Stanford Research Systems SR785, Operating Manual And Programming Reference

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2-70 Intrinsic Noise Sources
SR785 Dynamic Signal Analyzer
Intrinsic (Random) Noise Sources
Random noise finds its way into signals in a variety of ways. Good test design can
reduce these noise sources and improve the measurement stability and accuracy.
There are a variety of intrinsic noise sources which are present in all electronic signals.
These sources are physical in origin.
Johnson Noise
Every resistor generates a noise voltage across its terminals due to thermal fluctuations
in the electron density within the resistor itself. These fluctuations give rise to an open-
circuit noise voltage,
V
noise
(rms) = (4kTR
∆
f)
1/2
where k=Boltzmann's constant (1.38x10
-23
J/
°
K), T is the temperature in
°
Kelvin
(typically 300
°
K), R is the resistance in Ohms, and
∆
f is the bandwidth in Hz.
∆
f is the
bandwidth of the measurement (usually the FFT linewidth).
Shot Noise
Electric current has noise due to the finite nature of the charge carriers. There is always
some non-uniformity in the electron flow which generates noise in the current. This
noise is called shot noise. This can appear as voltage noise when current is passed
through a resistor. The shot noise or current noise is given by
I
noise
(rms) = (2qI
∆
f)
1/2
where q is the electron charge (1.6x10
-19
Coulomb), I is the RMS AC current or DC
current depending upon the circuit, and
∆
f is the bandwidth (usually the FFT linewidth).
1/f Noise
Every 10
Ω
resistor, no matter what it is made of, has the same Johnson noise. However,
there is excess noise in addition to Johnson noise which arises from fluctuations in
resistance due to the current flowing through the resistor. For carbon composition
resistors, this is typically 0.1 to 3 µV of rms noise per Volt applied across the resistor.
Metal film and wire-wound resistors have about 10 times less noise. This noise has a 1/f
spectrum and makes measurements at low frequencies more difficult.
Other sources of 1/f noise include noise found in vacuum tubes and semiconductors.
Total Noise
All of these noise sources are incoherent. The total random noise is the square root of the
sum of the squares of all the incoherent noise sources. Thus, the largest noise source
easily dominates all others in determining the noise floor of the measurement.