Signal bandwidth
One key element of any signal is the bandwidth it occupies. This is important because it defines the channel bandwidth required, and hence the number of channels that can be accommodated within a given segment of radio spectrum. With pressure on the radio spectrum increasing, the radio signal bandwidth is an important feature of any type of radio emission or transmission.
The bandwidth is governed by two major features:
- The type of modulation: Some forms of modulation use their bandwidth
more effectively than others. Accordingly, where spectrum usage is of importance, this alone may dictate the choice of modulation.
- The bandwidth of the modulating signal: A law called Shannon's law
determines the minimum bandwidth through which a signal can be transmitted. In general, the wider the bandwidth of the modulating signal, the wider the bandwidth required.
Modulating signal type
Apart from the form of modulation itself, the type of signal being used to modulate the carrier also has a bearing on the signal. Analogue and data are two very different forms of modulating signal and need to be treated differently. While different formats of actual modulation may be used, the type of signal being applied via the modulator also have a bearing on the signal.
Signals for high quality stereo broadcasting will be treated differently to signals that provide digital telemetry for example. As a result, it is often important to know the signal type that needs to be carried by the RF carrier.
By Ian Poole
Unit 2 Radio transmitters and receivers
Memorize the words: carrier wave – несущая волна
sine - синус
refer – отсылать; направлять; справляться; упоминать
slight – тонкий; хрупкий; легкий; незначительный
power supply – источник энергии
to amplify – усиливать
omnidirectional – действующий по всем направлениям
transverse – поперечный
conversely – наоборот
to exert – осуществлять; оказывать
back and forth – назад и вперед
Read the text and speak about the principle of operation of a transmitter:
Radio transmitters
A radio transmitter consists of several elements that work together to generate radio waves that contain useful information such as audio, video, or digital data.
- Power supply: provides the necessary electrical power to operate the
transmitter.
- Oscillator: creates alternating current at the frequency on which the
transmitter will transmit. The oscillator usually generates a sine wave, which is referred to as a carrier wave.
- Modulator: adds useful information to the carrier wave. There are two main
ways to add this information. The first, called amplitude modulation or AM, makes slight increases or decreases to the intensity of the carrier wave. The second, called frequency modulation or FM, makes slight increases or decreases to the frequency of the carrier wave.
- Amplifier: amplifies the modulated carrier wave to increase its power. The
more powerful the amplifier, the more powerful the broadcast.
- Antenna: converts the amplified signal to radio waves.
Read the text and define the role of antennas in radio communication: