared with FSK and PSK, for example, is that it has not got a constant envelope. This makes its processing (eg, power amplification) more difficult, since linearity becomes an important factor. However, it does make for ease of demodulation with an envelope detector.With bandlimiting of the transmitted ASK neither of these demodulation methods (envelope detection or synchronous demodulation) would recover the original binary sequence; instead, their outputs would be a bandlimited side. Thus further processing by some sort of decision-making circuitry for example – would be necessary.
Thus ASK demodulation is a two-stage process:
- recovery of the bandlimited bit stream
- regeneration of the binary bit stream.
Application of ASK:
- Amplitude-shift keying is used extensively for commercial terrestrial
- It is usefulness for satellite applications is limited.
- Space systems typically employ saturated power amplifiers.
When an amplitude-shifted keying signal is passed through such a nonlinear amplifier, sidelobes can grow large enough to interfere with the adjacent signals. As a result, the amount of bandwidth or power needed for signal transmission increases.
A comparison between ASK and FSK modulation techniques for radio links:
1) ASK transmitters are simpler than FSK
2) ASK transmitter current is 50% lower than FSK
3) SAW based ASK transmitters are more robust when exposed to extreme temperatures, vibration and shock
4) FSK transmission requires 1.5 times the bandwidth compared to ASK
5) ASK receivers are simpler than FSK
6) ASK receiver sensitivity is equal to or better than FSK
7) Properly implemented, ASK receiver performance in the presence of co-channel interference is generally better than FSK
8) Properly implemented, ASK receiver performance with amplitude flutter is equal to or better than FSK.
- Diode Clamper
- Diode Characteristics (Part 2)
- Low Pass Filters Circuit
- Series Circuit Diagrams
- Waveform Generator
- Full Wave Rectification (Part 2)
- Zener Diode Specification (Part 2)
- Atom Energy Levels
- Quadrature Amplitude Modulation
- Gaussian Minimum Shift Keying,
- Complex of Vectors
- Diode Clipper (Part 2)
- Semiconductor Materials
- Examples of Superposition Theorem in Circuit
- Series and Parallel Circuit Diagrams
