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 R7.
R1, R2, and C1 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.
