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Unformatted text preview: Connexions module: m10657 1 Communications: Using Direct Digital Synthesis * Matthew Berry This work is produced by The Connexions Project and licensed under the Creative Commons Attribution License † Abstract This module introduces Direct Digital Synthesis with prepared code that transmits arbitrary input as an FM radio signal. The module then explains how to program the DDS hardware and concludes with a simple Frequency Shift Keying exercise. 1 Introduction Direct Digital Synthesis ( DDS ) is a method for generating a desired waveform (such as a sine wave) by using the technique described in Figure 1 below. Figure 1: Direct digital synthesis (DDS) ( Couch ) Quantized samples of a desired waveform are stored in the memory of the microprocessor system. This desired waveform can then be generated by "playing out" the stored words into the digital-to-analog con- verter. The frequency of this waveform is determined simply by how fast the stored words are read from memory, and is thus programmable. Likewise, the phase and amplitude of the generated waveform are programmable. * Version 2.5: Feb 25, 2004 11:51 am US/Central † http://creativecommons.org/licenses/by/1.0 http://cnx.org/content/m10657/2.5/ Connexions module: m10657 2 The DDS technique is replacing analog circuits in many applications. For example, it is used in higher- priced communication receivers to generate local oscillator signals. It can also be used to generate sounds in electronic pipe organs and music synthesizers. Another application is its use by lab instrument manufacturers to generate output waveforms in function generators and arbitrary waveform generators ( Couch ). In this lab you will familiarize yourself with the capabilities of the Analog Devices AD9854 DDS. The DDS board is installed between the 6-channel card and the DSP card at some (not all) lab stations. You can tell which boxes have them by the way the 6-channel card sits higher inside the metal box. 2 Frequency Modulation (FM) Radio Exercise To get your feet wet and see a demonstration of the DDS, perform the following exercise. Copy the les FM.asm (downloadable here (p. 4)) and mod.asm from the v: \ ece320 \ 54x \ dds \ directory. Assemble and run the frequency modulation (FM) program FM.asm . Next, plug an audio source into one of the two DSP input channels that you've been using all semester. If you have a CD on you, pop it into the computer and use that. If not, use a music web site on the Internet as your audio source. Connect the computer to the DSP by using a male-male audio cable and an audio-to-BNC converter box (little blue box), both of which are in the lab. The computer has three audio outputs on the back; use the middle jack. Ask your TA if you can't nd the cable and/or box or don't see how to make the connection. Next, connect a dipole antenna to the output of the DDS (port...
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