Amplitude shift key

Electrical & Computers

There are three schemes of digital modulation. Amplitude shift key is one of the three modes like Amplitude Shift Keying (ASK), Frequency Shift Keying (FSK) and Phase Shift Keying (PSK).

In amplitude shift key, the strength of the carrier signal is varied to represent binary 1 or 0. Both frequency and phase remain constant while the amplitude changes. Bit duration is the period of time that defines one bit.

The peak amplitude of the signal during each bit duration, in amplitude shift key is constant and its value depends on the bit O or 1. The speeds of transmission using ASK is limited by the physical characteristics of the transmission medium.

This scheme requires low bandwidth, but is very susceptible to interference. Effects like multipath propagation, noise or path loss heavily influence the amplitude. Thus, in a wireless environment, constant amplitude cannot be guaranteed. amplitude shift key is usually not used for wireless radio transmission. But in the wired transmission scheme like the optical transmission, uses ASK. Here, a light pulse may represent a 1, while the absence of light represents other value. ASK can also be applied to wireless infrared transmission, using a directed beam or diffuse light.

FSK avoids most of the noise problems of amplitude shift key as the receiving device looks for specific frequency changes over a given number of periods, it can ignore voltage spikes. The limiting factors of FSK are the physical capabilities of the carrier ESK needs a larger bandwidth compared to ASK but is much less susceptible to errors.

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