Chapter 7. Analog Communication System Husheng Li The University of Tennessee

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.

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 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.

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.

Homework  Deadline: Nov. 18 th , 2013

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.

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 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.

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

FDM

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.

Example: FDMA Satellite System

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.

TDM

Synchronization Markers  Markers are needed for time synchronization

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

Crosstalk in PPM  The avoidance of crosstalk in PPM requires

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.

GSM  GSM is a hybrid of TDMA and FDMA

Homework  Deadline: Nov. 25 th , 2013

Phase Locked Loops  A PLL uses phase comparator

Phase Dynamics in PLL  The instantaneous angle in PLL is given by

Dynamics in PPL  The 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

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:

PLL Pilot Filter  We can use the following circuit to generate a sinusoid synchronized with the pilot:

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

Frequency-offset Loop  We can use the following PLL to realize the synthesize the sum of two frequencies:

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

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.

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

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

FM Detection  When 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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