A Low Cost DOS Function Generator

摘要

I recently began experimenting with narrow-band filters for use in a WWVB receiver and found that my ancient Hewlett Packard 3310A function generator was woefully inadequate as a test sig-nal source—the tuning rate was too high and the frequency stability too low. Rather than spend a lot of money for a new one I decided to try to build one. My first attempt was a Wien Bridge oscillator which was no better. Then I ran across a thread on the sci.electronics.design newsgroup discussing an Analog Devices direct digital synthesis (DDS) chip, the AD9833. After studying the data sheet a bit I decided this was just the component I needed. The AD9833 DDS IC is a low power programmable waveform generator capable of producing sine, triangle and square wave outputs at frequencies from zero to one-half the clock frequency. It can be programmed via a 3-wire serial interface—the parallel port of an IBM compatible PC can easily do the job and the necessary software is readily available. The internal frequency registers are 28 bits, which means that the resolution of the generated signal is the clock frequency divided by 2~(28)—for a 10 MHz clock that is less than 0.04 Hz. It is also inexpensive, less than $10 in unit quantities, and it provides much of the functionality of a commercial function generator at a fraction of the cost.