To transmit signals, it is necessary to convert, encode or modulate the signal into a form of energy that will travel through ...
fibre optics or
Modulation is the modification of a carrier signal to encode the information signal.
Demodulation, sometimes called detection, is the process of extracting the information signal from the modulated carrier.
AM, FM and Digital
The carrier wave is modified in a manner that is proportional to the information signal.
AM - The amplitude of the carrier is modified.
FM - The frequency of the carrier is modified.
There are several digital modulation methods. These achieve the same goal without the disadvantages of analogue AM and FM.
Radio Signals in Free Space
To transmit and receive radio signals, an antenna is needed.
The ideal size for an antenna is half a wavelength long.
Antennas longer than a few hundred metres are too big to be practicable. This sets a lower limit on the range of easily useable frequencies.
Antennas smaller than around a millimetre are also hard to make. This sets an upper limit on the range of frequencies that can be used easily.
Information signals are often at a low frequency. For example Audio covers the range from 20 to 20 000Hz.
At 20 000 Hz, the radio wavelength would equal C / F = 3x108 / 20000 = 15 kilometres. This is much too long to be transmitted using an antenna of a reasonable size. The solution is to use a carrier wave with a much shorter wavelength.
The carrier wave is a radio frequency signal at a frequency chosen so the antenna size is convenient.
The frequency bands are regulated because there is a shortage of bandwidth and there would be chaos without the regulations.
FM broadcast radio uses the 100MHz band. (3 metre wavelength)
This choice was not by chance.
The wavelength is 3 metres and a half wave antenna is 1.5 metres long.
The quarter wave 75cm antenna is just right to mount on a car and work efficiently.
This makes VHF FM car radio convenient.
On its own, the carrier contains no information.
The carrier has to be modified to allow the information signal to be sent. This is called MODULATION.
Baseband and Broadband
Baseband: The information signal is sometimes called baseband. The Plain Old Telephone Service (POTS) used baseband. The voice signals were carried on copper wires without any other processing. The disadvantage was the need for thousands of copper wire connections and frequent times when phone calls could not be made because all the lines were busy.
Broadband: Using multiplexing, many phone calls can be carried on a single cable. Each phone call is carried on a modulated carrier at a different frequency. SImilar techniques are used for digital broadband connections to homes and businesses.
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