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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