Q. Illustrates typical thermal noise waveform?

In general, any physical resistor or lossy device can be modeled by a noise source in series with a noiseless resistor, as shown in Figure. The noise source is usually characterized as a sample function of a random process. Since random processes involving probability and random variables are outside the scope of this text, we will resort to simpler explanations. Figure (a) illustrates a typical thermal noise waveform n(t). In view of the unpredictable behavior, since the average value of n(t) may be equal to zero, a more useful quantity is the rms value nrms so that the average noise power is given by

N = n²_rms/R, if n_rms is noise voltage

or

N = n²_rmsR, if n_rms is noise current

The spectrumof thermal noise power is uniformly spread over frequency up to the infrared region around 10¹² Hz, as shown in Figure (b). Such a distribution indicates that n(t) contains all electrical frequencies in equal proportion, and an equal number of electrons is vibrating at every frequency. By analogy to white light, which contains all visible frequencies in equal proportion, thermal noise is also referred to as white noise.