This application note discusses how to gain greater accuracy and less drift by deriving a sine wave from a digital source. This results in a stable, frequency-accurate since wave, which is ideal for servos, test equipment, and telecommunications systems.
Servos, test equipment, and telecommunications systems are among the applications that require stable, frequency-accurate sine-wave sources. Many such sine-wave oscillators are available, but finding one with a satisfactory level of absolute accuracy and drift can be a problem. You can get greater accuracy and less drift by deriving the sine wave from a digital source. Because square waves comprise a fundamental at the square-wave frequency plus an infinite number of odd harmonics, you can obtain the desired fundamental sinusoid by removing the harmonics with a lowpass filter. Switched-capacitor filters suit this application (Figure 1). IC3 is an 8th-order, lowpass Butterworth type.