Pulse Amplitude Modulation

Pulse Amplitude Modulation

Pulse amplitude modulation is defined as the data transmission by altering the amplitudes(power levels or voltage) of every pulse in a regular time sequence of electromagnetic pulses. The possible number of amplitudes can be infinite, but mostly it is some power of two so that the final output signal can be digital. For example, in level-4 PAM there are 22 discrete pulse amplitudes; in level-8 PAM there are 23 discrete pulse amplitudes.

Types of Pulse Amplitude Modulation

There are two major types of PAMs.

  • Single polarity PAM

Here, an appropriate fixed DC bias is integrated with the signal to assure that all pulses are positive.

  • Double polarity PAM

Here, the pulses are both negative and positive.

In some PAMs, the amplitude of every pulse is directly proportional to instantaneous modulating amplitude when pulse occurs. While in other PAM, the amplitude of every pulse is inversely proportional to instantaneous modulating amplitude when at occurrence of pulse. In some other systems, the intensity of every pulse is based on particular characteristics of modulating signal other than strength like instantaneous phase or frequency.

PAM is mostly applied in non-based modulating transmission of digital data and applications replaced by pulse-code modulation and pulse-position modulation.

Particularly all phone modems faster than 300 bit/s use quadrature amplitude modulation.

Advantages of Pulse Amplitude Modulation

  • PAM allows data to be transmitted more effectively, efficiently and quickly using conventional copper wires in greater volume.
  • The frequency modulations available are infinite; hence PAM formulas can be developed continually to allow increased data throughput over existing networks.
  • PAM is also the simplest form of modulation.

Practise This Question

A transmitter supplies 9 kW to the aerial when unmodulated. The power radiated when modulated to 40% is