通信原理英文.pptx

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Chapter 2 Signals
Classification of Signals
Deterministic signals and random signals
What is deterministic signal?
What is random signal?
Energy signals and power signals
Signal power: Let R = 1, then P = V2/R = I2R = V2 = I2
Signal energy：Let S represent V or I，if S varies with time，then S can be rewritten as s(t),
Hence, the signal energy E =  s2(t)dt
Energy signal satisfies
Average power: , then P = 0 for energy signal.
For power signal: P  0, ., power signal has infinite duration.
Energy signal has finite energy, but its average power equals 0.
Power signal has finite average power, but its energy equals infinity.
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Characteristics of deterministic signals
Characteristics in frequency domain
Frequency spectrum of power signal: let s(t) be a periodic power signal, its period is T0, then we have

where 0 = 2 / T0 = 2f0
∵ C(jn0) is a complex function, ∴ C(jn0) = |Cn|ejn
where |Cn| － amplitude of the component with frequency nf0
n － phase of the component with frequency nf0
Fourier series of signal s(t):
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【Example 】 Find the spectrum of a periodic rectangular wave.
Solution: Assume the period of a periodic rectangular wave is T , the width is , and the amplitude isV, then
Its frequency spectrum is
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Frequency spectrum figure
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【Example 】Find the frequency spectrum of a sinusoidal wave after full-wave rectification.
Solution：Assume the expression of the signal is
Its frequency spectrum:

The Fourier series of the signal is:
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f(t)
t
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Frequency spectral density of energy signals
Let an energy signal be s(t), then its frequency spectral density is

The inverse Fourier transform of S() is the original signal:
【Example 】Find the frequency spectral density of a rectangular pulse.
Solution: Let the expression of the rectangular pulse be
Then its frequency spectral density is