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Chapter 5 AM, FM, and Digital Modulated Systems Amplitude Modulation (AM)

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## Chapter 5 AM, FM, and Digital Modulated Systems Amplitude Modulation (AM)

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**Chapter 5**• AM, FM, and Digital Modulated Systems • Amplitude Modulation (AM) • Double Sideband Suppressed carrier (DSSC) • Single sideband signals (SSB) Hüseyin Bilgekul & Erhan Ince Eeng360 Communication Systems I Department of Electrical and Electronic Engineering Eastern Mediterranean University**Bandpass Signaling Review**Where Where • The modulated bandpass signal can be described by Modulation Mapping function:Convert m(t) →g(t) Ref :Table4-1 • The voltage spectrum of the bandpass signal is • The PSD of the bandpass signal is**Amplitude Modulation**• The Complex Envelope of an AM signal is given by Ac indicates the power level of AM and m(t) is the Modulating Signal • Representation of an AM signal is given by • Ac[1+m(t)] In-phase component x(t) • If m(t) has a peak positive values of +1 and a peak negative value of -1 AM signal 100% modulated • Envelope detection can be used if % modulation is less than 100%.**Amplitude Modulation**An Example of a message signal m(t) Waveform for Amplitude modulation of the message signal m(t)**Amplitude Modulation**B An Example of message energy spectral density. Carrier component together with the message 2B Energy spectrum of the AM modulated message signal.**AM – Percentage Modulation**• Definition: The percentage of positive modulation on an AM signal is • The percentage of negative modulation on an AM signal is • The percentage of overall modulation is If m(t) has a peak positive values of +1 and a peak negative value of -1 AM signal 100% modulated**AM Signal Waveform**% Positive modulation= 50% % Negative modulation =50% Overall Modulation = 50% Amax = 1.5Ac Amin = 0.5 Ac**AM – Percentage Modulation**Under modulated (<100%) 100% modulated Over Modulated (>100%) Envelope Detector Can be used Envelope Detector Gives Distorted signal**AM – Normalized Average Power**The normalized average power of the AM signal is If the modulation contains no dc level, then The normalized power of the AM signal is Discrete Carrier Power Sideband power**AM – Modulation Efficiency**• Definition : The Modulation Efficiency is the percentage of the total power of the modulated signal that conveys information. Only “Sideband Components” – Convey information Modulation Efficiency: Highest efficiency for a 100% AM signal : 50% - square wave modulation Normalized Peak Envelope Power (PEP)of the AM signal: Voltage Spectrum of the AM signal: Unmodulated Carrier Spectral Component Translated Message Signal**The peak envelope power (PEP) is**Example 5-1. Power of an AM signal Suppose that a 5000-W AM transmitter is connected to a 50 ohm load; Without Modulation Then the constant Acis given by If the transmitter is then 100% modulated by a 1000-Hz test tone , the total (carrier + sideband) average power will be The peak voltage (100% modulation) is (2)(707) = 1414 V across the 50 ohm load. The modulation efficiency would be 33% since < m2(t) >=1/2**Carrier Power**Sideband power Spectrum Modulation Efficiency Double Side Band Suppressed Carrier (DSBSC) • Power in a AM signal is given by • DSBSC is obtained by eliminating carrier component • If m(t) is assumed to have a zero DC level, then Power • Disadvantages of DSBSC: • Less information about the carrier will be delivered to the receiver. • Needs a coherent carrier detector at receiver**DSBSC Modulation**B An Example of message energy spectral density. No Extra Carrier component 2B Energy spectrum of the DSBSC modulated message signal.**Carrier Recovery for DSBSC Demodulation**• Coherent reference for product detection of DSBSC can not be obtained by the use of ordinary PLL because there are no spectral line components at fc.**Carrier Recovery for DSBSC Demodulation**• A squaring loop can also be used to obtain coherent reference carrier for product detection of DSBSC. A frequency divider is needed to bring the double carrier frequency to fc.**LSSB**USSB Single Sideband (SSB) Modulation • An upper single sideband (USSB) signal has a zero-valued spectrum for • A lower single sideband (LSSB) signal has a zero-valued spectrum for • SSB-AM – popular method ~ BW is same as that of the modulating signal. Note: Normally SSB refers to SSB-AM type of signal**–Hilbert transform of m(t) **Where and H(f) j f -j Single Sideband Signal • Theorem :A SSB signal has Complex Envelopeand bandpass form as: Upper sign (-) USSB Lower sign (+) LSSB Hilbert Transform corresponds to a -900phase shift**Using**Recall from Chapter 4 Single Sideband Signal Proof: Fourier transform of the complex envelope Upper sign USSB Lower sign LSSB Upper sign USSB If lower signs were used LSSB signal would have been obtained**SSB - Power**The normalized average power of the SSB signal Hilbert transform does not change power. SSB signal power is: Power of the modulating signal Power gain factor The normalized peak envelope (PEP) power is:**Generation of SSB**SSB signals have bothAM and PM. The complex envelope of SSB: For the AM component, For the PM component, Advantages of SSB • Superior detected signal-to-noise ratio compared to that of AM • SSB has one-half the bandwidth of AM or DSB-SC signals**Generation of SSB**• SSB Can be generated using two techniques • Phasing method • Filter Method • Phasing method This method is a special modulation type of IQ canonical form of Generalized transmitters discussed in Chapter 4 ( Fig 4.28)