A virtually jitter-free fractional-N divider for a Bluetooth radio

Using a fractional-N PLL as modulator in a transmitter has gained great interest recently. In order to maintain or even improve the performance of the transmitter, the jitter of the PLL has to be kept low. When a power-efficient zipper divider is employed in the PLL, its jitter must be considered because of jitter accumulation and scaling in the divider. Trying to apply the proven direct reclocking technique to combat jitter fails here due to the asynchronous nature of the zipper divider. This fact motivates us to devise a new and very effective technique called double-reclocking to remove jitter. It is demonstrated that the double-reclocking is even able to remove jitter that is too large for the direct reclocking scheme, if it worked here, to cope with.