MDS 663 D

6

Revision 062904

ICS663

Determining the Loop Filter Values

Calculating these values is best illustrated by an

example. Using the example in Figure 1, we can

synthesize 20 MHz from a 200 kHz input.

The phase locked loop may be approximately

described by the following equations:

Bandwidth

Damping factor,

where:

N = Total feedback divide from VCO,

including the internal VCO post divider

As a general rule, the bandwidth should be at least 20

times less than the reference frequency, i.e.,

In this example, using the above equation, bandwidth

should be less than or equal to 10 kHz. By setting the

be determined since all other variables are known. In

the example of Figure 1, N = 200, comprising the divide

by 2 on the chip (VCO post divider) and the external

divide by 100. Therefore, the bandwidth equation

becomes:

Choosing a damping factor of 0.7 (a minimal damping

factor than can be used to ensure fast lock time),

damping factor equation becomes:

value).

charge pump and should be approximately 1/20th the

To summarize, the loop filter components are:

⋅⋅

()

2

π N

⋅

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

=

ζ

2

------

⋅⋅

N

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

=

BW

REFIN

() 20

⁄

≤

10,000

⋅⋅

2

π 200

⋅

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

=

0.7

25 000

,

2

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

⋅⋅

200

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

=

⁄

≅