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PRINCIPLES OF COMMUNICATION SYSTEMS�BEC402

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Introduction to Communication System

  • Communication is the process of conveying or transferring information from one point to another
  • It is the process of establishing connection or link between two points for information exchange
  • Communication is a process by which the information/message is transmitted from one point to another, from one person to another, or from one place to another in the form of electrical signals, through some communication link.

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Introduction to Communication System

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MODULATION

  • It is the process of varying the characteristics of high frequency carrier in accordance with instantaneous values of modulating or message or baseband signal.

or

  • It is a frequency translation technique which converts baseband or low frequency signal to bandpass or high frequency signal.
  • Modulation is used in the transmitter

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NEED FOR MODULATION

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  • To reduce the height of antenna
  • Avoids mixing of signals
  • Allows multiplexing
  • Reduction in cost
  • Increases range of communication
  • Reduces noise and interference
  • Improves quality of reception

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Amplitude Modulation: Amplitude of the carrier is varied in accordance with the instantaneous values of modulating signal

Frequency Modulation: Frequency of the carrier is varied in accordance with the instantaneous values of modulating signal

Phase Modulation: Phase of the carrier is varied in accordance with the instantaneous values of modulating signal

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AMPLITUDE MODULATION

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  • Amplitude of the carrier signal is varied in accordance with the instantaneous values of modulating signal (message signal) where the frequency and phase of the carrier wave remains constant.
  • An increase or decrease in the amplitude of the modulating signal causes a corresponding increase or decrease in both the positive and the negative peaks of the carrier amplitude.
  • An imaginary line connecting the positive peaks and negative peaks of the carrier waveform gives the exact shape of the modulating information signal. This imaginary line on the carrier waveform is known as the envelope.

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  • The amplitude of the modulating signal should be less than the amplitude of the carrier.

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  • When the amplitude of the modulating signal is greater than the amplitude of the carrier, distortion will occur, causing incorrect information to be transmitted.

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  • One of the simplest amplitude modulators is the diode modulator.
  • Diode Modulator consists of a resistive mixing network, a diode rectifier, and an LC tuned circuit.
  • The carrier is applied to input resistor R2 and the modulating signal to the other Resistor R1.
  • The current through the diode is a series of positive-going pulses whose amplitude varies in proportion to the amplitude of the modulating signal as shown in figure.
  • These positive-going pulses are applied to the parallel-tuned circuit made up of L and C, which are resonant at the carrier frequency. Each time the diode conducts, a pulse of current flows through the tuned circuit.
  • The coil and capacitor repeatedly exchange energy, causing an oscillation, or “ringing,” at the resonant frequency.
  • The oscillation of the tuned circuit creates one negative half-cycle for every positive input pulse.
  • High amplitude positive pulses causes the tuned circuit to produce high- amplitude negative pulses. Low-amplitude positive pulses produce corresponding low-amplitude negative pulses. The resulting waveform across the tuned circuit is an AM signal

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  • Transistor Modulator consists of a resistive mixing network, a transistor, and a LC tuned circuit.
  • The carrier is applied to one of the input resistor and the modulating signal to the other Resistor.
  • The mixed signal appears across the third resistor. This network causes the two signals to be linearly mixed, i.e., algebraically added.
  • The mixed signal will be a input to transistor.
  • The base current controls a larger collector current, and therefore the circuit amplifies.
  • Rectification occurs because of the emitter-base junction. This causes larger half-sine

pulses of current in the tuned circuit.

  • The oscillation of the tuned circuit creates one negative half-cycle for every positive

input pulse.

  • High amplitude positive pulses cause the tuned circuit to produce high- amplitude

negative pulses. Low-amplitude positive pulses produce corresponding low-amplitude

negative pulses. The resulting waveform across the tuned circuit is an AM signal.

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High-Level AM: Collector Modulator.

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