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EE354 Spring 2021 Lab 5: Superheterodyne Receivers
In this lab you will be using MATLAB to design and simulate the functions of a superheterodyne receiver with AM and FM demodulation. Your design will be verified via hardware implementation in the subsequent lab.
Part 1: Pre-Lab Theory
The figure below is a block diagram of a basic superheterodyne receiver. Your objective is to design the system such that it meets the following specifications:
Transmitted Signals: 1. DSB-TC AM with audio modulation (fmax = 5 kHz) at 900 kHz. 2. Wideband FM with audio modulation (fmax = 15 kHz, β = 2.0) at 450 kHz.
Receiver IF Frequency: 100 kHz
At a minimum, your design must specify the following:
AM FM
Receiver RF frequency: ______Receiver RF frequency: ______
Receiver LO frequency (HSI): ______Receiver LO frequency (HSI): ______
Receiver LO frequency (LSI): ______Receiver LO frequency (LSI): ______
Image frequency (HSI): ______Image frequency (HSI): ______
Image frequency (LSI): ______Image frequency (LSI): ______
Image Reject Filter Center Freq: ______Image Reject Filter Center Freq: ______
Image Reject Filter BW: ______Image Reject Filter BW: ______
IF Filter BW: ______IF Filter BW: ______
IF RF RF IF mt( ) To Demod LO fC = XX MHz LO fC = 100 kHz BW = XX MHz BW = XX kHz
fLO fLO Figure 1: Block diagram of a DSB-TC Superheterodyne Communication System
------End of Pre-Lab ------
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Part 2: Simulation
The instructions below will step you through creating a superheterodyne receiver in Matlab and combining the IF output with your AM/FM demodulation code from class.
RF IF rtIF ( ) rt( ) ˆ mt( )
filter_bandpass.m LO
cos( 2π ft) LO Figure 2: Block diagram of a DSB-TC Superheterodyne AM Communication System Implementation in Matlab.
For your design, you may choose to implement either HSI or LSI. However, be aware that distractor signals abound!
*** Note 1: You will need the filter_bandpass.m function available from the course website, as well as the lab_5_signals.mat file.
*** Note 2: This particular lab will likely demonstrate why Matlab is such a major memory hog. The instructor highly recommends minimizing the number of intermediate variables you create and clearing things you no longer need. Additionally, consider only having one spectrum plot active at a time (comment out the others).
*** Note 3: The instructor highly recommends creating two different Matlab implementations. One for the AM signal and one for the FM signal.
1. Load up lab_5_signals.mat into Matlab. That file will place three variables into your workspace: • fs_rf: The sampling frequency used to generate the signals you will be working with. • s_all: The amplitude samples of the signals you will be working with. • time: The corresponding time vector to the amplitude samples.
2. Use the spec_analysis function and plot the spectrum of the combined signal to verify that you have the appropriate signals at the appropriate frequencies. You should be able to confirm which signal is AM and which is FM by examining the spectrum.
3. Using the block diagram as your guide, implement the superheterodyne receiver to generate rtIF ( ). It is highly recommended that you examine the output of each stage of the superhet receiver (at the output of the Image Reject Filter, output of the IF BPF, and LO signal). Confirm each time that you are getting the output you intended.
4. Did the IF signal show up at the desired frequency? Is the signal bandwidth and number/amplitude of sidebands correct? Why or why not? If you have errors, correct your design before proceeding.
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5. Once you have the desired rtIF ( ), implement your AM and FM demodulator to recover the original information signal. Output the resulting demodulated signal via the PC’s sound card.
Recall: Your sampling frequency is likely set based on the frequency and bandwidth the RF signal. As a result, the sampling frequency will be many times larger than the PC’s sound card can handle. To obtain an acceptable audio sampling frequency, we have to decimate (remove redundant samples from) the demodulated signal using the downsample command.
Things to consider for your lab report
RF Signal Spectrum IF Signal Spectrum IF Signal in Time Domain (demonstrating AM or FM modulation) Recovered mtˆ ( ) in the Time Domain Matlab code demonstrating downconversion and demodulation Discussion on filter parameters and why you chose those specific ones for your lab
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