Chapter 7. Analog Communication SystemHusheng LiThe University of Tennessee

Superheterodyne ReceiverFour 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.

Superhet Principle In the superhet principle, there are two distinct

amplification and filtering sections prior to demodulation: RF section and IF section.

Parameters of AM and FM

Illustration of Spectrum

Direct Conversion ReceiversDirect 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.

Image Signal RejectionThe 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.

HomeworkDeadline: Nov. 18th, 2013

Double Conversion ReceiverA 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.

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

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_IF

Operation of Spectrum AnalyzerThe number of resolvable spectral lines equalsThe 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.

MultiplexingThe basic multiplexing techniques include FDM,

TDM and CDM. The multiple access techniques include FDMA, TDMA and CDMA, and OFDM.

FDM

Crosstalk in FDMThe 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.

Example: FDMA Satellite System

Quadrature-Carrier MultiplexingQuadrature-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.

TDM

Synchronization MarkersMarkers are needed for time synchronization

Crosstalk and Guard TimesThe 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

Crosstalk in PPMThe avoidance of crosstalk in PPM requires

Comparison of TDM and FDMTDM 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.

GSMGSM is a hybrid of TDMA and FDMA

Homework Deadline: Nov. 25th, 2013

Phase Locked LoopsA PLL uses phase comparator

Phase Dynamics in PLLThe instantaneous angle in PLL is given by

Dynamics in PPLThe dynamics of the phase error are described

in the nonlinear differential equation:

where the loop gain is defined as

The steady state is given by

ConvergenceA necessary condition for the stead-state

solution is given by

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

PLL Pilot FilterWe can use the following circuit to generate a

sinusoid synchronized with the pilot:

Synchronous DetectionWhen there is no pilot (e.g., in DSB), we can

use the following Costas-PLL to lock the phase:

Frequency-offset LoopWe can use the following PLL to realize the

synthesize the sum of two frequencies:

Frequency MultiplicationWe can us the following PLL and frequency

divider to realize the multiplication of a frequency with an integer:

Adjustable Local OscillatorWe can use the following circuits to obtain

100kHz and 1.6MHz and adjustable LO that covers 9.90—9.99 MHz.

QuizUse the following figure to explain the purpose

of phase locked loop and its operation procedure:

Linearized PLLThe frequency domain model of PLL can be

obtained by linearizing the PLL:

FM DetectionWhen the input is FM signal, the PLL can be

approximated by a first-order lowpass filter:

The output is given by

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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