Communications: using direct digital synthesis

Introduction

Direct Digital Synthesis ( DDS ) is a method for generating a desired waveform (such as a sine wave)by using the technique described in below.

Quantized samples of a desired waveform are stored in the memory of the microprocessor system. This desired waveform can then begenerated by "playing out" the stored words into the digital-to-analog converter. The frequency of this waveform isdetermined simply by how fast the stored words are read from memory, and is thus programmable. Likewise, the phase andamplitude of the generated waveform are programmable.

The DDS technique is replacing analog circuits in many applications. For example, it is used in higher-pricedcommunication receivers to generate local oscillator signals. It can also be used to generate sounds in electronic pipe organsand music synthesizers. Another application is its use by lab instrument manufacturers to generate output waveforms infunction 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 installedbetween the 6-channel card and the DSP card at some (not all) lab stations. You can tell which boxes have them by the way the6-channel card sits higher inside the metal box.

Frequency modulation (fm) radio exercise

To get your feet wet and see a demonstration of the DDS, perform the following exercise. Copy the files FM.asm (downloadable here ) 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 inputchannels that you've been using all semester. If you have a CD on you, pop it into the computer and use that. If not, usea music web site on the Internet as your audio source. Connect the computer to the DSP by using a male-male audiocable and an audio-to-BNC converter box (little blue box), both of which are in the lab. The computer has three audiooutputs on the back; use the middle jack. Ask your TA if you can't find the cable and/or box or don't see how to make theconnection. Next, connect a dipole antenna to the output of the DDS (port \#1 on the back of the DDS board). A crude buteffective dipole antenna can be formed by connecting together a few BNC and banana cables in the shape of a T . There should be one or two of these concoctions in thelab. Once the connections are made, turn on the black receiver in the lab, and tune it to 104.9 MHz (wide band FM). Youshould be able to hear your audio source!