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~ Thermal Noise Calculator ~

Calculates the root mean square Thermal Noise Voltage Vn-rms in a given bandwidth at a specific temperature that is passively generated across the resistive part 'R' of a circuit element ~ There may be additional "excess noise" generated by current flowing through the resistance due to the type of material and its purity hence the "extra" pink noise from old carbon resistors or shot noise in active devices ~ Click here for other calculators


Boltzmann constant — k
1.3806x10-23
 J/K
Temperature — T
°C
High Pass Response –3dB — fl
Hz — Can be zero for d.c. coupled
Low Pass Response –3dB — fh
Hz — Every circuit has a low pass
fl and fh Filter order — m
assumed Butterworth response
Resistance — R
Ω
Calculated results using the values entered above
Noise Bandwidth — Bn
Noise Voltage — Vn-rms
nV  = √4kTBnR  =    nV/√Hz
Noise Level — ref. 1V
dBV = 20 log Vn-rms/1V
Noise Level — ref. 0.775V
dB(0.775) or dBu = 20 log Vn-rms/0.775V
Noise Level — ref. 1mW in 50Ω
dBm = 10 log (Vn-rms)2/50 x10-3
Reference voltage for calculating Signal/Noise ratio SNR etc.
ReferenceVoltage   V
dBr = 20 log (Vn-rms)/Vref

 The noise bandwidth is not the same as a -3dB bandwith and It is possible to enter fl higher than fh as can be done in practice using real cascaded and buffered filters but the math may not always work as expected like when fl = fh ~ The default values are for the Tektronix AA501 audio analyser or similar equipment with an IEC 22.4kHz audio filter with 3rd order Butterworth response

For more information about noise in components and amplifiers see:

The Art of Electronics 3rd edition chapter 8 ~ By Paul Horowitz and Winfield Hill ~ fellow cynics
Noise in Transistor Circuits ~ By P. J. Baxandall ~ Wireless World November 1968 ~ often referenced
Designing low–noise audio amplifiers ~ By Wilfried Adam ~ Wireless World June 1989
Introduction to low–noise amplifier design ~ By A. Foord ~ Wireless World April 1981
The design of Low-noise audio frequency amplifiers ~ By E. A. Faulkner ~ The Radio and Electronic Engineer July 1968 ~ This article along with the book 'Low-Noise Electronic System Design' By C. D. Motchenbacher and J. A. Connelly should answer most questions about electronic noise

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