The VFC in this circuit puts out an adequate sine-shaped output, but does not have good V-to-F linearity, and its frequency stability is not much better than 0.2%. An LM331 makes an excellent linear stable V-to-F converter, with a pulse output; but it can not make sines. But it can command, via a PLL, to force the sine VFC to run at the correct frequency. Simply connect the sine VFC of this circuit into one of the PLLs, instead of the LM331 VFC circuit. Then use a precise linear low-drift VFC based on the LM331 to establish the FIN to the PLL. If the voltage needed by the sine VFC to put out a given frequency drifts a little, that is okay, as the integrator will servo and make up the error. The use of a controlled sine wave generator in a test system was the first of many applications for a wide range phase locked loop circuit. This is a picture schematic:

*Source circuit : National Semicondutor Application*