Advantages of Shift Registers over DLLs for Flexible Low Jitter Multiphase Clock Generation

In this paper, we compare a shift register (SR) to a delay-locked loop (DLL) for flexible multiphase clock generation, and motivate why a SR is not only more flexible but often also better. For a given power budget, we show that a SR almost always generates less jitter than a DLL, assuming both are realized with current-mode logic. This is due to differences in jitter accumulation and the possibility to choose latch delays in a SR much smaller thanthe delays of DLL elements. For N-phase clock generation, a SR also functions as a divide-by-N and requires a voltage-controlled oscillator with N-times higher frequency. However, this does not necessary lead to more power consumption and can even have advantages like higher Q and less area for the inductors.