Standard Amplitude Modulation AM 1 Standard Amplitude Modulation

Standard Amplitude Modulation (AM) 1

Standard Amplitude Modulation (AM) � In 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 (ωct) along with the modulated signal m(t) cos (ωct). so there is no need to generate a carrier at the receiver and this is the idea of the standard AM. 2

Modulation �The modulating signal (information or baseband signal) �The carrier signal is �The modulated signal 3

Modulation 4

Modulation – case 1 When ( is nonnegative) for all value of t. � The envelop has the same 5 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 doesn’t require generation of a local carrier for demodulation.

Modulation – case 2 The condition is not satisfied. �The envelop shape is not as m(t). �So, we can’t recover m(t) from this envelop. �We can’t build a simple receiver. 6

Modulation Index � Let 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. 7

Spectrum �The spectrum of the modulated signal SAM(t) is the same as that of m(t) cos (ωct) plus tow additional impulses at + ωc and - ωc. �If then 8

Spectrum LSB 9 USB

Spectrum �The modulated signal spectrum centered at ωc is composed of three parts: �An impulse at the carrier frequency ωc �The upper sideband (USB), a portion that lies above ωc whose highest frequency component is at ωc + ωM � The lower sideband (LSB), a portion that lies below ωc whose lowest frequency component is at ωc - ωM � The bandwidth of the modulated waveform is twice the information signal bandwidth. 10

Transmission Power �The 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). 11

Demodulation �In the standard AM modulation, the modulation index should be in order to demodulate the received modulated signal by using an envelop detector. 12

Single-Sideband (SSB) Modulation 13

Introduction �Standard 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. 14

Introduction �Note 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) 15

Single-Sideband (SSB) Modulation �In single-sideband (SSB) modulation just only one sideband is transmitted. 16

Generation of SSB Signals �One 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) 17

Spectrum of SSB Signals e filter must have sharp cutoff characteristics to eliminate the undesired sideband 18

Demodulation �Demodulation of SSB signals can be achieved easily by using the coherent detector as used in the DSB demodulation, that is, by multiplying by a local carrier and passing the resulting signal through a low-pass filter. 19

Demodulation 20

Vestigial-Sideband (VSB) modulation 21

Vestigial-Sideband (VSB) modulation �Vestigial-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. 22

Generation of VSB Signals �A VSB signal can be generated by passing a DSB signal through a sideband-shaping filter [VSB filter]. 23

24

Demodulation of VSB �The m(t) can be recovered by synchronous or coherent demodulation , that is, by multiplying by a local carrier and passing the resulting signal through a low-pass filter. 25

26