DIGITAL FILTER FREQUENCY-SHIFT MODULATOR

Unity gain feedback places a second-order digital filter on the borderline of stability. The filter therefore oscillates in a numerical sense. Two feedback multipliers, each capable of determining a different central coefficient (and thus different oscillation frequencies) are alternatively inserted into an independent feedback path under control of an input baseband data signal whereby the output frequency is shifted in accordance with the input data. Phase discontinuities and amplitude variations due to the frequency shift are eliminated by extracting the numbers stored in the filter when a data transition occurs and reinserting new numbers representing samples of the new frequency wave, the new numbers further defining points on the new wave having the same instantaneous amplitude and phase as the wave samples defined by the extracted numbers.