Source of White Noise (Radio)

I was wondering, if you had an older radio that did not filter it out, there is always white noise on frequencies where no carrier exists. Even FM can be staticy, such as when I tune into a distant rock station I enjoy, depsite all the dogma that FM is so much “cleaner”. So, I was wondering: What is the source of all this white noise? I WAG it can only be celestial sources? (Googling only brought me explanations to the term “white noise” and “white noise machines”.) Yes, there are local sources of interference, like high voltage lines, but my question refers to CONSTANT sources of background white noise that is always, always there.

White noise or static is generated by:

  • Natural atmospheric processes such as lightning

  • Nearby electrical impulses in electrical switches, motors, vehicle ignition circuits, computers, and other man-made sources

  • Cosmic background noise at certain frequencies

There’s also noise in the amplifier stages of the radio. So, even with a perfect signal with zero modulation, if you turn up the volume enough, you will get white noise.

FM works by modulating the frequency (hence, frequency modulation, or FM). The FM receiver tracks the frequency and converts the frequency shifts into a signal, which is your audio signal. In order to track the frequency, it has to be strong enough to be distinguishable from the noise. An FM receiver will always have difficulty once the radio station starts to disappear into the background noise.

Similarly, two FM stations that are close in frequency can interfere with each other. The FCC spaces stations out around the country to minimize this, but in more populated areas where you have more stations you can run into more problems, especially once you get further away so that you are in an “in between” area where you are roughly equally distant from two distant stations.

If you have a strong clear signal with no competing signals nearby, FM is pretty good at eliminating noise, especially when compared to AM.

Yes. When you hear “stationary” white noise, you are undoubtedly hearing thermal noise generated within the radio itself. “Stationary” is a technical term that means loosely that the statistical properties of the noise are constant in time. Radio frequency interference (RFI) due to lightning, electrical equipment, and radio transmitters will typically give rise to bursts of noise or noise whose average power varies with time.

All electronic components with finite resistance produce noise, whose power is proportional to temperature. This “thermal noise” is due to the random motion and vibrations of electrons and atoms within the device, the same thing that gives rise to “Brownian Motion,” which was first explained by Einstein. Since the recieved power in a radio signal is typically very low, radios are built with tremendous amounts of amplification. If there is a strong enough radio signal present, the automatic gain control will reduce the gain so the signal will not saturate the amplifiers. If there is no strong signal, the amplification will be cranked up to the point that you can literally hear the random motion of the electrons in the radio receiver.

With very high quality electronics, low noise amplifiers, and excellent antennas, you can, in fact, hear the cosmic background radiation; but under more typical conditions it will be completely swamped by thermal noise in the receiver.

I should add, that there is also a quantum source of white noise that can also be significant. This is “shot noise” due to the fact that electricity flows in discrete amounts, one electron at a time.

Pretty cool stuff! Thanks, all, for the facts on this subject.