TopicsPrinciples of Amplitude Modulation (AM)AM AnalysisSingle Sideband AMPrinciples of Frequency Modulation (FM)FM AnalysisRadio ReceiverAdvantages & Disadvantages of Analog Modulation184.108.40.206.220.127.116.11
Why we Need Modulation?
3.1Principles of Amplitude Modulation
3.1Principles of Amplitude Modulation•Definition:•Amplitude of carrier frequency change proportionately to the value of the modulation signal.•Advantages:•Simple modulator circuits•Cheap :low-quality form of modulation used for commercial broadcasting of audio & video signal.•Disadvantages:•Poor performance due to noise•Inefficient use of transmitter power.•Application:•2 way radio communications, broadcasting, aircraft comm. & citizen band (CB) radio.
AM modulators are nonlinear devices◦2 input and 1 output: modulating signal and carrier signal.Several types of amplitude modulation◦Double Side Band Full Carrier (DSB-FC)◦Double Side Band Suppressed Carrier (DSB-SC)◦Single Side Band (SSB)AM generation is shown in Figure 2.1Modulated wave = AM envelope as shown in Figure 2.26Figure 2.1: Block diagram of Amplitude ModulationInformation, Vm(t)Carrier, Vc(t)ModulatorVAM (t)3.1Principles of Amplitude Modulation
7Figure 2.2 AM signal with the envelope3.1Principles of Amplitude Modulation
8AM in action3.1Principles of Amplitude Modulation
•AM begins with carrier vc, a sine wave with frequency c& amplitude Vc:•Modulating signal:•Then AM is:9Derivation of AM Equation3.1Principles of Amplitude ModulationRemember :Modulating signal uses the peak value of the carrier as a reference pointtfmVVVVVmtfVVVvVVmccenvcmmmcenvmcenv2sin]/[2sintfVvmmm2sintfVvccc2sin
10•Where m (modulation index)is defined as Vm/Vc, hence:•The voltage resulting AM wave envelope at any instant is:3.1Principles of Amplitude Modulation)2sin1(tfmVVmcenvtftfmVtfVvcmccenv2sin)2sin1(2sin
This yield, the upper and lower sidebands – frequency & amplitude.11CarrierLSBUSBUsing Trigo ID3.1Principles of Amplitude ModulationtffVtffVtfVvmcmmcmccAM)(2cos2)(2cos22sintffVmtffVmtfVvmccmcccc)(2cos2)(2cos22sin)cos()cos(2/1))(sin(sinbababa
AM modulators are non-linear device => non-linear mixing occurs.