11

SP8853A/B

Fig. 11 Standard form of third order loop filter

Fig. 12 Modified form of third order loop filter

−

+

R1

C1

R2

C2

FROM

CHARGE

PUMP

TO VCO

−

+

R1

C1

R2

Cx

FROM

CHARGE

PUMP

TO VCO

R3

C3

where

values can now be substituted in equation (3) to obtain a value

for

and

Example

Calculate values for a third order loop with parameters as

From equation (5):

2

tan 45

°1

t

500Hz32p

1

cos 45

°

∴t

=

3161·6

0·4142

From equation (4):

t

2

4

1

∴t

Using these values in equation (3):

t

7·96310

2

3

[A]

11

2

2

11

3

2









where A =

=

1

2

2

2

t





500141·6

6·832

1·1714





1

2

= 6·334310

2

∴t

Now, since

t

∴C

1·53310

2

5

For Fig. 11,

t

For Fig. 12,

t

Substituting for

t

t

t

3









=

t

or,

=

7·687310

2

0·0153310

2

6

∴R

t

3

=

1·318310

2

4

41627

∴C

For Fig. 12,

t

1·53310

2

5

∴C

or,

∴R

or,

t

7·687310

2

4

1·53310

2

8

t

Since the values of

components, either can be chosen and the other determined.

Assuming that

1·318310

2

4

∴C