This voltage to frequency converter (VFC) circuit uses 555 IC and 741 op-amp as the main components. Up to 20kHz oscillation can be produced by this circuit. Here is the schematic diagram:

Voltage To Frequency Converter circuit schematic

The zero adjustment is used to adjust the lowest frequency, short the input to ground and adjust this pot to obtain the lowest frequency limit. This circuit uses symmetric supply +5, 0 (ground), and -5V. The output is a squarewave with the frequency varies in accordance with the input voltage at 1okHz per Volt conversion. [Circuit’s schematic diagram source:]