Sine Wave in Frequency Domain

A complete sine wave in the time domain can be represented by one single spike in the frequency domain.

The time-domain and frequency-domain plots of a sine wave

A single-frequency sine wave is not useful in data communication. We need to send a composite signal, a signal composed of many simple sine waves.
According to Fourier analysis, any composite signal is a combination of simple sine waves with different frequencies, amplitudes and phases.

A composite periodic signal

Decomposition of a composite periodic signal in the time and frequency domain

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Information about Frequency and Phase

Units of Period and frequency


Frequency is the rate of change with respect to time.
Change in a short span of time means high frequency.
Change over a long span of time means low frequency.
The power we use at home has a frequency of 50-60 Hz.

If a signal does not change at all, its frequency is zero.
If a signal changes instantaneously, its frequency is infinite.


Phase describes the position of the waveform relative to time 0.

Fig.a shows a phase shift of 0 degrees.
Fig.b shows a phase shift of 90 degrees.
Fig.c shows a phase shift of 180 degrees.

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Periodic Analog Signals

Periodic analog signals can be classified as simple or composite. A simple periodic analog signal, a sine wave, cannot be decomposed into simpler signals. A composite periodic analog signal is composed of multiple sine waves.

Topics discussed in this section:
Sine Wave
Time and
Frequency Domain
Composite Signals

In data communications, we commonly use periodic analog signals and non-periodic digital signals.

A sine wave

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Analog and Digital Data, Analog and Digital Signals

Data can be analog or digital.
The term analog data refers to information that is continuous and digital data refers to the information that has discrete states.
Analog data take on continuous values and digital data take on discrete values.

Signals can be analog or digital. 

Analog signals can have an infinite number of values in a range and digital signals can have only a limited 
number of values.

Comparison of Analog and Digital Signals

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Sine Wave

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