Noise and Bandwidth

Author: N.H. Crowhurst

Noise energy can be reduced by limiting the frequency range

At an average room temperature, the noise energy of any resistor is .0165 micromicromicrowatt, or .000 000 000 000 000 000 0165 watt, per cycle. If we take the bandwidth as 10,000 cycles, the noise energy will be 165 micro-micromicrowatts. Once we know the value of the resistance, we can calculate the noise voltage it will produce, by the usual formula, E = \ P X R For example, for a 1-megohm resistor (1,000,000 ohms), the noise voltage for a bandwidth of 10,000 cycles is:

This suggests one convenient way to reduce effective noise when that is a^ severe problem. By reducing the frequency range at the high end, say from 20,000 cycles to 5,000 cycles, we lose 2 octaves of audio (and the top one doesn't have much in it anyway) and we reduce the noise in the ratio of 4 to 1, or 6 db. This technique is useful for communications work.