DP83241 datasheet(10/20 Pages) NSC

10 / 20 page

Loop Filter Calculations

Several constants need to be known in order to determine

the loop filter components They are the loop divide ratio N

the phase detector gain Kp the VCO gain Ko the loop

bandwidth Wo and the phase margin w The constants Kp

and Ko for the DP83241 are fixed at 80 mArad and 08

GradV respectively N is equal to the VCO frequency divid-

ed by the reference frequency wo is recommended to be

less than 120th of the reference frequency (times 2q

rads) Having found all these constants the following equa-

tions are used to find the component values

For w e 57 phase margin

R1 e (11 N wo)(Kp Ko)

C1 e (3 Kp Ko)(N wo2)

C2 e (03 Kp Ko((N wo2)

For a phase margin other than 57

R1 e (Cosec w a 1) ((N wo 2 Kp Ko))

C1 e (Tan w) ((2 Kp Ko)(N wo2))

C2 e (Sec w b Tan w) ((Kp Ko (N wo2))

The component equations above are not meant to provide

optimal solutions for all implementations

Let us now design an example system with the following

characteristics

125 MHz Crystal reference

250 MHz VCO

Since the VCO is twenty times the frequency of the refer-

ence frequency we get N e 20 We will set wo to be 130th

of the reference frequency or 262 x 106 Rad

From these values we get

C1 e 1400 pF C2 e 140 pF and R1 e 900X

Let us now design an example system with the following

characteristics

125 MHz Crystal reference

250 MHz External VCO with a gain of 40 MRadV

We will set wo to be 178th of the reference frequency or

1 0x106 Rad

From these values we get

C1 e 470 pF C2 e 47 pF and R1 e 68 kX

TLF10385 – 27

DP83241 Datasheet(PDF) 10 Page - National Semiconductor (TI)