temperature were the same numerically, since the equivalent is defined on that basis. In the base of other
equivalents, this often is not true.
5. If proper units are chosen, the equation for the noise power that can be delivered by a matched source at a
noise temperature, to (Pn = K2TB), is:
Pn = K2TB when Pn
=
noise in watts,
B
=
bandwidth of load in Hz,
T
=
temperature of source in degrees Kelvin,
K
=
Boltzman's constant,
-23
1.38 x 10 joules/degree Kelvin (or watt sec/degree Kelvin)
6. Since receiver bandwidths vary greatly, it is more convenient to express noise power in terms of noise per
unit of bandwidth.
Pn = KT
B
LEARNING EVENT 9: NOISE FIGURE
1. Some criterion is needed to rate receivers and receiving systems, indicating whether they are good, poor,
and so forth. The noise figure provides a numerical indicator as far as the noise performance is concerned. The
noise figure does not completely specify receiver performance since it says nothing about gain, bandwidth,
distortion, and so forth-all of which must be satisfactory as well.
2. The concept of noise figure has gone through many stages of development, and many slightly different
types of noise figure (spot noise figure, average noise figure) have been developed. This paragraph treats only
one type-average noise figure, the noise figure normally used in measuring a receiving system's performance.
The noise figure expresses the relative merit of a receiver in comparison with a so-called perfect receiver. The
perfect receiver is one that adds no noise to that produced by the antenna resistance and has a noise figure of 0
dB. The quantity normally is expressed as a power ratio converted to decibels, and the smaller the noise figure
the better the receiver. The noise figure to be considered is a single number characterizing the receiver and, in a
sense, is an average noise figure over the passband of the receiving system. Separate noise figures or spot noise
figures could be quoted at each frequency within the band, much the same as different gains can be quoted at
various frequencies for a simple amplifier. Just as it is common to refer to an amplifier as a 20-dB amplifier,
meaning that its maximum gain is 20 dB (100), so is it also common to quote a single noise figure as an average
over the whole passband. It is this average noise figure that will be of greatest interest as a criterion for rating a
system's performance. More specifically, it will be average standard noise figure.
3. The average standard noise figure gives a measure of the amount of noise that an amplifier (or any other
component) contributes to its output. The noise power at the
3-7
SS0031
Previous Page
