A square waveform filtered by a high-order low-pass filter in which -3-dB frequency
is lower than signal frequency will eliminate most harmonics of the waveform.
As a result, the filter outputs a fundamental sine waveform. This method is
applied to generate a sine waveform by using a switched-capacitor filter (MAX292)
(*see the figure*). This circuit offers
wide frequency range (0.1 Hz to 25 kHz), low distortion, and constant output
amplitude in the whole frequency range.

MAX292 (IC2) is an 8th-order low-pass Bessel filter. Its -3-dB corner frequency is determined by its clock frequency divided by 100. In other words, the corner frequency is 1 kHz if the clock 100 kHz. IC1 (74HC4060) is a 14-bit binary counter with a built-in oscillator. Its Q4 generates IC2's clock and Q10 sends a square waveform as IC2's input. Because the frequency ratio of the clock and input is 64:1 (which is lower than the -3-dB ratio), only the square waveform's fundamental can partially pass through the filter, with about 9-dB loss. The third harmonic will be down about 65 dB.

Thus, the filter outputs a pure sine waveform with approximately a 2.5-V p-p
amplitude. Because the ratio is a constant, output amplitude will also be constant
throughout the entire frequency range. IC2 contains an uncommitted op amp that's
used to offset output dc level by adjusting resistor R_{7}.

R_{1}, R_{2}, and C_{1} determine IC1 oscillation
frequency, which is 16 times the output frequency. Useful output-frequency range
is from 0.1 Hz to 25 kHz. By using the component values in the figure, it's
possible to achieve a frequency range of 80 Hz to 7.5 kHz.