SECTION II. NOISE MEASUREMENTS
LEARNING EVENT 8: NOISE TEMPERATURE
1. The noise voltage appearing across the terminals of a resistor is proportional to the temperature of the
resistor. The noise voltage is due to thermal agitation; that is, electron motion caused by the heating of the
electrons in the structure of the resistor. If the resistor is heated to a higher temperature, the noise voltage
increases; if the temperature is lowered, the noise voltage decreases. A useful measure of these noise voltages
is a quantity that is proportional to voltage squared, or power. The rule that this noise power or voltage
increases with temperature can be expressed in a more precise way if the noise is measured as a noise power
and the temperature is measured on an absolute scale.
Pn = K1T when Pn =
absolute temperature (in degrees Kelvin)
a. To compare noise temperature measurements without regard to the type of device or bandwidth
involved, it is necessary to use a standard noise temperature reference.
b. The Kelvin scale is used to show absolute temperatures. The Kelvin scale shows absolute temperature
because its zero point is referenced to the (theoretically) lowest possible temperature (absolute zero). Absolute
zero is the temperature at which all thermal agitation (molecular activity) theoretically ceases.
c. The standard noise temperature is defined as 290€ Kelvin (62.6€ Fahrenheit, 17€ Centigrade).