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Re: Can a coil replace a gridstopper?

Posted by TimFox (A) on January 29, 2007 at 11:04:34

In Reply to: Re: Can a coil replace a gridstopper? posted by Triode_Kingdom on January 28, 2007 at 17:23:01:

The equation for thermal noise voltage (not shot noise) in a resistance is

d(v^2) = 4kTR df

where k (Boltzmann's constant) = 1.37x10^-23 J/K
and T is the absolute temperature in Kelvins.

This was published almost simultaneously by Johnson and Nyquist, but is usually called Johnson noise. All resistors at the same temperature have the same noise power, but that power is a different voltage (or current), as a function of the resistance.

The square root of this equation is the noise spectral density in volts per root Hertz, which is a slightly non-rigorous way of discussing the statistical variance, which is (square volts)/(Hertz). For a white source (such as thermal or shot noise), multiply the noise density by the square root of the bandwidth. For a pink or blue noise density, integrate the variance (in square volts per Hz) over the frequency bandwidth and take the square root of the integral.

My mnemonic is that a 50 ohm resistor generates 0.9 nV/(root Hz), and the noise voltage density scales as the square root of the resistance.

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