Modulation
- Transmission of data for short ranges does not need any complex procedures.
- Example an audio frequency of range 20 to 20,000 Hz when transmitted over long distances some techniques are used to transmit the information without any loss.
- “For long-distance broadcast, the low-frequency baseband signal (input signal) is superimposed onto a high-frequency radio signal by a process called modulation”. (Tamil Nadu Text Book Corporation #)
- Carrier signal or radio signal which is a high-frequency signal is employed to carry the baseband signal in the modulation process.
- Carrier signal’s frequency is very high as a result it can be transmitted very long with less attenuation.
- The carrier signal is mostly a sine wave.
- In the Modulation process, three characteristics of the carrier signal are modified by the baseband signal.
- Frequency of radio waves and its characteristics are mentioned below:
- Those three characteristics of radio waves or signals are Amplitude, Phase, Frequency.
Amplitude Modulation (AM)
- “If the amplitude of the carrier signal is modified according to the instantaneous amplitude of the baseband signal, then it is called amplitude modulation.” (Tamil Nadu Text Book Corporation #).
- In AM, the phase and frequency are constant and amplitude is modified.
- AM is used in Radio transmission and also in TV broadcasting.
Advantages of AM
- Transmission and Reception is easy
- Requires lesser bandwidth
- The operating cost is less.
Disadvantages of AM
- High noise level
- The operating range is small
- Low efficiency
Frequency Modulation(FM)
- In FM, the frequency of the carrier signal is modified as per the instantaneous amplitude of the baseband signal.
- In FM the amplitude and phase of the carrier signal remain constant.
- When the baseband amplitude has increased the frequency of the carrier also gets increase and vice versa.
Advantages of FM
- The noise level is decreased and increases in signal-noise ratio.
- The operating range is high.
- As all the transmitted power is used, the efficiency of the transmission is high.
- FM reproduces better audio quality as the FM utilizes the entire frequencies in the human audible range.
Disadvantages of FM
- A wider channel is required for FM
- The transmitter and receivers of FM are expensive
- The area coverage is less compared to AM
Phase Modulation
- “The instantaneous amplitude of the baseband signal modifies the phase of the carrier signal keeping the amplitude and frequency constant is called Phase Modulation”. (Tamil Nadu Text Book Corporation #).
- The phase modulation is similar to FM, except the phase of the carrier signal is modified.
- The frequency shift in carrier wave frequency subsists in phase modulation as well.
- The frequency shift depends on the amplitude of the modulating signal and the frequency of the signal.
Advantages of PM
- FM signal produced from PM signal which is very stable.
- It is extremely stable.
- Phase modulation provides high transmission speed on a given bandwidth.
