the system doubles the measured power; halving the bandwidth halves the power. This means that the power
available from the resistor depends on bandwidth as well as temperature. The previous expression for noise
power can be rewritten to include the bandwidth.
c. This indicates that specifying the amount of noise power available from a resistor does not mean too
much unless we also know the bandwidth of the measuring system. This is where the noise temperature becomes
valuable, inasmuch as it gives a measure of the noise power available from the resistor. The noise temperature
does not depend on the bandwidth of the measuring system. Further measurements might be made to determine
whether changing the resistance of the resistor while maintaining the same temperature would yield different
noise powers. The results of this experiment would give a negative result; therefore, the power does not depend
on the value of the resistance.
d. Many other sources of noise behave in much the same manner as the resistor discussed. In the case of
the resistor, the thermal noise temperature and noise temperature were the same numerically, since the equivalent
is defined on that basis. In the case of other equivalents, this often is not true.
e. If proper units are chosen, the equation for the noise power that can be delivered by a matched source
at a noise temperature, T (Pn - K2TB), is:
f. Since receiver bandwidths vary greatly, it is more convenient to express noise power in terms of noise
per unit of bandwidth.
4-9.
NOISE FIGURE
a. Some criterion is needed to rate receivers and receiving systems, indicating whether they are good,
poor, etc. 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, etc., all of which must be satisfactory as well.
b. The concept of noise figure has gone through many stages of development, and many slightly
different types of noise figure (spot noise figure, average
344 L4
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