Voltage‐To‐Frequency Converters
Fernando Vidal‐Verdú, Rafael De Jesus Navas-Gonzalez, Angel Rodriguez-Vazquez · Encyclopedia of RF and Microwave Engineering · 2005
Abstract Voltage‐to‐frequency‐converters (VFCs) do typically employ relaxation oscillators to generate an output signal whose frequency depends linearly on an input voltage. VFCs are data converters; they are designed to codify the input voltage by means of the output frequency. Hence, they have strong requirements regarding linearity, dynamic range, and stability. On the other hand, compared to other converters, they have the advantage of low cost, particularly for large resolutions; 13 bit resolution is typical, and more than 20 bits are feasible. However, in such a case several milliseconds may be necessary to complete the conversion—the smaller the resolution, the shorter the conversion time. The serial frequency modulated output provided by VFCs is particularly well suited for telemetry, to send the outcome of a measurement by wire, radio, or fiberoptic links. After the frequency‐modulated signal is received, it can be easily codified into a digital word, or converted back into a voltage by a frequency‐to‐voltage converter. This type of output simplifies galvanic isolation through the use of either an optocoupler or a magnetic coupler. The serial output is also useful for increasing system compactness if throughput requirements are not high. Although many smart monolithic sensors have serial frequency‐modulated outputs, a VFC is needed in case we have an output voltage like that of a Wheatstone bridge after being amplified by an instrumentation amplifier, or the output of a generating sensor such as a thermocouple. This article deals first with the concepts behind voltage to frequency conversion, and then describes the main types of VFCs on the market, their limitations and main applications. Frequency‐to‐voltage converters are also briefly described.