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Revision 3.6

36 - 47

AS3977

Data Sheet - A p p l i c a t i o n I n f o r m a t i o n

Example:

Wanted frequency 425 MHz by 16 MHz crystal frequency.

The values of P and M can be taken from table

P=1

→ Defined with the BSEL Bit’s BSEL<1:0>= Hex “2”

M=5

→ INT<9>= 5-4 = 1

The Setting of DIVR<2:0> = 2 results in a reference frequency divider of 4.

The Division ratio will now be:

(EQ 18)

And can be split into the values:

N = whole numbered part of

(EQ 19)

As the Rest is zero, we have to set AC=M and N to decrease by one.

N=20

INT < 3:0 >= N-10 = 20-10=10 = Hex „A“

AC=5

INT < 7:4 >= AC-1= 5-1=4 Hex „4“

9.8.2 Optional Frequency Calculation with Overlapping Fractional Bands by Using the Bit INT<8>

frequency trimming independently on the required output frequency and selected crystal. Note that setting INT<8> high increases the PLL in-

band noise by approximately 3dB. Therefore it is recommended to use this option only if the required frequency band did not fit to the crystal

reference.

Steps for INT<8>=1

DR

f

RF

Pf

REF

⋅

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

433 92

,

R

1.4

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

108 48

,

DR

INT

DR

FRAC

+

==

=

=

DR

INT

3

–

()

M

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

108

3

–

()

5

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

21

==

FRAC

FRAC

FRAC

FRAC

FRAC

FRAC

FRAC

FRAC

FRAC

FRAC

FRAC

FRAC

Frequency Band

INT=N-2

INT=N-1

INT=N

INT=N+1

INT8=1

INT8=0

Modulation on the Integer border with

INT<8> = 0 not possible

Modulation on the Integer border with

INT<8> = 1 not possible