CONVERTERS FOR DIFFERENT VARIABLES TO FREQUENCY-TIME PARAMETERS OF THE ELECTRIC SIGNAL
Apart from the various types of frequency sensors considered below there is another big group of analog sensors with current, voltage or resistance output. These are, for example, thermocouples, potentiometric sensors etc. When using the frequency signal as the sensor's informative parameter, it is expedient to use voltage (current)-to-frequency converters of the harmonious or pulse signal in the measuring channel. Such converters are frequently used in practice due to their good performance: linearities and the stability of transformation characteristics, accuracy, frequency ranges, manufacturability and simplicity.
2.1 Voltage-to-Frequency Converters (VFCs)
The operational principle of majority VFC consists in alternate integration of input voltage and generation of pulses when the integrator's output voltage equals to the reference voltage. VFC with continuous integration have special advantages. Such converters do not require additional time for returning the integrator to an initial condition at the beginning of a new conversion cycle.
Initially such converters were developed based on magnetic materials with a rectangular hysteresis loop. These were the elementary magneto-transistor multivibrators with input frequency directly proportional to the input voltage [65,66]. Various modifications with improved characteristics and possibilities [67–69] were further developed. For example, a pulse-width ...