Slide 1

Standard Amplitude Modulation (AM)nalhareqi-20131Standard Amplitude Modulation (AM)nalhareqi-20132In the DSP-SC demodulation, a receiver must generate a local carrier in frequency and phase synchronism with the carrier used for modulation.This call for a sophisticated receiver and could be quit costly.The other alternative is for the transmitter to transmit a carrier Ac cos (ct) along with the modulated signal m(t) cos (ct). so there is no need to generate a carrier at the receiver and this is the idea of the standard AM.Modulationnalhareqi-20133The modulating signal (information or baseband signal)

The carrier signal is

The modulated signal

Modulationnalhareqi-20134

Modulation case 1nalhareqi-20135When ( is nonnegative) for all value of t.The envelop has the same shape as m(t).Hence, we can recover m(t) from this envelop.At the receiver, the detection is extremely simple and inexpensive operation, which doesnt require generation of a local carrier for demodulation.

Modulation case 2nalhareqi-20136The condition is not satisfied. The envelop shape is not as m(t).So, we cant recover m(t) from this envelop.We cant build a simple receiver.

Modulation Indexnalhareqi-20137Let Am the peak amplitude of m(t) and Ac is the carrier amplitude.We define modulation index as:

When : this mean that for all t and the SAM(t) (modulated signal) can be demodulated by the envelop detector. When (overmodulation): this mean that A + m(t) is not 0 for all t and the option of envelop detection is no longer viable.

7Spectrumnalhareqi-20138The spectrum of the modulated signal SAM(t) is the same as that of m(t) cos (ct) plus tow additional impulses at + c and - c .If

then

Spectrumnalhareqi-20139

USBLSBSpectrumnalhareqi-201310The modulated signal spectrum centered at c is composed of three parts:An impulse at the carrier frequency c The upper sideband (USB), a portion that lies above c whose highest frequency component is at c + M The lower sideband (LSB), a portion that lies below c whose lowest frequency component is at c - MThe bandwidth of the modulated waveform is twice the information signal bandwidth.

Transmission Powernalhareqi-201311The advantage of envelop detection in the standard AM has its price.Recall, the transmission power and the channel bandwidth are the two primary communication resources and should be used efficiently.The transmission of the carrier wave represent a waste of power.In the standard AM, only a fraction of the total transmitted power is actually for m(t). Demodulationnalhareqi-201312In the standard AM modulation, the modulation index should be in order to demodulate the received modulated signal by using an envelop detector.

Single-Sideband (SSB) Modulationnalhareqi-201313Introductionnalhareqi-201314Standard AM and DSB-SC modulations has two sidebands: LSB and USB. These two methods waste the channel bandwidth because they both require a transmission bandwidth equal to twice the message bandwidth.

Introductionnalhareqi-201315Note that the USB and the LSB are symmetric about the carrier frequency.Hence, given the spectra of either sideband, we can determine the other.So, the transmission of either sideband is sufficient to reconstruct the message signal m(t) at the receiver.Thus, the bandwidth of the transmitted signal ( modulated signal) will be the bandwidth of the modulating signal ( baseband signal)Single-Sideband (SSB) Modulationnalhareqi-201316In single-sideband (SSB)modulation just only one sideband is transmitted.

Generation of SSB Signalsnalhareqi-201317One way to generate an SSB signal is to:generate a DSB signal first, and then suppress one of its sidebands by filtering ( band-pass filter designed to pass one of the sidebands of the modulated signal)

Spectrum of SSB Signalsnalhareqi-201318

The filter must have sharp cutoff characteristics to eliminate the undesired sidebandDemodulationnalhareqi-201319Demodulation of SSB signals can be achieved easily by using the coherent detector as used in the DSB demodulation, that is, by multiplyingby a local carrier and passing the resulting signal through a low-pass filter.

Demodulationnalhareqi-201320

Vestigial-Sideband (VSB) modulationnalhareqi-201321Vestigial-Sideband (VSB) modulationnalhareqi-201322Vestigial-Sideband (VSB) modulation is a compromise between SSB and DSB modulations. In this modulation scheme, one sideband is passed almost completely, whereas just a trace, or vestige, of the other sideband is retained.The typical bandwidth required to transmit a VSB signal is about 1.25 that of SSB. VSB is used for transmission of the video signal in commercial television broadcasting.Generation of VSB Signalsnalhareqi-201323A VSB signal can be generated by passing a DSB signal through a sideband-shaping filter [VSB filter].

nalhareqi-201324

Demodulation of VSBnalhareqi-201325The m(t) can be recovered by synchronous or coherent demodulation , that is, by multiplyingby a local carrier and passing the resulting signal through a low-pass filter.

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