Voltage to frequency converter (VFC) is an oscillator whose frequency is linearly proportional to control voltage. There are two common VFC architectures: the current steering multivibrator and the charge-balance VFC. For higher linearity, the charge-balancing method is preferred. The charge balanced VFC may be made in asynchronous or synchronous (clocked) forms. The synchronous charge balanced VFC or "sigma delta" (Σ-Δ) VFC is used when output pulses are synchronized to a clock. The charge balance VFC is more complex, more demanding in its supply voltage and current requirements, and more accurate. It is capable of 16 to 18 bit linearity. Σ-Δ modulator can be used for synchronous VFC (SVFC). The synchronous behaviour is good in many applications, but the output of SVFC is not a pure tone (plus harmonics) like a conventional VFC, but it contains components harmonically related to the clock frequency. The SVFC produces a change in probability density of output pulses N and N+1 clock cycles after the previous output pulse. In this paper, the modified SVFC (MSVFC) is described. This MSVFC works similarly as conventional SVFC but it has a pure tone on output (for constant input voltage). Therefore, it is possible to measure the period of MSVFC output (this does not work for SVFC).
Keywords:
voltage to frequency converter, charge balanced, sigma delta modulator
4th International Conference on Advanced A/D and D/A Conversion Techniques and their Applications (together with 7th IMEKO TC4 Workshop on ADC Modelling and Testing)
