Pulse Amplitude Modulation

Pulse Amplitude Modulation (PAM) is a digital modulation technique used in communication systems to transmit digital data over a communication channel. In PAM, the amplitude of a series of pulses is varied in proportion to the amplitude of the analog signal that is to be transmitted. The resulting pulse train is then transmitted over the communication channel.

Pulse Amplitude Modulation
Pulse Amplitude Modulation

The basic concept of PAM involves converting an analog signal into a digital signal by sampling it at regular intervals and then quantizing it to a finite number of levels. The resulting digital signal is then used to modulate the amplitude of a series of pulses, with each pulse representing one sample of the digital signal.

The pulse train produced by PAM can be transmitted using various communication techniques such as time division multiplexing (TDM), frequency division multiplexing (FDM), or code division multiplexing (CDM). The receiver demodulates the pulse train to recover the original digital signal by detecting the amplitude of each pulse and quantizing it back to its original level.

PAM is widely used in various communication systems, including digital audio systems, computer networking, and digital television broadcasting. It is a simple and efficient modulation technique that provides a high level of accuracy and reliability in transmitting digital signals.

Pulse Modulation
  1.  Carrier is a train of pulses
  2.  Example: Pulse Amplitude Modulation (PAM), Pulse width modulation (PWM) ,Pulse Position Modulation (PPM)

Types of Pulse Modulation:

  1.  The immediate result of sampling is a pulse-amplitude modulation (PAM) signal
  2.  PAM is an analog scheme in which the amplitude of the pulse is proportional to the amplitude of the signal at the instant of sampling
  3.  Another analog pulse-forming technique is known as pulse-duration modulation (PDM). This is also known as pulse-width modulation (PWM)
  4.  Pulse-position modulation is closely related to PDM 

PAM Generation:

The carrier is in the form of narrow pulses having frequency fc. The uniform sampling takes place in multiplier to generate PAM signal. Samples are placed Ts sec away from each other.

PAM Modulator
PAM Modulator

  1.  The circuit is simple emitter follower.
  2.  In the absence of the clock signal, the output follows input.
  3.  The modulating signal is applied as the input signal.
  4.  Another input to the base of the transistor is the clock signal.
  5.  The frequency of the clock signal is made equal to the desired carrier pulse train frequency.
  6.  The amplitude of the clock signal is chosen the high level is at ground level(0v) and
  7.  low level at some negative voltage sufficient to bring the transistor in cutoff region.
  8.  When clock is high, circuit operates as emitter follower and the output follows in the input modulating signal.
  9.  When clock signal is low, transistor is cutoff and output is zero.
  10.  Thus the output is the desired PAM signal.

PAM Demodulator:

  •  The PAM demodulator circuit which is just an envelope detector followed by a second order op-amp low pass filter (to have good filtering characteristics) is as shown below 

PAM Demodulator
PAM Demodulator