Presentation on theme: "Chapter 5 AM, FM, and Digital Modulated Systems"— Presentation transcript:
1 Chapter 5 AM, FM, and Digital Modulated Systems Amplitude Modulation (AM)Double Sideband Suppressed carrier (DSSC)Assymetric Sideband SignalsSingle sideband signals (SSB)Frequency Division Multiplexing (FDM)Huseyin BilgekulEeng360 Communication Systems IDepartment of Electrical and Electronic EngineeringEastern Mediterranean University
2 Bandpass Signaling Review The modulated bandpass signal can be described byWhereModulation Mapping function: Convert m(t) →g(t) Ref : Table 4-1The voltage spectrum of the bandpass signal isThe PSD of the bandpass signal isWhere
3 Amplitude ModulationThe Complex Envelope of an AM signal is given byAc indicates the power level of AM and m(t) is the Modulating SignalRepresentation of an AM signal is given byAc[1+m(t)] In-phase component x(t)If m(t) has a peak positive values of +1 and a peak negative value of -1AM signal 100% modulatedEnvelope detection can be used if % modulation is less than 100%.
4 Amplitude Modulation An Example of a message signal m(t) Waveform for Amplitude modulation of the message signal m(t)
5 Carrier component together with the message Amplitude ModulationBAn Example of message energy spectral density.Carrier component together with the message2BEnergy spectrum of the AM modulated message signal.
6 AM – Percentage Modulation Definition: The percentage of positive modulation on an AM signal isThe percentage of negative modulation on an AM signal isThe percentage of overall modulation isIf m(t) has a peak positive values of +1 and a peak negative value of -1AM signal 100% modulated
7 AM Signal Waveform % Positive modulation= 50% % Negative modulation =50%Overall Modulation = 50%Amax = 1.5AcAmin = 0.5 Ac
8 AM – Percentage Modulation Under modulated (<100%)100% modulatedEnvelope DetectorCan be usedGives Distorted signalOver Modulated (>100%)
9 Discrete Carrier Power AM – Normalized Average PowerThe normalized average power of the AM signal isIf the modulation contains no dc level, thenThe normalized power of the AM signal isDiscrete Carrier PowerSideband power
10 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 informationModulation Efficiency:Highest efficiency for a 100% AM signal : 50% - square wave modulationNormalized Peak Envelope Power (PEP) of the AM signal:Voltage Spectrum of the AM signal:Unmodulated Carrier Spectral ComponentTranslated Message Signal
11 Example 5-1. Power of an AM signal Suppose that a 5000-W AM transmitter is connected to a 50 ohm load;WithoutModulationThen the constant Ac is given byIf the transmitter is then 100% modulated by a 1000-Hz test tone , the total (carrier + sideband) average power will beThe peak voltage (100% modulation) is (2)(707) = 1414 V across the 50 ohm load.The peak envelope power (PEP) isThe modulation efficiency would be 33% since < m2(t) >=1/2
12 Double Side Band Suppressed Carrier (DSBSC) Carrier PowerSideband powerPower in a AM signal is given byDSBSC is obtained by eliminating carrier componentIf m(t) is assumed to have a zero DC level, thenSpectrum Power Modulation Efficiency Disadvantages of DSBSC:Less information about the carrier will be delivered to the receiver.Needs a coherent carrier detector at receiver
13 No Extra Carrier component DSBSC ModulationBAn Example of message energy spectral density.No Extra Carrier component2BEnergy spectrum of the DSBSC modulated message signal.
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 fc.
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 fc.
16 Single Sideband (SSB) Modulation An upper single sideband (USSB) signal has a zero-valued spectrum forA lower single sideband (LSSB) signal has a zero-valued spectrum forSSB-AM – popular method ~ BW is same as that of the modulating signal.Note: Normally SSB refers to SSB-AM type of signalUSSBLSSB
17 Single Sideband Signal Theorem : A SSB signal has Complex Envelope and bandpass form as:Upper sign (-) USSBLower sign (+) LSSB– Hilbert transform of m(t) WhereandHilbert Transform corresponds to a -900 phase shiftH(f)f-jj
18 Single Sideband Signal Proof: Fourier transform of the complex envelopeUpper sign USSBLower sign LSSBUsingRecall from Chapter 4Upper sign USSBIf lower signs were used LSSB signal would have been obtained
20 SSB - Power The normalized average power of the SSB signal Hilbert transform does not change power.SSB signal power is:Power of the modulating signalPower gain factorThe normalized peak envelope (PEP) power is:
21 Generation of SSB Advantages of SSB SSB signals have both AM and PM. The complex envelope of SSB:For the AM component,For the PM component,Advantages of SSBSuperior detected signal-to-noise ratio compared to that of AMSSB has one-half the bandwidth of AM or DSB-SC signals
22 Generation of SSB SSB Can be generated using two techniques Phasing methodFilter MethodThis method is a special modulation type of IQ canonical formof Generalized transmitters discussed in Chapter 4 ( Fig 4.28)
23 Generation of SSB Filter Method The filtering method is a special case in which RF processing (with asideband filter) is used to form the equivalent g(t), instead of usingbaseband processing to generate g(m) directly. The filter method is themost popular method because excellent sideband suppression can beobtained when a crystal oscillator is used for the sideband filter.Crystal filters are relatively inexpensive when produced in quantity atstandard IF frequencies.