STW81103 datasheet(20/53 Pages) STMICROELECTRONICS

20 / 53 page

Circuit description

STW81103

20/53

5.4

A and B counters

The 5-bit A-counter and 12-bit B-counter, in conjunction with the selected dual modulus

(16/17 or 19/20) prescaler, allow the generation of output frequencies that are spaced only

by the reference frequency divided by the reference division ratio. The division ratio and the

VCO output frequency are given by the following formulas:

N = B x P + A

where

FVCO: output frequency of VCO

P: modulus of dual modulus prescaler (16 or 19 selected through the digital interface)

B: division ratio of the main counter

A: division ratio of the swallow counter

Fref: input reference frequency

R: division ratio of the reference counter

N: division ratio of the PLL

For the VCO divider to work correctly, B absolutely must be greater than A, which can take

any value ranging from 0 to 31. The value range of N is either from 256 to 65551 (if P=16) or

from 361 to 77836 (P=19).

Figure 16.

VCO divider diagram

VCO

()

------------------------------

ref

5-bit

12-bit

To PFD

modulus

VCOBUF+

VCOBUF-

Prescaler

16/17 or 19/20

B-counter

A-counter

STW81103 Datasheet(PDF) 20 Page - STMicroelectronics