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Eeng 360 1 Chapter 5 AM, FM, and Digital Modulated Systems Amplitude Modulation (AM) Double Sideband Suppressed carrier (DSSC) Assymetric Sideband Signals Single sideband signals (SSB) Frequency Division Multiplexing (FDM) Huseyin Bilgekul Eeng360 Communication Systems I Department of Electrical and Electronic Engineering Eastern Mediterranean University

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Eeng 360 2 The modulated bandpass signal can be described by Bandpass Signaling Review The voltage spectrum of the bandpass signal is The PSD of the bandpass signal is Where Modulation Mapping function: Convert m(t) g(t) Ref : Table 4-14-1

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Eeng 360 3 Amplitude Modulation The Complex Envelope of an AM signal is given by A c indicates the power level of AM and m(t) is the Modulating Signal A c [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 Representation of an AM signal is given by Envelope detection can be used if % modulation is less than 100%.

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Eeng 360 4 An Example of a message signal m(t) Waveform for Amplitude modulation of the message signal m(t) Amplitude Modulation

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Eeng 360 5 Amplitude Modulation An Example of message energy spectral density. Energy spectrum of the AM modulated message signal. B 2B Carrier component together with the message

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Eeng 360 6 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 AM Percentage Modulation If m(t) has a peak positive values of +1 and a peak negative value of -1 AM signal 100% modulated

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Eeng 360 7 AM Signal Waveform A max = 1.5A c A min = 0.5 A c % Positive modulation= 50% % Negative modulation =50% Overall Modulation = 50%

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Eeng 360 8 AM Percentage Modulation Under modulated (100%)

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Eeng 360 9 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 PowerSideband power

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Eeng 360 10 AM Modulation Efficiency Translated Message Signal 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

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Eeng 360 11 Example 5-1. Power of an AM signal Suppose that a 5000-W AM transmitter is connected to a 50 ohm load; Then the constant A c is given by Without Modulation 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 peak envelope power (PEP) is The modulation efficiency would be 33% since =1/2

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Eeng 360 12 Double Side Band Suppressed Carrier (DSBSC) Power in a AM signal is given by Carrier Power Sideband power DSBSC is obtained by eliminating carrier component If m(t) is assumed to have a zero DC level, then Spectrum Power Disadvantages of DSBSC: Less information about the carrier will be delivered to the receiver. Needs a coherent carrier detector at receiver Modulation Efficiency

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Eeng 360 13 DSBSC Modulation An Example of message energy spectral density. Energy spectrum of the DSBSC modulated message signal. No Extra Carrier component B 2B

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Eeng 360 14 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 f c.

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Eeng 360 15 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 f c.

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Eeng 360 16 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 USSB LSSB

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Eeng 360 17 Single Sideband Signal Theorem : A SSB signal has Complex Envelope and bandpass form as: Upper sign ( - ) USSB Lower sign ( + ) LSSB Hilbert transform of m(t) Where and Hilbert Transform corresponds to a -90 0 phase shift H(f) f -j j

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Eeng 360 18 Single Sideband Signal Proof: Fourier transform of the complex envelope Using Recall from Chapter 4 If lower signs were used LSSB signal would have been obtained Upper sign USSB Lower sign LSSB Upper sign USSB

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Eeng 360 19 Single Sideband Signal

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Eeng 360 20 SSB - Power The normalized average power of the SSB signal Hilbert transform does not change power. SSB signal power is: The normalized peak envelope (PEP) power is: Power gain factor Power of the modulating signal

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Eeng 360 21 Generation of SSB SSB signals have both AM 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

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Eeng 360 22 Generation of SSB SSB Can be generated using two techniques 1.Phasing method 2.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) of Generalized transmitters discussed in Chapter 4 ( Fig 4.28)

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Eeng 360 23 Generation of SSB Filter Method The filtering method is a special case in which RF processing (with a sideband filter) is used to form the equivalent g(t), instead of using baseband processing to generate g(m) directly. The filter method is the most popular method because excellent sideband suppression can be obtained when a crystal oscillator is used for the sideband filter. Crystal filters are relatively inexpensive when produced in quantity at standard IF frequencies. standard IF frequencies.

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Eeng 360 24 Weavers Method for Generating SSB.

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Eeng 360 25 Generation of VSB

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Eeng 360 26 Frequency Divison Multiplexing