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4-1 YEDITEPE UNIVERSITY ENGINEERING FACULTY COMMUNICATION SYSTEMS LABORATORY EE 351 – COMMUNICATION SYSTEMS I EXPERIMENT 4: DEMODULATION of DSBSC and SSB Objective: Examining product demodulation schemes of DSBSC and SSB signal. Discussing their practical realization issues. Equipment: ¾ Master Signals Module ¾ Phase Shifter Module ¾ Headphone Amplifier Module ¾ Quadrature Phase Splitter Module ¾ Multiplier Module ¾ VCO Module ¾ Adder Module ¾ Oscilloscope General Information: Demodulation process is applied to received signal at receiver end of a communication system. After modulating a message signal at transmitter, the signal is fed to the transmission channel to carry the information to the receiver end. Due to the non-ideal characteristics of the transmission channel and noise, the received signal will not be exacty the same as the transmitted signal. The message signal must be reconstructed by the help of received signal. One of the most important steps of signal reconstruction process is demodulation at receiver end. Generally speaking, there is an inverse symmetry between modulation steps and demodulation steps. One of the demodulation methods for DSBSC and/or SSB signals is product demodulation. The received (modulated) signal is in the form that the spectrum of the message signal riding on a carrier frequency, f c . A product demodulator locates the original spectrum of the message signal to the baseband by multiplying the received signal with a local oscillator signal whose frequency is either equal to the carrier frequency ( synchronous or coherent demodulation ) or not ( asynchronous or incoherent demodulation ). In this experiment, two different types of synchronous demodulation is examined: one is the synchronous product demodulator shown in Fig 4.1, and the other is the phasing type SSB demodulator shown as the ideal in Fig 4.2 and as the practical in Fig 4.3 . Asynchronous demodulation also for DSBSC and SSB is examined as shown in Fig 4.4 .
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