A proven way to send an analog signal a long distance is to transport the signal as a frequency using a voltage-to-frequency converter (VFC), a special circuit whose output is a frequency that is proportional to its input voltage.

In principle, it is relatively easy to send a frequency signal over a long transmission path without interference through optical, coaxial, or radio links. The frequency is then reconverted to an analog voltage by an FVC that is usually tailored to perform its inverse function, often using a phase-locked loop (PLL)

This project uses an odd approach to design and build a hobby-level VFC module using a handful of easily available and inexpensive electronic components.

A schematic diagram of the VFC module is shown in Fig. 3. Please note, a well-regulated 5V DC power source is absolutely necessary to run this VFC circuit. The heart of the circuit is the venerable CMOS micropower phase-locked loop IC HEF4046B.

The HEF4046B (IC1) includes a VCO sub-system in addition to phase comparators and other elements. It can generate a square wave with a 50% duty cycle. The frequency is reasonably close to the 0Hz-1kHz range, which is curbed by the 100nF capacitor (C1) and the 10k resistor (R1).