chapter 7. analog communication system husheng li the university of tennessee
TRANSCRIPT
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Chapter 7. Analog Communication SystemHusheng LiThe University of Tennessee
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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.
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Superhet Principle In the superhet principle, there are two distinct
amplification and filtering sections prior to demodulation: RF section and IF section.
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Parameters of AM and FM
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Illustration of Spectrum
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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.
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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.
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HomeworkDeadline: Nov. 18th, 2013
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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.
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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
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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
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Operation of Spectrum AnalyzerThe 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.
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MultiplexingThe basic multiplexing techniques include FDM,
TDM and CDM. The multiple access techniques include FDMA, TDMA and CDMA, and OFDM.
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FDM
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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.
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Example: FDMA Satellite System
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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.
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TDM
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Synchronization MarkersMarkers are needed for time synchronization
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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
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Crosstalk in PPMThe avoidance of crosstalk in PPM requires
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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.
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GSMGSM is a hybrid of TDMA and FDMA
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Homework Deadline: Nov. 25th, 2013
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Phase Locked LoopsA PLL uses phase comparator
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Phase Dynamics in PLLThe instantaneous angle in PLL is given by
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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
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ConvergenceA necessary condition for the stead-state
solution is given by
When the error is very close to zero, we have the following approximation:
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PLL Pilot FilterWe can use the following circuit to generate a
sinusoid synchronized with the pilot:
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Synchronous DetectionWhen there is no pilot (e.g., in DSB), we can
use the following Costas-PLL to lock the phase:
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Frequency-offset LoopWe can use the following PLL to realize the
synthesize the sum of two frequencies:
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Frequency MultiplicationWe can us the following PLL and frequency
divider to realize the multiplication of a frequency with an integer:
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Adjustable Local OscillatorWe can use the following circuits to obtain
100kHz and 1.6MHz and adjustable LO that covers 9.90—9.99 MHz.
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QuizUse the following figure to explain the purpose
of phase locked loop and its operation procedure:
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Linearized PLLThe frequency domain model of PLL can be
obtained by linearizing the PLL:
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FM DetectionWhen the input is FM signal, the PLL can be
approximated by a first-order lowpass filter:
The output is given by
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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.
1 Time(seconds)
0-1