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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. -1 0 1 Time (seconds)