TLC2933

HIGH-PERFORMANCE PHASE-LOCKED LOOP

SLAS136A – APRIL 1996 – REVISED JUNE 1997

14

POST OFFICE BOX 655303

APPLICATION INFORMATION

low-pass-filter (LPF) configurations

Many excellent references are available that include detailed design information about LPFs and should be

consulted for additional information. Lag-lead filters or active filters are often used. Examples of LPFs are shown

in Figure 17. When the active filter of Figure 17(c) is used, the reference should be applied to FIN-B because

of the amplifier inversion. Also, in practical filter implementations, C2 is used as additional filtering at the VCO

input. The value of C2 should be equal to or less than one tenth the value of C1.

R1

C1

T1 = C1R1

(a) LAG FILTER

R1

C1

T1 = C1R1

T2 = C1R2

R2

(b) LAG-LEAD FILTER

R2

C1

R1

T1 = C1R1

T2 = C1R2

(c) ACTIVE FILTER

A

–

VI

VO

VI

VO

VI

C2

VO

C2

Figure 17. LPF Examples for PLL

the passive filter

The transfer function for the low-pass filter shown in Figure 17(b) is;

V

O

V

IN

+

1

) s

1

) s

where

T1

+ R1

(2)

Using this filter makes the closed-loop PLL system a type 1 second-order system. The response curves of this

system to a unit step are shown in Figure 18.

the active filter

When using the active filter shown in Figure 17(c), the phase detector inputs must be reversed since the filter

adds an additional inversion. Therefore, the input reference frequency should be applied to the FIN-B terminal

and the output of the VCO divider should be applied to the input reference terminal, FIN-A.

The transfer function for the active filter shown in Figure 17(c) is:

F(s)

s

(3)

Using this filter makes the closed-loop PLL system a type 2 second-order system. The response curves of this

system to a unit step are shown in Figure 19.