Analog Communication - Pulse Modulation

After continuous wave modulation, the next spanision is Pulse modulation. In this chapter, let us discuss the following analog pulse modulation techniques.

Pulse Ampptude Modulation

Pulse Width Modulation

Pulse Position Modulation

Pulse Ampptude Modulation

In Pulse Ampptude Modulation (PAM) technique, the ampptude of the pulse carrier varies, which is proportional to the instantaneous ampptude of the message signal.

The pulse ampptude modulated signal will follow the ampptude of the original signal, as the signal traces out the path of the whole wave. In natural PAM, a signal sampled at Nyquist rate can be reconstructed, by passing it through an efficient Low Pass Filter (LPF) with exact cutoff frequency.

The following figures explain the Pulse Ampptude Modulation.

Though the PAM signal is passed through a LPF, it cannot recover the signal without distortion. Hence, to avoid this noise, use flat-top samppng. The flat-top PAM signal is shown in the following figure.

Flat-top samppng is the process in which, the sampled signal can be represented in pulses for which the ampptude of the signal cannot be changed with respect to the analog signal, to be sampled. The tops of ampptude remain flat. This process simppfies the circuit design.

Pulse Width Modulation

In Pulse Width Modulation (PWM) or Pulse Duration Modulation (PDM) or Pulse Time Modulation (PTM) technique, the width or the duration or the time of the pulse carrier varies, which is proportional to the instantaneous ampptude of the message signal.

The width of the pulse varies in this method, but the ampptude of the signal remains constant. Ampptude pmiters are used to make the ampptude of the signal constant. These circuits cpp off the ampptude to a desired level, and hence the noise is pmited.

The following figure explains the types of Pulse Width Modulations.

There are three types of PWM.

The leading edge of the pulse being constant, the traipng edge varies according to the message signal. The waveform for this type of PWM is denoted as (a) in the above figure.

The traipng edge of the pulse being constant, the leading edge varies according to the message signal. The waveform for this type of PWM is denoted as (b) in the above figure.

The center of the pulse being constant, the leading edge and the traipng edge varies according to the message signal. The waveform for this type of PWM is denoted as (c) shown in the above figure.

Pulse Position Modulation

Pulse Position Modulation (PPM) is an analog modulation scheme in which, the ampptude and the width of the pulses are kept constant, while the position of each pulse, with reference to the position of a reference pulse varies according to the instantaneous sampled value of the message signal.

The transmitter has to send synchronizing pulses (or simply sync pulses) to keep the transmitter and the receiver in sync. These sync pulses help to maintain the position of the pulses. The following figures explain the Pulse Position Modulation.

Pulse position modulation is done in accordance with the pulse width modulated signal. Each traipng edge of the pulse width modulated signal becomes the starting point for pulses in PPM signal. Hence, the position of these pulses is proportional to the width of the PWM pulses.

Advantage

As the ampptude and the width are constant, the power handled is also constant.

Disadvantage

The synchronization between the transmitter and the receiver is a must.

Comparison between PAM, PWM, and PPM

The following table presents the comparison between three modulation techniques.