Radio 1985, 2
By using an op-amp and a 555 timer you can easily make a simple voltage to frequency converter that has good parameters (see figure below).
The timer is used in typical configuration, except that the timing resistor is replaced with the current source based on the op-amp DA1 (741). This solution allowed to obtain a good linearity (better than 3%).
With the value of components used in this circuit the output frequency will increase linearly from 0 to 21 kHz while the input voltage increases from 0 to 5 V (the input voltage controls the output frequency with the relationship 4.2 kHz/V).
In order to get good linearity, the values of resistors R1-R3, R5 may not deviate more than 0.5%.
Linearni prevodnik napeti kmitocet. - Amaterske Radio, 1984, N 4. p. 152.