Our simulations using the validated multisegment simulation program with integrated circuit emphasis (SPICE) model show that the terahertz (THz) spectrometers using plasmonic field effect transistors could be used for the frequency to digital conversion. The THz SPICE unified charge control model that uses the distributed channel resistances, capacitances, and Drude inductances is validated by technology computer-aided design simulations. The THz spectrometers using Si plasmonic field effect transistors (or TeraFETs) determine the frequency of the impinging radiation by measuring the gate bias where the responsivity changes sign. This crossover frequency is sensitive to the channel length and nearly insensitive to the gate bias in the weak and moderate inversion regions. This unique feature allows the use of multiple THz spectrometers with different channel lengths to implement the frequency-to-digital converters in the sub-THz and THz frequency range. The simulations predict the operation frequency from 110 GHz up to 4 THz for the frequency-to-digital converters using Si TeraFETs with feature sizes from 20 to 130 nm.