Did you ever wonder how radio signals are created?
I just started a new project to make a radio signal generator. This will be a DIY piece of computer-controlled test equipment. Most people take radio signals for granted, but have no idea how they are created.
Radio signals are all around us. AM, FM and television broadcasting use radio signals, as do mobile phones. Your microwave oven uses radio signals to cook your food. You computers and tablets use digital clocks to control their timing, and these clocks emit radio signals. Nature also creates radio signals such as from lightning and distant quasars. In fact, our universe is surrounded by radio signals emitted shortly after the Big Bang. These are referred to as cosmic microwave background.
Radio signals are a form of invisible electromagnetic radiation. Light is the visible form. Our modern understanding of radio signals began with James Clerk Maxwell. His accomplishment during the 19th century was to formulate the theory of electromagnetic radiation. This unification of light and electrical phenomena led to the understanding of radio waves and how they work. Maxwell’s efforts were just as important as was Newton’s work in physics. Later, during the 20th century, we came to understand that the electromagnetic force is one of the four fundamental forces in the universe.
Radio signals are actually easy to create
Man made radio signals are created by passing electricity through a tuned circuit. Traditionally, tuned circuits have used a combination of capacitors and coils, also called inductors. (See top left.) A capacitor is an device that can store an electrical field. Capacitors are usually made of opposing metal plates positioned near each other, but not touching. If you apply positive voltage to one plate and a negative voltage to the other plate, the capacitor will build up an electrical charge. On the other hand, an inductor or coil is a device that can store a magnetic field. When you pass an electric current through an inductor, it temporarily stores energy in a magnetic field.
By selecting the appropriate values of inductance and capacitance, you create a tuned or resonant circuit. Resonance leads to oscillation, where voltage and current swing back and forth, kind of like a pendulum. The electrical and magnetic fields support each other. With the addition of a few other components for regulation and feedback, this simple arrangement of a coil, capacitor and voltage can produce a stable periodic vibration – a radio signal.
When attached to an antenna, this periodic signal (in its basic form, a sine wave) creates a dance between the increasing and decreasing electrical and magnetic fields, which push out from the antenna and radiate into space.
As an aside, trigonometry is very important for radio work. The sine and cosine curves describe how energy passes through capacitors and coils. This, in turn, is why were refer to basic radio signals as “sine waves”. If you want to do any sort of electronic engineering, don’t skip your trig classes!
If you look inside an older radio or television, you will find a coil and a capacitor, similar to that shown above. The variable capacitor is connected to your tuning knob. As you turn the knob, you change the capacitance, which changes the tuned frequency. Because radio signals are based on physics, most of these circuits work forwards and backwards, i.e. for transmitting and receiving.
However, if you look inside a newer radio or television, you may not see a traditional tuned circuit. Instead, you may see a direct digital synthesizer, or DDS. Since basic radio signals are sine waves, a DDS stores a sine-cosine lookup table in memory. When you send a digital command to the chip to produce a certain frequency, the computer on the chip basically just looks up a sine wave of a certain frequency in memory, and produces it.
All modern electronic devices use crystals, which provide a tuned circuit for one frequency. You will find a crystal to keep time inside almost all of your computers. They are also used as a reference frequency for direct digital synthesis. Shorty, I will describe my project to create radio signals with a DDS signal generator.