The circuit in Fig 1 generates a 50-Hz to 1-kHz sinusoidal waveform that exhibits lower than 60-dB THD. The input voltage controls the output frequency with the relationship 1 kHz/V. The output amplitude is invariant with frequency over the entire operating range. IC1 is a V/F converter with an input control range of 0 to 1V. You adjust R5 so that the output frequency equals 1 kHz with a 1V input.IC3 is a switched-capacitor lowpass filter. The clock frequency, provided by IC1, determines the filter's 3-dB corner frequency in the ratio of 100:1. Thus, the corner frequency is 100 Hz if IC3's clock frequency is 10 kHz. Meanwhile, IC2 divides the clock frequency by 64 to provide the input to IC3. Because the input frequency is higher than 1/100th of the clock frequency, only the fundamental of the square-wave input can partially pass through the filter. The fundamental passes through the filter with about an 8-dB loss. The total harmonic content is lower than 60 dB.
A built-in operational amplifier amplifies the filter's output. You can adjust R7 to change the output amplitude. Because the ratio of IC3's input and the clock frequency is always a constant, the output amplitude remains constant throughout the entire operating-frequency range.