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Experiment 5 – Amplitude modulation Preliminary discussion In an amplitude modulation (AM) communications system, speech and music are converted into an electrical signal using a device such as a microphone. This electrical signal is called the message or baseband signal. The message signal is then used to electrically vary the amplitude of a pure sinewave called the carrier. The carrier usually has a frequency that is much higher than the message’s frequency. Figure 1 below shows a simple message signal and an unmodulated carrier. It also shows the result of amplitude modulating the carrier with the message. Notice that the modulated carrier’s amplitude varies above and below its unmodulated amplitude. Figure 1 5-2 © 2007 Emona Instruments Experiment 5 - Amplitude modulation Figure 2 below shows the AM signal at the bottom of Figure 1 but with a dotted line added to track the modulated carrier’s positive peaks and negative peaks. These dotted lines are known in the industry as the signal’s envelopes. If you look at the envelopes closely you’ll notice that the upper envelope is the same shape as the message. The lower envelope is also the same shape but upside-down (inverted). Figure 2 In telecommunications theory, the mathematical model that defines the AM signal is: AM = (DC + message) × the carrier When the message is a simple sinewave (like in Figure 1) the equation’s solution (which necessarily involves some trigonometry that is not shown here) tells us that the AM signal consists of three sinewaves: One at the carrier frequency One with a frequency equal to the sum of the carrier and message frequencies One with a frequency equal to the difference between the carrier and message frequencies In other words, for every sinewave in the message, the AM signal includes a pair of sinewaves – one above and one below the carrier’s frequency. Complex message signals such as speech and music are made up of thousands sinewaves and so the AM signal includes thousands of pairs of sinewaves straddling carrier. These two groups of sinewaves are called the sidebands and so AM is known as double-sideband, full carrier (DSBFC). Importantly, it’s clear from this discussion that the AM signal doesn’t consist of any signals at the message frequency. This is despite the fact that the AM signal’s envelopes are the same shape as the message. Experiment 5 – Amplitude modulation © 2007 Emona Instruments 5-3 The experiment In this experiment you’ll use the Emona DATEx to generate a real AM signal by implementing its mathematical model. This means that you’ll add a DC component to a pure sinewave to create a message signal then multiply it with another sinewave at a higher frequency (the carrier). You’ll examine the AM signal using the scope and compare it to the original message. You’ll do the same with speech for the message instead of a simple sinewave. Following this, you’ll vary the message signal’s amplitude and observe how it affects the modulated carrier. You’ll also observe the effects of modulating the carrier too much. Finally, you’ll measure the AM signal’s depth of modulation using a scope. It should take you about 1 hour to complete this experiment. Equipment Personal computer with appropriate software installed NI ELVIS plus connecting leads NI Data Acquisition unit such as the USB-6251 (or a 20MHz dual channel oscilloscope) Emona DATEx experimental add-in module two BNC to 2mm banana-plug leads assorted 2mm banana-plug patch leads 5-4 © 2007 Emona Instruments Experiment 5 - Amplitude modulation Procedure Part A - Generating an AM signal using a simple message 1. Ensure that the NI ELVIS power switch at the back of the unit is off. 2. Carefully plug the Emona DATEx experimental add-in module into the NI ELVIS. 3. Set the Control Mode switch on the DATEx module (top right corner) to PC Control. 4. Check that the NI Data Acquisition unit is turned off. 5. Connect the NI ELVIS to the NI Data Acquisition unit (DAQ) and connect that to the personal computer (PC). 6. Turn on the NI ELVIS power switch at the back then turn on its Prototyping Board Power switch at the front. 7. Turn on the PC and let it boot-up. 8. Once the boot process is complete, turn on the DAQ then look or listen for the indication that the PC recognises it. 9. Launch the NI ELVIS software. 10. Launch the DATEx soft front-panel (SFP). 11. Check you now have soft control over the DATEx by activating the PCM Encoder module’s soft PDM/TDM control on the DATEx SFP. Note: If you’re set-up is working correctly, the PCM Decoder module’s LED on the DATEx board should turn on and off.