Understanding AM Modulation: Spectrum Analysis in Lab #10
School
National Career College**We aren't endorsed by this school
Course
BA 225
Subject
Electrical Engineering
Date
Dec 12, 2024
Pages
5
Uploaded by BarristerGuineaPigMaster1197
Lab #10 AM RECEIVER, TRANSMITTER AND SPECTRUM A Very Brief Introduction to Communications The Digilent Analog Discovery Studio Oscilloscope also includes an FFT (Fast Fourier Transform) feature. The Digilent Oscilloscope FFT is basically a Spectrum Analyzer. You need to understand a few basic things from communications to appreciate what the Digilent Oscilloscope FFT displays. Amplitude modulation (AM) covers a class of modulation systems in which the amplitude of the main carrier is the characteristic that is varied. AM is sometimes simplistically described as varying the amplitude of the carrier from zero power to a peak power level. The modulation process produces sidebands, which are bands of frequencies on both sides of the carrier frequency. AM results in three products in the frequency range of interest: (1) the carrier (2) the lower sideband (LSB) (3) the upper sideband (USB) Thus, if a carrier of 10 MHz were modulated by a 1000 Hz sine wave, the outputs would be as shown below. Answer the following two questions: (1) Radio station KCSP in Kansas City is located at 610 kHz on the AM broadcasting band. What is its carrier frequency? (2) If KCSP is broadcasting a 1000 Hz sinusoidal tone, at what frequency are the upper and lower sidebands located?
Introduction In this lab, we will be examining Amplitude Modulation and a simple Spectrum Analyzer. A sinusoidal signal is Amplitude Modulated with another sinusoid. All spectrums are analyzed with the FFT feature in the Digilent Oscilloscope in the Digilent Analog Discovery Studio system. Equipment and Components 1- 15,000 ohm resistor 1- 100,000 ohm resistors 741 Op Amp Digilent Analog Discovery Studio system The Basic Transmitter On the Wavegen Generator Channel 1 select Modulation in the drop down menu. Check the AM checkbox if necessary and uncheck the FM checkbox. Set the Carrier Type to Sine, Frequency to 50 kHz, Amplitude/Index to 1V. Set the AM Type to Sine, Frequency to 2 kHz, Amplitude Index to 10%. For both Carrier and AM set offset at 0%, Symmetry at 50%, and Phase at 0o. The Wavegen 1 should look like this:
~ FFT (Fast Fourier Transform) Click the FFT icon on the Digilent oscilloscope. Attach the output of the Wavegen1 to the input of the Digilent Oscilloscope Channel 1. On the FFT menu there are various options which may be adjusted such as: Window, Type, Start, Center, Stop, Span, Top, Units, and Bottom. For descriptions of these settings see the Help feature under Scope->Views->FFT, and Spectrum->Traces->Magnitude, and Spectrum->Traces->Trace. Experiment with all of these settings and note their function for your Lab Report. You also may need to adjust the Channel 1 Range setting. Adjust the various Oscilloscope FFT settings mentioned until you obtain a display similar to the following. [Some settings to start with are: Window->Rectangular, Type->Sample, Start->40 kHz, Center->50 kHz, Stop->60 kHZ, Span->20 kHz, Top-> 40 dBV, Units-> 40 dBV, Bottom-> -160 dBV.] The frequency spectrum of the received signal is displayed. Note that the high peak is at 50 kHz, the frequency of the sine wave that you are transmitting and receiving. ~ ~
There should be peaks on each side of the central peak. The two side peaks (sidebands) should be 2 kHz on either side of the central peak. These are the upper sideband and lower sideband resulting from Amplitude Modulation. Snip the Oscilloscope screen so that it can be pasted into your lab report. Adjust the AM frequency of Wavegen1 to different values around 2 kHz and note how the upper and lower sidebands move closer to and away from the carrier. The Receiver A high gain amplifier is built using a 741 Op Amp in a simple configuration. Tie the 15 kiloohm resistor to pin 2 with the other end connected to ground. Use the 100 kiloohm resistor for the feedback resistor between pins 2 and 6. Power is from +12 volts to pin 7 and from -12 volts to pin 4. Nominally the Op Amp has a gain of 6.67. The Wavgen1 output is connected to pin 3. The Op Amp output pin 6 is connected to the Digilent oscilloscope Channel 1 input. Here is the schematic: Adjust the settings on the Digilent oscilloscope FFT screen. You also may need to adjust Channel 1 Range setting. Adjust the various Oscilloscope FFT settings mentioned until you obtain a display somewhat similar to the following:
Note the extra peaks in the spectrum caused by the op amp. Snip the Oscilloscope FFT screen so that it can be pasted into your lab report. (1)Include snips of your Spectrum results. (2)Describe the function of the various FFT controls in your Lab Report such as: Window, Type, Start, Center, Stop, Span, Top, Units, and Bottom (3)Find a commercial spectrum analyzer on the Internet and discuss its features and price in your Lab Report. Here is a place to start: https://www.keysight.com/us/en/product/N9322C/basic-spectrum-analyzer-bsa-9-khz-7-ghz.html The Keysight Basic Spectrum Analyzers cost about $11,000 to $15,000. [Keysight was spun off from Agilent in 2014. Agilent was spun off from Hewlett-Packard in 1999.]