phase margin in the fractional-N PLL

[khsung]

Let's assume that I am designing a fractional-N PLL.
2nd order MASH sigma-delta modulator is used.
The division ratio is selected as one of the four values(4,5,6, and 7) according to the output of the sigma-delta modulator output.
Let's assume that the effective division ratio is 5.3.

In the above case, when i calculate the phase margin,

Should i calculate the phase margin assuming N=5.3?

or

Should i calculate the phase margin assuming N=4,5,6, and 7 (four times) and then choose the worst phase margin?

whis is right?

 

[vbhupendra]

you should check PM with Highest division ratio (in ur case 7).

 

[xiaorui2007]

Hi vbhupendra
Why should we use highest division ratio? You mean that we should use worst case for phase margin? Then if we use transfer function to do analyse for total pll, which division ratio should we use?

thanks very much!

 

[eng_Semi]

you should check your phase margin using the arithmetic mean or the geometric mean of N

Arithmetic mean = (Nmax + Nmin)/2
Geometric mean = sqrt(Nmin*Nmax)