A frequency-voltage converter (FVC) is a circuit which responds to the frequency of its input and delivers an output voltage which is linearly proportional to that input frequency. It is unusual for an integrated circuit to be designed as an FVC, but most voltage-frequency converters (VFCs) may be reconfigured as FVCs. There are three obvious FVC architectures: leaky integrators, phase-locked loops (PLLs), and counter-timer/DAC systems. All have their advantages and disadvantages, all are widely used, and all are widely misunderstood. This module considers the architecture and characteristics of these.