Sources produce signals without having input. We like to think of these as having controllable parameters, like amplitude and frequency. Examples would be oscillators that produce periodic signals like sinusoids and square waves and noise generators that yield signals with erratic waveforms (more about noise subsequently). Simply writing an expression for the signals they produce specifies sources. A sine wave generator might be specified by y (t)= Asin (2πf0t) u (t), which says that the source was turned on at t =0 to produce a sinusoid of amplitude A and frequency f0.
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