lec11_noise

lec11_noise - Noise & Noise Rejection Noise Sources...

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Slide 1 ME365 Noise & Noise Rejection Noise & Noise Rejection Noise Sources Characterizing Noise Signal-to-Noise Ratio (SNR, S/N) Power Spectral Density Modes of Interference Noise Rejection: Filtering Modulation Shielding & Grounding Differential Amplifier Averaging
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Slide 2 ME365 Noise & Noise Rejection Noise Noise Noise is any unsteady component of the measurement signal that causes the instantaneous value of the signal to differ from the instrument’s rendition of the “true value”. Time x ( t ) x t s t n t ( ) ( ) ( ) Actual Signal Noise
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Slide 3 Noise Sources Internal Temperature-induced random noise in conductors (Johnson noise) Random current fluctuations in transistors (Shot noise) Flicker noise External AC Interference Communication Interference Switching Interference Mechanical/Structural Vibration ME365 Noise & Noise Rejection
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Slide 4 ME365 Noise & Noise Rejection Average Signal Power Average power (in Watts) dissipated when voltage signal is connected to a 1 W resistor: Assume the actual signal s ( t ) and the noise n ( t ) are independent of one another: Root Mean Square (RMS) voltage of a signal is: Ex: For x ( t ) = A sin( w t + f ), the average power of x ( t ) is: Noise Characterization x t T x t dt T 2 2 0 1 ( ) ( ) x t s t n t 2 2 2 ( ) ( ) ( ) Signal Power Noise Power  x t x t ( ) ( ) rms 2 A 2 /2
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Slide 5 ME365 Noise & Noise Rejection Signal-to-Noise Ratio (SNR, S/N) Characterizes the “noisiness” of a particular signal, x ( t ) = s ( t ) + n ( t ): SNR is usually expressed in decibels (dB): The uncertainty of a measurement is the inverse of the SNR. Noise Characterization S N Signal Power Noise Power s t n t 2 2 ( ) ( ) S N Signal Power Noise Power or S N Signal RMS Noise RMS dB dB rms rms s t n t s t n t 10 10 20 20 10 10 2 2 10 10 log log ( ) ( ) log log ( ) ( )
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Slide 6 ME365 Noise & Noise Rejection Noise Characterization System “Noisiness” The “noisiness” of a system (device) is called the Noise Figure ( NF ). It is defined to be the ratio between the input and output SNR: It is desirable to design a measurement system (device) that has a noise figure ( NF ) close to 1. NF SNR SNR NF NF dB ( ) ( ) log ( ) INPUT OUTPUT 10 10 Measurement System (Device) Input x ( t ) Output y ( t )
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ME365 Noise & Noise Rejection Random Noise – Observe “random” signal for several observation periods: Signal is different from period to period. The average power for each observation period is about the
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lec11_noise - Noise & Noise Rejection Noise Sources...

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