High speed, high accuracy fractional-N frequency synthesizer using nested mixed-radix digital Δ-Σ modulators

If the modulus of the DΔΣM in a fractional-N frequency synthesizer is a power of two, then the output frequency is constrained to be a rational multiple of the phase detector frequency (fPD), where the denominator of the rational multiplier is a power of two. If the required output frequency is not related to fPD in this way, one is forced to approximate the ratio by using a small programmable modulus DΔΣM or a very large power of two modulus. Both solutions involve additional hardware. In addition, the programmable modulus solution can suffer from spurs, while the large power of two lacks accuracy. This paper presents a new solution, based on mixed-radix algebra, where the required ratio is formed by combining two different moduli. The programmable modulus solves the accuracy problem, while the large power of two modulus minimizes the spur content. In addition, the phase detector can be clocked at high speed.