tsb_nph,
Thanks for your reply, thanks for the paper, I also have that paper and read it.
But I find if load resistor is realized by active device like MOS transistor, they more or less have some non linearity. And one problem occur (as shown in the attached figure): The non-linear load resistor makes the crossover point of input and output at different voltage level. And this makes delay between clk1 and clk2 not equal to delay between clk2 and clk3! Since delay is mesured at the same voltage level.