There are many applications that require stability in the operation, such as telecommunications, servos, and test equipment. They are using frequency-accurate sine-wave sources, but to find frequency-accurate sine-wave sources with satisfactory level of absolute accuracy and drift can be a problem, even if there are many such sine-wave oscillators are available.
To get less drift and greater accuracy ws can derive the sine wave from a digital source. We can gain desired fundamental sinusoid by removing the harmonics with a lowpass filter,because square waves consist of a fundamental at the square-wave frequency plus an infinite number of odd harmonics. This application fits using switched-capacitor filters. IC3 is an 8th-order, low-pass Butterworth type.
Switching or sweeping the frequency applied at C1 has a proportional effect on the sine-wave generator output Because the filter’s input and clock frequencies have a fixed ratio of 1:128. Switching or sweeping the frequency
will not affect output amplitude because this band is well below the smoothing filter’s 25kHz corner frequency. The frequencies that represent a potential cause of aliasing in this circuit—the odd-numbered harmonics that exceed half the clock rate—have insignificant amplitudes. So we do not have problem with Alias frequencies.
