June 19, 1997

Simple circuit generates clean sine waves

Med Dyer, Jabra Corp, San Diego, CAGenerating a square wave at a particular frequency is fairly simple. Many techniques for doing so exist, using counters, flip-flops, and--everyone's favorite--the venerable 555 timer. Generating a sine wave, however, is not nearly as simple or inexpensive. Figure 1 shows a technique for generating a high-quality sine wave from a square-wave source. Using textbook methodology, you can easily convert a square wave into a sine wave: Feed a square wave at the desired frequency into an appropriate four- to six-pole lowpass filter. The filter's output is a sine wave; the higher the order of the filter, the purer the sine wave. In practice, this conversion technique is difficult to implement, because such filters require several components and are difficult to adjust.

Fortunately, an alternative exists. Switched-capacitor filters, such as the MF6 sixth-order Butterworth lowpass filter from National Semiconductor (Santa Clara, CA), provide high-order rolloff that depends on an external frequency rather than on a collection of discrete components. By clocking such a device at an appropriate rate, you can convert a square-wave input to a pure sine-wave output. In Figure 1, the 555 timer generates a square wave at approximately 1 kHz and routes the square wave to the MF6 filter. The filter can operate in two clocking configurations. In the first, a logic-level clock at 100 times the desired cutoff frequency drives the clock input. This configuration is useful for programming applications and is suitable for high-frequency use.

The second clocking configuration uses an on-chip Schmitt trigger with an external capacitor and a resistor to generate the required 100f clock input. This second configuration is inexpensive and is suitable for fixed-frequency applications, including this example. The 1-kHz square-wave input at the MF6, combined with a 126-kHz clock, results in a 1-kHz sine wave at the output of the MF6. The sine wave is clean with harmonics that are down by more than 60 dB. Note that varying the RC combinations at the 555 and the MF6 varies the source frequency and cutoff, respectively. (DI #2045)

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