STW81101AT datasheet(19/48 Pages) STMICROELECTRONICS

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STW81101

Circuit description

19/48

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, make it possible to generate output frequencies which are

spaced only by the reference frequency divided by the reference division ratio. Thus, the

division ratio and the VCO output frequency are given by these formulas:

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 reference counter

N: division ratio of PLL

For a correct working of the VCO divider, B must be strictly higher than A. A can take any

value ranging from 0 to 31. The range of N can vary from 256 to 65551 (P=16) or from 361

to 77836 (P=19).

Figure 16.

VCO divider diagram

VCO

) F

ref

(

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

PC00403

5-bit

12-bit

To PFD

modulus

VCOBUF+

VCOBUF-

Prescaler

16/17 or 19/20

B-counter

A-counter

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