Signal can be modulated in terms of:
(a) Amplitude Modulation (AM). When the amplitude of high frequency carrier wave is changed in accordance with the intensity of signal, it is called amplitude modulation. In amplitude modulation only the amplitude of the carrier wave is changed in accordance with intensity of signal However, the frequency of modulated wave is remains the same. i.e carrier frequency. Fig shows the principle of amplitude modulation. Fig. ‘b’ shows the audio electrical signal whereas fig ‘a’ shows carrier wave constant amplitude. fig ‘c’ shows amplitude modulated wave form(AM).Note that the amplitude of both positive and negative half cycles of carrier wave are changed in accordance with the signal. For instance, when the signal is increasing in the positive sense, the amplitude of carried wave also increases. On the other hand, during negative half cycle of the signal, the amplitude of carrier wave decreases. An electronic circuit called modulator does amplitude modulation.
(b) Frequency Modulation (FM). The type of modulation in which frequency (number of alternations per second) carrier is varied with amplitude of modulating signal (speech). It is used in Television Broadcasting System, Satellite Communication System etc.
(c) Phase Modulation (PM). The type of modulation in which a 90° phase shifted carrier is added in amplitude modulated wave. It is used in Satellite communication, Fibre optics transmission etc.
Likewise detection can also be in terms of amplitude, frequency or phase modulation. Amplitude modulation has the advantage of simplicity of equipment where as frequency modulation and phase modulation has advantage of less noise, better clarity but range is comparatively low.
Types of Radiated Wave
Continuous Wave (CW). It consists of radiated waves during the time when key is pressed and no radiation during the time the key is not pressed.
Modulated Continuous Wave (MCW). It consists of changing the amplitude of radiated wave during the periods of information, i.e., when the key is pressed.
Radio Telephony (RT). It is the form of MCW, in which the variation of waves amplitude is due to human speech.
Frequency of radio system ranges in a wide spectrum. It is used in various fields. These are tabulated in the following page:
|VHF||03 – 30 K Hz||Long distance point to point communication.|
|Low Frequency||30 – 300 K Hz||Marine, Navigational Aids|
|Medium Frequency||300 – 3000 K Hz||Broadcasting, Marine|
|High Frequency||3000 – 30000 K Hz||Communication of all types|
|VHF||30 – 300 M Hz||TV, FM Broadcasting, Radar, Air Navigation and aviation communication|
|UHF||300 – 3000 M Hz||Radar, Microwave relays, short distance communication|
|Super High Frequency||3000 – 30000 M Hz||Radar, Radio relays, Navigation, Experiments|
|Extremely High Frequency||30000 – 3000000 MHz (30–300 GHz)||Experimental|