AM, FM and Pulse Modulation
- [Paris, France - Civil Engineering Discoveries]
- Eelectromagnetic (EM) Waves
When you listen to the radio, watch TV, or cook dinner in a microwave oven, you are using electromagnetic (EM) waves. Radio waves, television waves, and microwaves are all types of EM waves. They only differ from each other in wavelength. Wavelength is the distance between one wave crest to the next. Waves in the EM spectrum vary in size from very long radio waves the size of buildings, to very short gamma-rays smaller than the size of the nucleus of an atom.
EM waves can be described by their wavelengths, energy, and frequency. All three of these things describe a different property of light, yet they are related to each other mathematically. This means that it is correct to talk about the energy of an X-ray or the wavelength of a microwave or the frequency of a radio wave.
In fact, X-rays and gamma-rays are usually described in terms of energy, optical and infrared light in terms of wavelength, and radio in terms of frequency. This is a scientific convention that allows the use of the units that are the most convenient for describing whatever energy of light you are looking at. After all - there is a huge difference in energy between radio waves and gamma-rays. Here's an example. Electron-volts, or eV, are a unit of energy often used to describe light in astronomy. A radio wave can have an energy of around 4 x 10-10 eV - a gamma-ray can have an energy of 4 x 109 eV. That's an energy difference of 1019 (or ten million trillion) eV!
We already know that when we talk about wavelength, we are talking about the distance between two peaks of a wave. Wavelength is usually measured in meters (m). Frequency is the number of cycles of a wave to pass some point in a second. The units of frequency are thus cycles per second, or Hertz (Hz). Radio stations have frequencies. They are usually equal to the station number times 1,000,000 Hz. For instance, - the local Washington, DC station HFS has a frequency of 99.1 million Hz in the FM radio band.
- AM Radio and FM Radio
Radio works by transmitting and receiving electromagnetic waves. The radio signal is an electronic current moving back and forth very quickly. A transmitter radiates this field outward via an antenna; a receiver then picks up the field and translates it to the sounds heard through the radio.
in FM (Frequency Modulation), a radio wave is known as the "carrier" or "carrier wave" is modulated in frequency by the signal that is to be transmitted. The amplitude and phase remain constant.
AM (Amplitude Modulation) is a technique used in electronic communication, most commonly for transmitting information through a radio carrier wave. It works by continuously changing the strength of the transmitted signal in relation to the information being sent. For example, The changes in the strength of the signal may be utilized to specify the sounds to be reproduced by the speaker, or the light intensity of television pixels.
In AM radio, the strength (amplitude) of the signal is changed (modulated) to make the sounds. In FM (Frequency Modulation) radio, it is the speed (frequency) of the signal that is changed. When you tune in your radio, the dial number indicates the kilo or megaHertz at which the signal is being broadcast.
Radio signal uses specific frequency, or how quickly the waves of the field move up and down. Hertz is a measurement of the number of wave cycles per second - AM is expressed in kiloHertz, while FM radio is expressed in megaHertz. The station power affects the range of the signal, or how far it can travel. The strongest AM power allowed in the United States is 50,000 watts.
- Digital Pulse Modulation
- Electromagnetic Theory
Electricity and magnetism were once thought to be separate forces. However, in 1873, Scottish physicist James Clerk Maxwell developed a unified theory of electromagnetism. The study of electromagnetism deals with how electrically charged particles interact with each other and with magnetic fields.
There are four main electromagnetic interactions:
- The force of attraction or repulsion between electric charges is inversely proportional to the square of the distance between them.
- Magnetic poles come in pairs that attract and repel each other, much as electric charges do.
- An electric current in a wire produces a magnetic field whose direction depends on the direction of the current.
- A moving electric field produces a magnetic field, and vice versa.
Maxwell also developed a set of formulas, called Maxwell's equations, to describe these phenomena.
- Radio Waves and Fields
Understanding any type of radio is impossible without also having a general understanding of the purpose of radio: to send and receive information by using radio waves. Radio waves are just another form of light that travels at the same speed; 186,000 miles per second. Radio waves can get to the Moon and back in 2 ½ seconds or circle the Earth in 1/7 second.
The energy in a radio wave is partly electric and partly magnetic, appearing as an electric field and a magnetic field wherever the wave travels. (A field is just energy stored in space in one form or another, like a gravitational field that you experience as weight.). These fields make charged particles - such as the electrons in a wire - move in sync with the radio wave.
These moving electrons are a current, just like in an AC power cord except that they form a radio current that your radio receiver turns into, say, audible speech. This process works in reverse to create radio waves. Transmitters cause electrons to move so that they, in turn, create the radio waves.
Antennas are just structures in which the electrons move to create and launch radio waves into space. The electrons in an antenna also move in response to radio waves from other antennas. In this way, energy is transferred from moving electrons at one station to radio waves and back to moving electrons at the other station.
- Animation of A Half-Wave Radio Dipole
Animation of a half-wave dipole antenna radiating radio waves, showing the electric field lines. The antenna in the center is two vertical metal rods connected to a radio transmitter (not shown). The transmitter applies an alternating electric current to the rods, which charges them alternately positive (+) and negative (−). Loops of electric field leave the antenna and travel away at the speed of light; these are the radio waves. In this animation the action is shown slowed down enormously.
[More to come ...]
