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Noise and Interference

Noise and Interference. Any signal or phenomena which degrades signal to noise ratio (S/N). External Thermal noise (raw or amplified) On-channel unwanted emissions (interference) Atmospheric noise Man made noise Internal Distortion Harmonics Shot Noise (carrier transit noise)

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Noise and Interference

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  1. Noise and Interference • Any signal or phenomena which degrades signal to noise ratio (S/N). • External • Thermal noise (raw or amplified) • On-channel unwanted emissions (interference) • Atmospheric noise • Man made noise • Internal • Distortion • Harmonics • Shot Noise (carrier transit noise) • Flicker (1/f) Noise • For now, we will concentrate on Thermal and Shot Noise, as they: • Are pervasive • Are of similar magnitude • Have similar spectral properties

  2. Spectral Density • Thermal and shot noise are broad spectrum (white) random noise processes, characterized by “flat spectral density”, which has two important implications: • Equal energy available at all frequencies • Noise power entering a system is proportional to the system’s acceptance bandwidth (usually, the bandwidth of one received channel). • PN(watts) = ( ? ) x BW(hz) Fundamental Property of White Noise: Spectral Density N0 (watts/Hz = joule) N0 = Pwr/BW = kT for thermal noise. PN(watts) = kTB k = Boltzman’s constant = 1.38 x 10-23 (j/K) ; T = Absolute Temp (K)

  3. Thermal Noise Voltage PN(watts) ~ R R B For frequencies within B For frequencies outside of B Ideal Bandpass Filter Vn Voltage Spectral Density: Current Spectral Density:

  4. Shot Noise Shot Noise is represented by a random noise current generated in a semiconductor junction , in parallel with the junction dynamic resistance rd = 0.026v/IDC (for silicon). IN ~ rd q = 1.6 x 10-19 Coul (electron charge) Since noise currents and voltages are random, they must always be considered as RMS values when multiple noise sources are present: Thevenin is alive and well in Prescott Arizona, . . . and the rest of the universe.

  5. Example + 10 VDC - 20 kW Ideal Bandpass Filter B = 100 kHz B Vn

  6. Ishot ~ rd 20 kW ~ ~ 20 kW B 100 kHz Open Circuit Noise Voltage Vn Ith Ideal Bandpass Filter Vth

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