Basic block diagram of communication system:
Figure 1. Block diagram of Communication System
Input transducer: The device that converts a physical signal from source to an electrical, mechanical or electromagnetic signal more suitable for communicating
- Transmitter: The device that sends the transducer signal
- Transmission channel: The physical medium on which the signaln is carried
- Receiver: The device that recovers the transmitted signal fromn the channel
- Output transducer: The device that converts the received signal back into a useful quantity
Analog modulation:
- The purpose of a communication system is to transmit information signals (baseband signals) through a communication channel The term baseband is used to designate the band of frequenciesn representing the original signal as delivered by the input transducer
- For example, the voice signal from a microphone is a baseband signal,n and contains frequencies in the range of 0-3000 Hz
- The “hello” wave is a baseband signal
Figure 2. Hello Wave
- Since this baseband signal must be transmitted through a communication channel such as air using electromagnetic waves, an appropriate procedure is needed to shift the range of baseband frequencies to other frequency ranges suitable for transmission, and a corresponding shift back to the original frequency range after reception. This is called the process of modulation and demodulation.
- Remember the radio spectrum
Figure 3.Radio Spectrum
- For example, an AM radio system transmits electromagnetic waves with frequencies of around a few hundred kHz (MF band).The FM radio system must operate with frequencies in the range of 88- 108 MHz (VHF band).
- Since the baseband signal contains frequencies in the audio frequency range (3 kHz), some form of frequency-band shifting must be employed for the radio system to operate satisfactorily.
- This process is accomplished by a device called a modulator.
- The transmitter block in any communications system contains the modulator device
- The receiver block in any communications system contains the demodulator device
- The modulator modulates a carrier wave (the electromagnetic wave) which has a frequency that is selected from an appropriate band in the radio spectrum
- For example, the frequency of a carrier wave for FM can be chosen from the VHF band of the radio spectrum
- For AM, the frequency of the carrier wave may be chosen to be around a few hundred kHz (from the MF band of the radio spectrum) The demodulator extracts the original baseband signal from the receivedn modulated signal
- To Summarize: Modulation is the process of impressing a low-frequency information signal (baseband signal )onto a higher frequency carrier signal Modulation is done to bring information signals up to the Radio Frequency (or higher) signal.
Basic analog communication system:
Figure 4. Basic Analog Communication System
Types of analog modulation:
- Amplitude Modulation (AM)
Amplitude modulation is the process of varying the amplituden of a carrier wave in proportion to the amplitude of a baseband signal. The frequency of the carrier
Figure 5.Amplitude Modulation
remains constant
- Frequency Modulation (FM)
Frequency modulation is the process of varying the frequencyn of a carrier wave in proportion to the amplitude of a baseband signal. The amplitude of the carrier remains constant
Figure 6.Frequency Modulation
- Phase Modulation (PM)
Phase modulation is the one in which the instantaneous phase in the carrier signal is varied in accordance with the instantaneous amplitude of the modulating signal. The amplitude and the frequency of the carrier signal remains unaltered after PM.
AM VS FM:
- AM requires a simple circuit, and is very easy to generate.
- It is simple to tune, and is used in almost all short wave broadcasting.
- The area of coverage of AM is greater than FM (longer wavelengths (lower frequencies) are utilized-remember property of HF waves)
- However, it is quite inefficient, and is susceptible to static and othern forms of electrical noise.
- The main advantage of FM is its audio quality and immunity to noise.
- Most forms of static and electrical noises are naturally AM, and an FM receiver will not respond to AM signals.
- The audio quality of a FM signal increases as the frequency deviation increases (deviation from the center frequency), which is why FM broadcast stations use such large deviation.
- The main disadvantage of FM is the larger bandwidth it requires.