Digital Signals and Waveforms

Electronic circuit can be divided into two broad catagories: Digital and Analog.
Digital electronics involves quantities with discrete values and analog electronic involves quantities with continuous values.

Fig. Analog Signal

An analog quantity is one having continious values. Most things that can be measured quantitavely occur in nature in analog form.Example of analog quantities are time, pressure, distance, sounds etc.

Fig. Digital Signal

An digital quantity is one having a discrete set of values. It is mainly used for processing within the computer. Digital data can be processed and transmitted more efficiently and reliably then analog data. It has high accuracy.

Digital Waveform:

Digital waveforms consists of voltage levels that are changing back and forth between the HIGH and LOW levels of states.

Positive going pulse is generated when the voltage or current goes from its normally LOW level to HIGH level and then back to its LOW level.

Negative going pulse is generated when the voltage goes from its normally HIGH level to its LOW level and back to its HIGH level.






A digital wavefom is made up of a series of Pulses. A pulse has two edges:

1. Leading edge: Occurs first time at t0
2. Trailing edge: Occurs last time at t1

• For a positive going pulse, the leading edge is a rising edge and trailing edge is a falling edge.
• For a negative going pulse, the leading edge is a falling edge and tariling edge is a rising edge.

Non-ideal Pulse

Rise time (tr):
time required for a pulse to go from Low to High level.

Fall time (tf):
time required for a pulse to go from High level to Low level.

Amplitude:
height of pulse from its baseline.

Pulse width (tw):
it is a measure of duration of the pulse and is often defined as the time interval between 50% points on the rising and falling edges.

Waveform Characteristics

Most waveforms encountered in digital system are composed of series of pulses, sometimes called pulse trains, can be classified as
Periodic
Non-Periodic

A periodic pulse waveform is one, that repeats itself at a fixed interval called a Time Period (T). The frequency (f) is the rate at which it repeats itself and is measured in Hertz (Hz)

Period = T1 = T2 = T3 = ......... = Tn

frequency (f) = 1/T

A non-periodic pulse waveform doesnot repeat itself at fixed interval and may be composed of pulses with ramdomly differing pulse width and randomly differing time interval between the pulses.




The frequency of a pulse is the reciprocal of the period. The relation between frequency and time period is expressed as
f = 1/T
T = 1/f

Duty Cycle:

An important characteristics of a periodic digital waveform is its duty cycle, which is the ratio of the pulse width (tw) to the period (T). It can be expressed as a percentage.

Duty Cycle = (tw/T)*100%