Faculty of Engineering Electrical Engineering Department Communication Engineering I Lab (EELE 3170) Eng. Adam M. Hammad

Faculty of Engineering Electrical Engineering Department Communication Engineering I Lab (EELE 3170) Eng. Adam M. Hammad

EXPERIMENT #5 DSB-SC AND SSB MODULATOR

Theory The amplitude-modulated signal is simple to produce but has two practical drawbacks: The bandwidth of AM signal is twice of modulating signal Most of the power is transmitted in the carrier, not in the information bearing sidebands. Since the AM signal is hidden the double sidebands and the carrier signal does not contain any information, We can suppress the carrier. By doing carrier suppression 67% of transmission power can be saved

Double Side Band Suppressed Carrier DSB-SC is a kind of amplitude modulation in which the carrier frequency component is absent. Amplitude modulation called DSB-FC (DSB-Full Carrier) DSB-SC signal can be generated using: Balanced modulator (MC1496 ) Ring modulator Used in Television Broadcasting Disadvantages of DSBSC: Less information about the carrier will be delivered to the receiver, so there is an increase of complexity Needs a coherent carrier detector at receiver

Single Side Band Suppressed Carrier Both of two side bands are exactly one image of the each other Single sideband contains entire information of the signal. SSB signal is produced by passing the DSB signal through a highly selective band pass filter. This filter selects either the upper or the lower sideband. Transmission bandwidth can be cut by half, if one sideband is entirely suppressed but there is a considerable increase in equipment complexity. Applications 1. SSB transmission is used in applications where the Power saving. 2. SSB transmission is also used in applications in which bandwidth requirements are low

AM Modulation The first term represents the double sideband signals The second term represents the carrier signal.

Different Frequency Spectrums of AM Modulation.

DSB-SC and SSB Modulation

Spectrum of DSB-SC and SSB Each sideband pair {whether upper or lower} contains the complete information or the original message.

Implementation of DSB-SC Modulator We utilize balanced modulator MC1496 to design the DSB-SC modulated signal Balanced modulator is used for generation double side band suppress carrier signal and SSB signals. A balanced modulator has two inputs: 1. Single-frequency carrier 2. Modulating signal. For the modulator to operate properly, the amplitude of the carrier must be sufficiently greater than the amplitude of the modulating signal (approximately six to seven times greater). Modulation index is smaller or equal to 1, the larger modulation index, the better transmission efficiency. 85 %

Circuit Diagram of DSB-SC Modulation By Utilizing MC1496

Continued.. The carrier signal is inputted from pin 10 and the audio signal is inputted from pin 1. R8 determine the gain of the whole circuit R9 determine the magnitude of bias current. By adjusting VR1 or changing input amplitude of audio signal, we can control the percentage modulation of amplitude modulation, which means we can adjust the output become the DSB-SC By adjusting VR2, we can control the magnitude of the output amplitude, which is also the gain.

Continued.. DSB-SC signal in blue, original message shown in red

Implementation of SSB Modulation SSB modulator is combination of two DSB-SC modulators. SSB Generation Methods : 1. Phase shift method Phase difference of each audio signal and carrier signal of the two DSB-SC modulators is 90 degrees. 2. Selective filtering method DSB-SC signal passes trough band pass filter.

Block Diagram of SSB Modulator Phase Shift Method Phase shift method

Continued..

Block Diagram of SSB Modulator Selective Filtering Method It is the most common method of generation SSB. The basic idea: Using m(t) to generate DSB-SC (m(t) * cos ωct) DSB-SC goes through a BPF For successful implementation of this method, we must have BW << fc m(t) must have little or no low-frequency content

DSB-SC Modulation Using MATLAB Without Function

Results

DSB-SC Modulation Using MATLAB With Function

Results

Answer these questions Compare AM, DSB-SC and SSB What is the advantage of DSB-SC over AM system? What is the advantage of SSB over DSB-SC system? What are the advantages of suppressing the carrier? How can we obtain a DSB-SC signal? How can we obtain a SSB signal? Explain Phase Shift method for SSB generation.