ECE4371, Fall, 2009FM Modulator and DemodulatorFM Direct ModulatorFM Direct Modulator cont.Indirect FMIndirect FM cont.Slide 7Analysis of Indirect FMA simple electronic implementation of frequency multiplierArmstrong FM ModulatorFM DemodulatorFM Slope DemodulatorFM Slope Demodulator cont.Slope DetectorHard LimiterRatio DetectorZero Crossing DetectorFM Demodulator PLLPLL FMPhase and Frequency AcquisitionSlide 21Slide 22Slide 23Superheterodyne ReceiverAdvantage of superheterodyne receiverFM BroadcastingFc=19KHz. (a) Multiplexer in transmitter of FM stereo. (b) Demultiplexer in receiver of FM stereo.TV FM broadcastingSatellite RadioXM vs. SirusECE4371, Fall, 2009Zhu HanDepartment of Electrical and Computer EngineeringClass 6Sep. 10th, 2007FM Modulator and DemodulatorFM Modulator and DemodulatorFM modulator–Direct FM–Indirect FMFM demodulator–Direct: use frequency discriminator (frequency-voltage converter)–Ratio detector–Zero crossing detector–Indirect: using PLLSuperheterodyne receiverFM broadcasting and Satellite radioFM Direct ModulatorFM Direct ModulatorDirect FM–Carrier frequency is directly varied by the message through voltage-controlled oscillator (VCO)–VCO: output frequency changes linearly with input voltage–A simple VCO: implemented by variable capacitor–Capacitor Microphone FM generatorFM Direct Modulator cont.FM Direct Modulator cont.Direct method is simple, low cost, but lack of high stability & accuracy, low power application, unstable at the carrier frequencyModern VCOs are usually implemented as PLL ICWhy VCO generates FM signal?0Capacitance changes with the applied voltage: ( ) ( )C t C Cm t= +D02000000LC oscillator frequency: 1 1( )2 2 ( )1 1 ( ) ( )22 ( )2 ( )if tLC LC L Cm tCm t O tCLCf Cf m tCf f m tp pp= =+ D� �D= - +� �� �D� -= - DIndirect FMIndirect FMGenerate NBFM first, then NBFM is frequency multiplied for targeted Δf.Good for the requirement of stable carrier frequencyCommercial-level FM broadcasting equipment all use indirect FMA typical indirect FM implementation: Armstrong FMBlock diagram of indirect FMIndirect FM cont.Indirect FM cont.First, generate NBFM signal with a very small β1 1 1 1( ) cos(2 ) sin(2 )sin(2 )c c mv t A f t A f t f tp b p p= -m(t)Indirect FM cont.Indirect FM cont.Then, apply frequency multiplier to magnify β–Instantaneous frequency is multiplied by n–So do carrier frequency, Δf, and β–What about bandwidth?right lefti if n f=Analysis of Indirect FMAnalysis of Indirect FM121 22. Nonlinear device outputs frequencies: ( ) ( ) ( ) ( ) ( )fno nnf nk m tv t a v t a v t a v t+= + + + +L L1011. Input: ( ) cos 2 2 ( ) ,max | ( ) | where ( ) ( ), 1tc ffi fv t A f t k m dk m tf t f k m tWp p t tb� �= +� �� �= + =�=1103. Bandpass filter select new carrier ( ) cos 2 2 ( )ctc ff nfs t A nf t nk m dp p t t=� �= +� �� ��%1max | ( ) |where new ( ) ( ),fi fnk m tf t nf nk m tWb= + =……C1:100pF, L1:2.7μH. D:1N914L2:.22μH, L3:1.8μH, L4:330μHC2:120pF, C3:10pF.30 MHz output. X3 (x5)A simple electronic implementation of frequency multiplierA simple electronic implementation of frequency multiplierArmstrong FM ModulatorArmstrong FM ModulatorInvented by E. Armstrong, an indirect FMA popular implementation of commercial level FMParameter: message W=15 kHz, FM s(t): Δf=74.65 kHz.Can you find the Δf at (a)-(d)?Solution:(a) 14.4 Hz. (b) 72 14.4 1.036 kHz.(c) 1.036 kHz. (d) 72 1.036 74.65 kHz.f ff fD = D = � =D = D = � =FM DemodulatorFM DemodulatorFour primary methods–Differentiator with envelope detector/Slope detectorFM to AM conversion–Phase-shift discriminator/Ratio detectorApproximates the differentiator–Zero-crossing detector–Frequency feedbackPhase lock loops (PLL)FM Slope DemodulatorFM Slope DemodulatorPrinciple: use slope detector (slope circuit) as frequency discriminator, which implements frequency to voltage conversion (FVC)–Slope circuit: output voltage is proportional to the input frequency. Example: filters, differentiatorfreqency in s(t) voltage in x(t)10 2020 40 Hz jHz jppM MFM Slope Demodulator cont.FM Slope Demodulator cont.Block diagram of direct method (slope detector = slope circuit + envelope detector)0( ) cos 2 2 ( ) , where ( ) ( )tc c f i c fs t A f t k m d f t f k m tp p t t� �= + = +� �� ��10Let the slope circuit be simply differentiator:( ) 2 2 ( ) sin 2 2 ( )( ) 2 2 ( )tc c f c fo c c fs t A f k m t f t k m ds t A f k m tp p p p t tp p� �� �=- + +� �� �� �� ��- +� ��so(t) linear with m(t)Slope DetectorSlope DetectorMagnitude frequencyresponse of transformer BPF.Hard LimiterHard LimiterA device that imposes hard limiting on a signal and contains a filter that suppresses the unwanted products (harmonics) of the limiting process.Input SignalOutput of hard limiterBandpass filterRemove the amplitude variations))(cos()()(cos)()(tfcidaamktwtAttAtv))(cos(4)(tfcodaamktwte )(5cos51)(3cos31)(cos4)( ttttvoRatio DetectorRatio DetectorFoster-Seeley/phase shift discriminator–uses a double-tuned transformer to convert the instantaneous frequency variations of the FM input signal to instantaneous amplitude variations. These amplitude variations are rectified to provide a DC output voltage which varies in amplitude and polarity with the input signal frequency. –ExampleRatio detector –Modified Foster-Seeley discriminator, not response to AM, but 50%Zero Crossing DetectorZero Crossing DetectorFM Demodulator PLLFM Demodulator PLLPhase-locked loop (PLL)–A closed-loop feedback control circuit, make a signal in fixed phase (and frequency) relation to a reference signalTrack frequency (or phase) variation of inputsOr, change frequency (or phase) according to inputs–PLL can be used for both FM modulator and demodulatorJust as Balanced Modulator IC can be used for most amplitude modulations and demodulationsPLL FMPLL FMRemember the following relations–Si=Acos(wct+1(t)), Sv=Avcos(wct+c(t))–Sp=0.5AAv[sin(2wct+1+c)+sin(1-c)]–So=0.5AAvsin(1-c)=AAv(1-c)–Section 2.14Phase and Frequency Acquisition Phase and Frequency Acquisition(a) Block diagram for a PLL FM demodulator; (b) PLL FM demodulator using the XR-2212 PLL(a) Block