Fref of delta-sigma modulator in PLL

A 3-order MASH modulator in fractional-N pll, each stage uses a 20-bit accumulator. How fast can it work? And generally what frequency does it work at?
Thanks!

it works at the PLL reference frequency, which is about few of MHz

Thanks! I know the PLL Fref is the frequency it works.(The real working frequecny should be the divided vco frequency,right?) I want to know if my Fref is about 20MHz, does it need special adder? Or normal adder can reach it? And how about 4th-order MASH?

how can i realize a 20 bit accumulator via matlab function to be used in simulink to simulate the mash 111