Fundamentally, a receiver is limited in sensitivity by noise that competes with the signal we want. A receiver is also limited in handling strong signals by its nonlinearities, which produce intermodulation products that block reception. Noise is a random voltage or current that is present whether a signal is there or not. We distinguish noise from interference, which is an unwanted signal coupled into the circuit, and from fading, which is a variation in the signal level, caused by interference between radio waves arriving by different paths. There are many different sources of noise. Several forms are caused by bias currents. In diodes, the random arrival times of electrons cause shot noise. Another current noise is 1/f noise, where power varies inversely with the frequency. This 1/f noise is found in contacts, and it is associated with energy states at interfaces called traps. It can often be reduced by improving the fabrication process. However, even in the absence of bias currents there is noise associated with resistors. It is called Johnson noise after John Johnson at the Bell Telephone Laboratories, who first measured it.

Noise

On an oscilloscope, noise makes a trace appear as a band that evokes the feeling of grass. We can write the noise as a function of time V(t), but we would not be able to predict its value at a future time.