Pulse Amplitude Modulation

Pulse Amplitude Modulation

Pulse Amplitude Modulation is “pulse shaping”. Essentially, communications engineers realize that the shape of the pulse in the time domain can positively or negatively affect the characteristics of that pulse in the frequency domain. There is no one way to shape a pulse, there are all sorts of different pulse shapes that can be used, but in practice, there are only a few pulse shapes that are worth the effort. These chapters will discuss some of the common pulses, and will develop equations for working with any generic pulse.

Square Wave

The most logical way to transmit a digital signal is through a stream of pulses. One distinct pulse for a digital “1”, and another distinct pulse for a digital “0”. Intuitively, a square pulse will transmit this data, and there are a number of different ways to transmit the data using

The square wave is a basic choice for transmitting digital data because it is easy to transmit, and is generally easy to receive. If we take the fourier transform of a square wave, we get a sinc function. A sinc function is a never-ending function, which means that a square wave in the time domain has a very wide bandwidth. When using a square wave, there will always be a trade-off, because high-frequency components of the square wave will be attenuated by the channel, and the resultant waveform will be more prone to error on the other end.

Pulse Amplitude Modulation

Unipolar Square Wave

A unipolar square wave is a wave where a logical 1 is transmitted using a square pulse of height A. Then a logical 0 is transmitted with a 0 voltage.

Bipolar Square Wave

A bipolar square wave is a square wave where a 1 is transmitted with a pulse of height A/2, and a 0 is transmitted with a pulse of -A/2.