Download presentation

Presentation is loading. Please wait.

Published byWilson Greene Modified over 4 years ago

1
Modulación Analógica (AM-FM) Cx Eléctricas 09 – E.Tapia

2
Modulación de Onda CC (CW) Representación en dominios t-f Efectos del ruido en los receptores correspondientes

3
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

4
Modulation Carrier is sinusoidal wave Amplitude, frequency, or phase are varied with a modulating wave - signal

5
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

6
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

7
Frequency Domain

8
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

9
Linear Modulation

10
DSB-SC- (Double SideBand-Supressed Carrier)

11
Coherent Detection

12
Note that Non coherent detection may lead to null quadrature effect Need coherent local oscillator at demodulation >> complexity >> the price

13
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

14
VSB – Vestigial Sideband Modulation

15
More on VSB

16
Frequency Modulation (FM) f is the frequency deviation is the modulation index defined as f /f m

17
Which is the FM angle? << 1 radian is known as narrowband FM >> 1 radian is known as wideband FM

18
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

19
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

20
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

21
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)

22
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

23
Noise in AM From the SNR at the channel (C, AM) we desire the SNR at the output, demodulator – envelope

24
Phasorial Analysis

25
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

26
Threshold effect in AM Detectors

27
Noise Effects in FM Limiter: clipp and round so that amplitude is independent of the carrier amplitude at the receiver input.

28
Noise Model for FM R(t) is Rayleigh Phase is uniform

29
Signal Model for FM

30
Signal and Noise in FM

31
Discriminator Output Provided the carrier to noise is high

32
FM Discriminator: S2N

33
Cont’ The carrier power has noise quoting effect in FM Recall that The average signal transmitted power is k f 2 P

34
How can we improve S2N in FM?

35
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

36
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.

37
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

38
Alternatevely Clicks are heard after the low pass filter

39
Threshold Effect As is decreased the rate of clicks grows Rate of clicks is high threshold occurs

40
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

41
FM Threshold Reduction FM demodulator with negative feeback (FMFB) or PLL

42
FM Threshold Reduction The VCO output The phase comparator output

43
FM Threshold Reduction (cont)

46
Linear Model of the PLL-FM Demodulator

47
PreEmphasis - Deemphasis Pre at transmitter De- at the receiver

48
Pre-emphasis & De-emphasis Pre at transmitter De- at the receiver

49
Conclusions

Similar presentations

OK

Prof. Brian L. Evans Dept. of Electrical and Computer Engineering The University of Texas at Austin EE445S Real-Time Digital Signal Processing Lab Fall.

Prof. Brian L. Evans Dept. of Electrical and Computer Engineering The University of Texas at Austin EE445S Real-Time Digital Signal Processing Lab Fall.

© 2018 SlidePlayer.com Inc.

All rights reserved.

To make this website work, we log user data and share it with processors. To use this website, you must agree to our Privacy Policy, including cookie policy.

Ads by Google