Digital 90-degree phase shifter without pll or dll

A novel digital 90-degree phase shifter is described that avoids the use of conventional PLL or DLL. The phase shifter is suitable for use in DDR-type high data-rate communication. Two key novel circuits in the phase shifter are discussed, one of which has fast lock-in process and the other of which has wide work range and ability against variation of PVT. Simulation shows that the phase shifter can work at high frequency and is insensitive to variation of PVT