Radio Frequency Filtration
Introduction
Music starts out as air vibrations. These are not directly affected by electromagnetic (EM) waves, except while they are passing through an audio system in the form of electronic signals. Planet Earth has long had natural EMI, in the form of various electric and magnetic storms; both those occurring in the atmosphere and those occurring on the “surface” of the Sun and Jupiter in particular. Since 1900, the planet has increasingly abounded in human- made EMl babble, comprising electromagnetic energy fields and waves, some continuous, some pulsed, and others random. As stray signals nearly always have nothing to add to the music at hand, and most are profoundly unmusical, and as EMI permeates almost everywhere above ground unless guarded against, music signals require “pro-active” protection.
EM waves used for radio broadcasting and communications mainly start in earnest at 150 kHz (in the United Kingdom and continental Europe) and above, and continue to frequencies l0,000 times higher. However, special radio transmissions (for submerged submarines, national clocks, and caving) may use frequencies below 100 kHz and even those below 20 kHz.
Requirement
All active devices are potentially susceptible to EMI. BJTs, all kinds of field effect transistors (FETs), and also valves can all act as rectifiers at RF, demodulating radio transmissions. However, this is very much more likely with BJTs, as the nonlinearity of a BJT’s forward biased base-emitter junction that gives rise to rectification is triggered by considerably lower levels of RF voltage or field strength. All kinds of FETs and valves are relatively “RF proof” in comparison. Oxidized copper, generally dirty contacts, crystalline soldered joints, or wrong metal-to-metal interfaces can all act as RF detectors as well, through rectification.