DELAY-LOCKED LOOP

摘要

A delay locked loop (DLL) is described in which a phase detector compares the phase of the output of the DLL with that of a reference input. The output of the phase comparator drives a differential charge pump which functions to integrate the phase comparator output signal over time. The charge pump output controls a phase shifter with unlimited range that adjusts the phase of the DLL output so that the output of the phase comparator is high 50 % of the time on average. Because the DLL adjusts the phase shifter until the output of the phase detector is high 50 % of the time, on average, the relationship of the DLL output clock to the input reference clock depends only on the type of phase detector used. For example, when a data receiver is used as the phase detector in the DLL, the output of the DLL is a clock signal which can be used as a sampling clock for data receivers elsewhere in the system, and is timed to sample data at the optional instant independent of temperature, supply voltage and process variations. Alternatively, a quadrature phase detector may be employed to generate a clock signal that possesses a quadrature 90° relationship with a reference clock signal input. This may be used, for example, to generate a transmit clock for a data transmission device. Furthermore, the DLL is controlled to minimize dither jitter while minimizing acquisition time. In addition, duty cycle correcting amplifiers are employed to produce a DLL output clock that has a desired duty cycle, for example 50 %. Additionally, the inputs to the charge pump are reversed in alternate quadrants of the phase plane in order to enable unlimited phase shift with a finite control voltage range.