通信原理英文版课件：Ch2 Continuous-Wave Modulation part2

1、ReviewModulationDefinitionBaseband signals (modulating wave), carrier, and modulated waveContinuous-wave modulationAmplitude modulation (AM)Angle modulation (includes PM and FM)1Review (contd)Amplitude modulationVirtues: SimpleLimitations: Wasteful of power and bandwidth2Review (contd)Upper sideband

2、 and lower sidebandSpectrum of AM wave3Review (contd)Linear modulation schemesDSB, SSB, and VSB 4Review (contd)DSB:Coherent detectionCostas receiverQuadrature-carrier multiplexingVSB:5Review (Contd)FDMAngle modulationPMFMTwo kinds of single-tone FM signalsNarrowband FMWideband FM6Review (contd)Trans

3、mission bandwidth of FM signalsThe Carsons ruleThe universal curveGeneration of FM signalsDirect FM (VCO)Indirect FM (narrowband wideband)Detection of FM signalsDirect method (frequency discriminator)Indirect method (phase-locked loop)72.8 Nonlinear Effects in FM SystemsTwo basic forms of nonlineari

4、tyStrong, e.g. square-law modulators, hard-limiters, and frequency multipliersWeak: arises due to imperfection (we consider weak nonlinearity in this section)Memoryless channelOutput is an instantaneous function of inputNo energy storageConclusionsFM is not affected by amplitude nonlinearitiesFM is

5、sensitive to phase nonlinearities82.8 Nonlinear Effects in FM Systems (Contd)92.9 Superheterodyne ReceiversFigure 2.32 Basic elements of an AM radio receiver of the superheterodyne type.102.10 Noise in CW Modulation SystemsAssumptionsAWGN channelIdeal band-pass filtering and a proper ideal demodulat

6、or at the receiverFigure 2.33 Receiver model.11Signal-to-Noise Ratios (SNRs)N0: Average noise power per unit bandwidth measured at the front end of the receiver.Figure 2.34 Idealized characteristic of band-pass filtered noise.Average noise power is N0BT.n(t) is the filtered noise,12SNRs (Contd)Messa

7、ge signal and noise should be additively at the receiver output. This is satisfied with coherent detection. For envelope detection (in AM) and frequency discrimination (in FM), noise power should be relatively low.13SNRs (Contd)(SNR)C(SNR)I(SNR)ODefining “figure of merit” to compare different CW mod

8、ulation systems:142.11 Noise in Linear Receivers Using Coherent DetectionAM demodulationCarrier is suppressedCoherent detection (linear)Carrier is transmittedEnvelope detection (nonlinear)Considering coherent detection of DSB-SC signalAssuming perfect synchronization15Noise in Linear Receivers Using

9、 Coherent Detection (Contd)Figure 2.36 Model of DSB-SC receiver using coherent detection.16Noise in Linear Receivers Using Coherent Detection (Contd)The message bandwidth is W, therefore the average noise power is WN0.Average power of the message signal is:Average power of the modulated signal s(t)

10、is:17Noise in Linear Receivers Using Coherent Detection (Contd)Message signal and noise are additive at the receiver output.18Noise in Linear Receivers Using Coherent Detection (Contd)Average power of the message signal at receiver output is:Average power of the noise at receiver output is:19Noise i

11、n Linear Receivers Using Coherent Detection (Contd)Figure of merit of SSB is the same as that of DSB-SCNo tradeoff is offered between noise performance and channel bandwidth202.12 Noise in AM Receivers Using Envelope DetectionAverage power of the modulated signal s(t) is:Average power of noise in th

12、e message bandwidth is:21Noise in AM Receivers Using Envelope Detection (Contd)Figure 2.37 Model of AM receiver.22Noise in AM Receivers Using Envelope Detection (Contd)Figure 2.38 Phasor diagram for AM wave plus narrowband noise for (a) the case of high carrier-to-noise ratio, (b) the case of low ca

13、rrier-to-noise ratio.23The Case of HighCarrier-to-Noise RatioThe DC term Ac is not related to the message signal m(t) and may be removed by using a blocking capacitor.Noise performance of AM is always worse than DSB-SC.24Example 2.4: Single-Tone ModulationThe maximum figure of merit is 1/3.25The Cas

14、e of LowCarrier-to-Noise RatioLost of message signal since (t) is uniformly distributed over 2 radiansThreshold effect:Below a carrier-to-noise value (the threshold), the noise performance deteriorates much more rapidly than proportionately to the carrier-to-noise ratio.Nonlinear detector has a thre

15、shold effectCoherent detector does not have a threshold effect26The Case of Low Carrier-to-Noise Ratio (Contd)Figure 2.39 Output signal-to-noise ratio of an envelope detector for varying carrier-to-noise ratio.High signal-to-noise ratio:output SNR is proportional to input SNRLow signal-to-noise rati

16、o:output SNR is proportional to the squared input SNRWeak signal suppression:The detector favors strong signals and penalizes weak signals.272.13 Noise in FM ReceiversFigure 2.40 Model of an FM receiver.Bandwidth is BT, BT 1/3, that is 0.471, FM offers improved noise performance over AM.36Capture Ef

17、fectConcerns interference from another FM modulated signal with close frequencyWhen there is a stronger signal, the receiver locks onto that signalWhen the signals are of nearly equal strength, the receiver fluctuates between them37FM Threshold EffectWhen the carrier-to-noise ratio decreases, the FM

18、 receiver breaks.Individual clicks and crackling or sputtering sounds will be heardThe clicks arise from changes of (t) by 2 radians.38FM Threshold Effect (Contd)Figure 2.44 Illustrating impulselike components in (t) d(t)/dt produced by changes of 2 in (t); (a) and (b) are graphs of (t) and (t), res

19、pectively.39FM Threshold Effect (Contd)Figure 2.45 Dependence of output SNR on input carrier-to-noise ratio for FM receiver. The carrier-to-noise ratio is defined byIn practice, the threshold effect is avoided when = 20 (13 dB).40FM Threshold Effect (Contd)When configuring an FM systemThe transmissi

20、on bandwidth BT is determined by the modulation index and message bandwidth W;The average transmitted power is determined by the threshold value and the average noise power per unit bandwidth N0.41FM Threshold ReductionUsing feedback to reduce thresholdFM demodulator with negative feedback (FMFB dem

21、odulator) (5 7 dB)Phase-locked loop demodulator (2 3 dB)Figure 2.46 FM threshold extension.42FM Threshold Reduction (Contd)Figure 2.47 FM demodulator with negative feedback.By adopting feedback, only frequency variation is tracked. As a result, noise bandwidth is significantly reduced.43Pre-Emphasis

22、 andDe-Emphasis in FMFigure 2.48 Power spectral density of (a) noise at FM receiver output, and (b) a typical message signal.PSDs of message signals usually decrease with frequency;PSD of noise at receiver output increases with frequency.Slightly reducing bandwidth intolerable distortionPre-emphasis

23、 and de-emphasis better solution44Pre-Emphasis andDe-Emphasis in FM (Contd)Figure 2.49 Use of pre-emphasis and de-emphasis in an FM system.The improvement factor45Example 2.6Figure 2.50 (a) Pre-emphasis filter. (b) De-emphasis filter.Typically value of I in commercial FM broadcasting is 22 (about 13