"Modulation is the process of superimposing a low frequency signal on a high frequency carrier signal."
"The process of modulation can be defined as varying the RF carrier wave in accordance with the intelligence or information in a low frequency signal."
"Modulation is defined as the precess by which some characteristics, usually amplitude, frequency or phase, of a carrier is varied in accordance with instantaneous value of some other voltage, called the modulating voltage."
The term carrier is applied to the voltage whose characteristic is varied and the term modulating voltage (signal) is used for the voltage in accordance with which the variation is made.
The modulation is the process of up shifting the message frequency to a range more useful for transmission.
Need For Modulation
- If two musical programs were played at the same time within distance, it would be difficult for anyone to listen to one source and not hear the second source. Since all musical sounds have approximately the same frequency range, form about 50 Hz to 10KHz. If a desired program is shifted up to a band of frequencies between 100KHz and 110KHz, and the second program shifted up to the band between 120KHz and 130KHz, Then both programs gave still 10KHz bandwidth and the listener can (by band selection) retrieve the program of his own choice. The receiver would down shift only the selected band of frequencies to a suitable range of 50Hz to 10KHz.
- A second more technical reason to shift the message signal to a higher frequency is related to antenna size. It is to be noted that the antenna size is inversely proportional to the frequency to be radiated. This is 75 meters at 1 MHz but at 15KHz it has increased to 5000 meters (or just over 16,000 feet) a vertical antenna of this size is impossible.
- The third reason for modulating a high frequency carrier is that RF (radio frequency) energy will travel a great distance than the same amount of energy transmitted as sound power.
Types of Modulation
The carrier signal is a sine wave at the carrier frequency. Below equation shows that the sine wave has three characteristics that can be altered.
Instantaneous voltage (E) =Ec(max)Sin(2πfct + θ)
The term that may be varied are the carrier voltage Ec, the carrier frequency fc, and the carrier phase angle θ. So three forms of modulations are possible.
Amplitude modulation is an increase or decrease of the carrier voltage (Ec), will all other factors remaining constant. Read More
Frequency modulation is a change in the carrier frequency (fc) with all other factors remaining constant. Read More
Phase modulation is a change in the carrier phase angle (θ ). The phase angle cannot change without also affecting a change in frequency. Therefore, phase modulation is in reality a second form of frequency modulation. Read More