Chapter 7. Analog Communication System - PowerPoint Presentation

Slide1

Chapter 7. Analog Communication System

Husheng Li

The University of TennesseeSlide2

Superheterodyne Receiver

Four tasks of the receiver:

Demodulation

Carrier frequency tuning

Filtering

Amplification of signal

In theory, all of the foregoing requirements could be met with a high-gain tunable

bandpass

amplifier. In practice, it is hard to achieve both selective and

tuneable

.Slide3

Superhet Principle

In the

superhet

principle, there are two distinct amplification and filtering sections prior to demodulation: RF section and IF section.Slide4

Parameters of AM and FMSlide5

Illustration of SpectrumSlide6

Direct Conversion Receivers

Direct conversion receivers (DC) are a class of tuned-RF (TRF) receivers that consist of an RF amplifier followed by a product detector and suitable message amplification.Slide7

Image Signal Rejection

The DC’s chief drawback is that it does not reject the image signal that is present in the opposite sideband and is thus more susceptible to noise and interference.Slide8

Homework

Deadline: Nov. 18

th

, 2013Slide9

Double Conversion Receiver

A double-conversion receiver takes the

superhet

principle one step further by including two frequency converters an two IF sections. The second IF is always fixed-tuned, while the first IF and second LO may be fixed or tunable.Slide10

Receiver Specification

Receiver sensitivity is the minimum input voltage necessary to produce a specified signal-to-noise radio (SNR) at the output of the IF section. A good-quality shortwave radio typically has sensitivity of 1

uV

for a 40dB SNR.

Dynamic range (DR) is

Selectivity specifies a receiver’s capability to discriminate against adjacent channel signals.

The noise figure indicates how much the receiver degrades the input signal’s S/N.

Image Rejection is Slide11

Scanning Spectrum Analyzers

If the LO in a

superhet

is replaced by a VCO, then the

predetection

portion acts like a

votlage

-tunable

bandpass

amplifier with center frequency f0=

f_LO

+/-

f_IFSlide12

Operation of Spectrum Analyzer

The number of resolvable spectral lines equals

The IF output produced by a single line takes the form of a

bandpass

pulse with time duration

A rapid sweep rate may exceed the IF pulse response. Hence, we have

Hence, the accurate resolution (small B) calls for a slow rate and long observation time.Slide13

Multiplexing

The basic multiplexing techniques include FDM, TDM and CDM. The multiple access techniques include FDMA, TDMA and CDMA, and OFDM.Slide14

FDMSlide15

Crosstalk in FDM

The major practical problem of FDM is crosstalk, the unwanted coupling of one message into another.

Intelligible crosstalk arises primarily because of nonlinearities in the system which cause one message signal to appear as modulation on another subcarrier.

The crosstalk may also come from imperfect spectral separation by the filter bank.Slide16

Example: FDMA Satellite SystemSlide17

Quadrature-Carrier Multiplexing

Quadrature-carrier multiplexing, also known as quadrature amplitude modulation (QAM), utilizes carrier phase shifting and synchronous detection to permit two DSB signals to occupy the same frequency band.Slide18

TDMSlide19

Synchronization Markers

Markers are needed for time synchronizationSlide20

Crosstalk and Guard Times

The filter design in TDM should be avoid inter-channel crosstalk.

A guard time is needed to avoid the crosstalk in TDM.

The crosstalk reduction factor is Slide21

Crosstalk in PPM

The avoidance of crosstalk in PPM requiresSlide22

Comparison of TDM and FDM

TDM is readily implemented with high-density VLSI circuitry where digital switches are extremely economical.

TDM is invulnerable to the usual causes of crosstalk in FDM.

TDM may or may not be advantageous when the transmission medium is subject to fading.

Most systems are hybrids of FDMA and TDMA.Slide23

GSM

GSM is a hybrid of TDMA and FDMASlide24

Homework

Deadline: Nov. 25

th

, 2013Slide25

Phase Locked Loops

A PLL uses phase comparatorSlide26

Phase Dynamics in PLL

The instantaneous angle in PLL is given bySlide27

Dynamics in PPL

The dynamics of the phase error are described in the nonlinear differential equation:

w

here the loop gain is defined as

The steady state is given bySlide28

Convergence

A necessary condition for the stead-state solution is given by

When the error is very close to zero, we have the following approximation:Slide29

PLL Pilot Filter

We can use the following circuit to generate a sinusoid synchronized with the pilot:Slide30

Synchronous Detection

When there is no pilot (e.g., in DSB), we can use the following Costas-PLL to lock the phase:Slide31

Frequency-offset Loop

We can use the following PLL to realize the synthesize the sum of two frequencies:Slide32

Frequency Multiplication

We can us the following PLL and frequency divider to realize the multiplication of a frequency with an integer:Slide33

Adjustable Local Oscillator

We can use the following circuits to obtain 100kHz and 1.6MHz and adjustable LO that covers 9.90—9.99

MHz.Slide34

Quiz

Use the following figure to explain the purpose of phase locked loop and its operation procedure:Slide35

Linearized PLL

The frequency domain model of PLL can be obtained by linearizing the PLL:Slide36

FM Detection

When the input is FM signal, the PLL can be approximated by a first-order

lowpass

filter:

The output is given bySlide37

Software Project

Topic: Analyze the spectrum of AM and FM signals.

Software:

Matlab

Tasks:

1. Consider triangle series shown in the right figure. Consider carrier frequency 200Hz. Choose the modulation indices by yourself.

2. Sample the modulated signal. Plot the time domain curves of the AM and FM modulated signals. Determine the sampling rate by yourself.

3. Read the introduction to discrete Fourier transform (DFT) in

Matlab

(

http://www.mathworks.com/help/matlab/math/discrete-fourier-transform-dft.html

) and the function

fft

(

http://www.mathworks.com/help/matlab/ref/fft.html

).

4. Use function

fft

in

Matlab

to obtain the spectrum of the modulated signal. Compare the spectrum of AM and FM.

5. Change the parameters such as modulation index and draw conclusions on the impact of

these parameters.

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(seconds)

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