One of the main parts of frequency–time-domain smart sensors is the frequency (period, duty-cycle or time interval)-to-code converter. This unit directly influences such sensor metrological characteristics, as the accuracy and the conversion time as well as the power consumption. In spite of the fact that the frequency can be converted into digital code more precisely in comparison with other informative parameters of the signal, in practice, it is not a trivial task of simple time-window counting. Besides, very often a consumer or a sensor manufacturer is not an expert in the area of frequency-time measurements. Let us consider the most popular frequency-to-code conversion discrete methods and give an analysis of metrological performances, conversion frequency ranges and requirements for realization with the aim of choosing the most-appropriate conversion method.
The formation and development of this perspective began after the publication in 1947 of the first patent on the electronic ADC with narrow time intervals offered by Filipov and Negnevitskiy in 1941. This converter was based on the discrete counting method of normalized impulses of the reference frequency (Patent 68785, USSR).
Today, there are many patents concerning various conversions and measuring methods of frequency–time parameters of electrical signals. The main methods are: