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Modulación Analógica (AM-FM) CX Eléctricas 2011 E.Tapia.

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Presentation on theme: "Modulación Analógica (AM-FM) CX Eléctricas 2011 E.Tapia."— Presentation transcript:

1 Modulación Analógica (AM-FM) CX Eléctricas 2011 E.Tapia

2 As can be seen from the diagram, UMTS FDD is designed to operate in paired frequency bands, with uplink in the MHz band, and downlink in the MHz band. UMTS TDD is left with the unpaired frequency bands MHz, and MHz. UMTS

3 3 450 PremiumDigitalizados BásicosDigitalizados Qam Espectro Banda de Directa425 Espectro Banda de Retorno CM 64 Qam Básicos Analógicos DOCSIS - BANDA DE RETORNO:

4 Frecuencia en MHz TELEMETRÍA ( CHEETAH) 7,5 MHz WEB TV (ENSAYOS) 8,9 MHz Distribución de servicios en la banda de retorno ( MHz) SET-TOP-BOX (ACC4000) 10,7 MHz TELEFONIA ( ENSAYOS) 38 a 42 MHz CABLEMODEM ( DOCSIS) 24,5 a 36 MHz SET-TOP-BOX (DCT) 13,8 MHz BW 200 KHz DOCSIS - BANDA DE RETORNO:

5 TDT- Television Digital Terrestre

6

7 Modulación de Onda CC (CW) Representación en dominios t-f Efectos del ruido en los receptores correspondientes

8 Modulation -Demodulation Ix transmission in presence of noise Ix bearing signals or baseband signals Transmitter-Channel-Receiver Frequency shifting on Tx – Modulation using a carrier Frequency shift back on Rx –Demodulation

9 Modulation Carrier is sinusoidal wave Amplitude, frequency, or phase are varied with a modulating wave - signal

10 Amplitude Modulation Message signal m(t) and carrier c(t) are independent Carrier amplitude is varied about a mean value (Ac), linearly with m(t) K a is the modulation sensiviy measured in 1/volt

11 Some issues on AM Overmodulation  Leads to envelope distortion. The demodulator will track a false envelope and information will be lost. f c >>>> W – the message bandwidth  Easy envelope visualization and tracking

12 Frequency Domain

13 Note that Mod-Demod are implemented using non-linear devices Demod are often envelope detectors AM Power and AM Bandwith  Not efficient at power use (tx of c(t))  Sidebands are related each other >>>> just one is needed  Hence >>>> avoid c(t) transmission and duplicate sidebands

14 Linear Modulation

15 DSB-SC- (Double SideBand-Supressed Carrier)

16 Coherent Detection

17 Note that Non coherent detection may lead to null quadrature effect Need coherent local oscillator at demodulation >> complexity >> the price

18 SSB MOdulation DSB-SC + Filtering for Sideband Removal Highly selective filters from cristal oscillators Coherent detector >> low power pilot carrier addition is added at transmission

19 VSB – Vestigial Sideband Modulation

20 More on VSB

21 Dealing with Noise

22 Radio Spectrum AM problems  static interference from household appliances and lighting  limited audio quality (frequency response and dynamic range)  nighttime interference between many stations (co-channel interference), because of ionospheric refraction, especially in rural areas

23 Modulación Angular

24 Frecuencia Instantánea

25 Phase Modulation

26 Frequency Modulation (FM) Apex (radio band) - Wikipedia, the free encyclopedia

27 Frequency Modulation (FM)  f is the frequency deviation  is the modulation index defined as  f /f m

28 Which is the FM angle?  << 1 radian is known as narrowband FM  >> 1 radian is known as wideband FM

29 FM- Cont’

30 FM is non-linear

31 FM with modulating tone

32 Narrowband vs. Wideband FM

33 Narrowband FM

34 Compare to AM

35 Narrowband FM Generation

36 Errors coming from the Aproximation

37 Demodulacion Directa FM (Discrimador de Frecuencia)

38 Discriminador de Frecuencia

39 Equivalente pasabajo de H1(f)

40 Detecting (I)

41 Detecting (II)

42 Detecting (III)

43 Detecting (IV)

44 Chapter 2. Puntos 2.1 a 2.9

45 Narrowband Noise Representations

46 Noise in CW Modulation Chanel Model is AWGN  Power spectral density is N o /2 Receiver model defined by a bandpass filter and a demodulator model

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58 Tono + ruido pasabanda

59 Calculando pdf conjunta

60 Pdf conjunta de envolvente y fase

61 Marginales

62 Chapter 1. Puntos 1.10 a 1.13 Consulta suplementaria CioffiChap2PassBand.pdf

63 Modulación Analógica frente al ruido

64 SNRs SNR I (Input)  Ratio of the average power of the modulated signal s(t) to the average power of the filtered noise SNR o (Output)  Ratio of the averaged power of the demodulated signal to the power of noise measured at the receiver output SNR c (Channel)  Ratio of the averaged power of the modulated signal to the average power of noise in the message bandwith both at the receiver input

65 Noise in DSB Coherent Detection s(t) is the DSB component of x(t) C is system dependent scaling factor m(t) sample from stationary process of zero mean and S(f) Hence compute SNR C, DSB

66 Figure of Merit in Coherent Detection The quadrature component of noise is rejected in coherent detection The average power of filtered noise n(t) is Same for n I (t)

67 Figure …. The same holds for SSB NO way to improve SNR by increasig bandwith use in DSB w.r.t SSB The effect of modulation is just frequenxy shifting

68 Noise in AM From the SNR at the channel (C, AM) we desire the SNR at the output, demodulator – envelope

69 Phasorial Analysis

70 Figures of Merit Always << 1for AM envelope receivers Equal to 1 for DSB, SSB Caused by waste of power on carrir transmission Existence of threshold effect

71 Threshold effect in AM Detectors

72

73 Noise Effects in FM Limiter: clipp and round so that amplitude is independent of the carrier amplitude at the receiver input.

74 Noise Model for FM R(t) is Rayleigh Phase is uniform

75 Signal Model for FM

76 Signal and Noise in FM

77 Discriminator Output Provided the carrier to noise is high

78 FM Discriminator: S2N

79 Cont’ The carrier power has noise quoting effect in FM Recall that  The average signal transmitted power is k f 2 P

80 How can we improve S2N in FM?

81 The conclusion FM provides a mechanism for the exchange of improved noise performance by increased transmission bandwidth FM can also reject other FM signals closed to the carrier frequency provided interferent signal are weaker w.r.t. the target FM input

82 Threshold Effect in FM Assumption  Carrier to Noise ratio at the discriminator input >> 1 Violation to this assumption  FM receiver breaks. From breaks to sputtering sounds. The formula does not hold.

83 No signal but Noise Ac >> n I, n Q Ac << n I, n Q  P 1 noves to the origin and random phase is observed is around

84 Alternatevely Clicks are heard after the low pass filter

85 Threshold Effect As  is decreased the rate of clicks grows Rate of clicks is high threshold occurs

86

87 Designing an FM System Given D (  )  Compute B T Given B T and N 0 (Noise power per unit bandwidth)  Determine A C to keep above the threshold

88 FM Threshold Reduction FM demodulator with negative feeback (FMFB) or PLL

89 FM Threshold Reduction The VCO output The phase comparator output

90 FM Threshold Reduction (cont)

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93 Linear Model of the PLL-FM Demodulator

94 PreEmphasis - Deemphasis Pre at transmitter De- at the receiver

95 Pre-emphasis & De-emphasis Pre at transmitter De- at the receiver

96 Conclusions

97 Chapter 2. Puntos 2.10 a 2.14


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