STW81103 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'STW81103' PDF : 53 Pages View PDF
Circuit description
STW81103
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=BxP+A
FVCO
=
(---B-----×------P------+-----A----)
R
×
Fref
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
VCOBUF-
Prescaler
VCOBUF+
16/17 or 19/20
modulus
To PFD
5-bit
A-counter
12-bit
B-counter
20/53
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