Journey to the Bottom of Your Rig, Radio Fundamentals Explored. Original article by Houston, Long, Keating, et al, now with comments by Tom Farley. Reprinted with permission.
Pages: (1) (2) (3) (4) (5) (6) (7) (8) Modulation page // Oscillator Page
Peeling off, Detecting, or Modulation
The authors mentioned how older radio receivers "peeled" the incoming, low frequency audio signals right off the much higher radio frequencies. Let's start at the beginning.
We can't hear radio signals without help. Radio frequency signals are way beyond our range of hearing. We can hear voice or audio signals up to about 15,000 to perhaps 20,000 cycles per second. Think of a tuning fork or a piece of fine crystal. Strike either and hear them resonate, vibrating at thousands of cycles. A radio frequency, though, can oscillate at millions of times a second! So, we need to process a radio signal before we can hear what the radio wave carries. One part in processing is called detection or demodulating the carrier. But before we understand demodulating, we need to know how voice signals get turned into electrical signals Hang in there, it is simpler than it sounds.
The most important principle in radio and telephony is the concept of variable resistance, pictured above; it is how everything gets started. Your voice is sound in motion. Speaking causes sound waves. Hold a piece of binder paper by its corners close to your mouth. Loudly and firmly say "I don't understand any of this!" Feel that paper vibrate? That's sound in motion.
Telephone and radio transmitters convert that acoustic pressure into electrical pressure. That's why the electrical tester above shows a rise and fall as sound waves rise and fall. A radio or telephone receiver at the other end of our call then takes these variations in the current and throws the process into reverse. It works a speaker by the changes in the electrical signal, that is, a speaker now vibrates in sympathy with the varrying current it receives. Now let's add some terms to what we've already learned.
With radio we produce a so called "carrier" for our call to travel on. This invisible electrical path is a very high radio frequency. Speaking into the radio's transmitter varies or modulates the carrier wave. We impress the voice signal, something using low audio frequencies, onto a much higher radio signal. Get it? Isn't radio pHun? Now that we know about modulating, we can get back to learning about detecting or demodulating. But first, one last comment.
The radio technique I just described is called A.M. or amplitude modulation. I didn't want to scare anyone by calling it by its real name. A.M. simply means that a carrier wave is modulated in proportion to the strength of a signal. The carrier rises and falls instantaneously with each high and low of the conversation. Just like we've seen, the voice signal produces an immediate and equivalent change in the carrier. Okay, back to the article and remember, when you see a reference to a detector, think demodulator!
From The Big Dummy's Guide to C.B. Radio, courtesy of The Book Publishing Company P.O. Box 99,Summertown, TN 38483 (888) 260-8458, (1976). Editors: White Lightning (Albert Houston) WB4BWR, Stringbean WA4LXC (Mark Long), Minnesota Mumbler WB4KDH (Jeffrey Keating), Ratchet Jaw K4IAP (William Hershfield), Buffalo Bill WA4KCF (William Bradley) Illustrations by Mark Schlichting and Peter Hoyt.
