Thanks for the files. Actually I am searching for VCO (delay cell) based on interpolation. By the way, there is subchapter in Razavi's book but the discussion / analysis is limited. There are two references referred by Razavi. I found S.K.Enam & A.A.Abidi paper, but for B.Lai & R.C Walker, "A monolithic 622 Mb/sec Clock Extraction and Data Retiming Circuit," ISSCC Dig. Tech. Papers, pp144-145, Feb, 1991. paper still didn't found it.