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. 18th, 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 goodquality 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 interchannel 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. 25th, 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/discretefourier-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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