You would have now realised that in order to transmit radio waves over long distances there is a need to use higher frequencies with lesser wavelength. However, the signal waves like audio or speech, have lower frequencies and higher wavelength. Therefore, we need to superimpose the signal wave on the carrier wave for transmission over long distances. Modulation is the process of super imposing intelligence available on the signal wave on to the carrier wave. Since, carrier wave is a high frequency wave containing high energy levels the modulated wave can travel longer distances.
Amplitude and frequency of a carrier wave remains constant since they do not carry any intelligence. In case you are not aware, the change in amplitude or frequency is what makes a sound carry a meaning. Such variation is available in the signal wave. Signal wave is a low energy wave containing voice or data intelligence. The simplest type of modulation is called the Keyed Modulation. Intelligence is superimposed on carrier wave by just switching on and off the carrier wave. Morse code is an example of a basic keyed modulation.
Amplitude modulation is one of the types of modulation commonly used in AM Radio. Amplitude Modulation the process of modifying amplitude of carrier wave. In amplitude modulation, the carrier wave is modulated using the amplitude of the signal wave.
Depth of Modulation of a Modulated waveform
At time we feel a difference in the clarity of audio reception. Depth of Modulation indicates the clarity of modulation and thereby the clarity of audio. It is measured as a difference in depth of Modulation (DDM). The difference in depth of Modulation is the ratio between amplitudes of modulated and unmodulated carrier wave. DDM is calculated from minimum and maximum amplitudes given by the formula.
Depth of Modulation = (Max – Min) / (Max + Min) x 100
Frequency Modulation of a Wave
Frequency modulation is the process of modifying the frequency of carrier wave. Frequency of carrier wave is modulated using the amplitude of the signal wave. Frequency Modulated wave requires a wider band width as compared to Amplitude Modulated wave. FM transmitters are simpler and use less power than AM transmitters. However, FM Receivers are more complex than AM receivers. Frequency modulation ensures better signal clarity due to lesser static noise.
Phase Modulation of a Wave
Phase Modulation is the process of modifying the phase of the carrier wave. Amplitude of signal wave is used to modify the phase of carrier wave. There are two types of phase modulation called the analogue type or digital type. Analogue type modifies phase of carrier by short time frequency changes.
Phase Shift Keying
Digital phase modulation type is also called Phase shift keying. Digital type modifies phase of carrier using binary numbers. Binary numbers are used to represent each of the 360 degree phase shift. Digital phase modulation has superior noise reduction and reduce fading. Global Positioning System (GPS) and Microwave Landing Systems (MLS) use Phase Shift Keying technology.
Pulse Modulation of a Wave
Pulse modulation is the process of transmitting carrier in short pulses. Actually, it is almost like keyed modulation, where you just switch on and switch off the carrier wave. Radar and Data link systems use pulse modulation for their operation. In the case of pulse modulation, the transmitted pulses are of very short duration and are measured accurately. Duration of pulses are measured in microseconds (109 seconds). Intervals between pulses are measured in milliseconds (106 seconds)