TLC2933_13 Datasheet, PDF (14/24 Pages) Texas Instruments – HIGH-PERFORMANCE PHASE-LOCKED LOOP

TLC2933
HIGHĆPERFORMANCE PHASEĆLOCKED LOOP
SLAS136B − APRIL 1996 − REVISED JANUARY 2002
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
VI
T1 = C1R1
VO
C1
(a) LAG FILTER
R1
VI
T1 = C1R1
T2 = C1R2
VO
R2
C2
C1
VI
(b) LAG-LEAD FILTER
C2
R2 ^C1
−A
VO
R1
T1 = C1R1
T2 = C1R2
(c) ACTIVE FILTER
Figure 17. LPF Examples for PLL
the passive filter
The transfer function for the low-pass filter shown in Figure 17(b) is;
VO
VIN
+
1
1)
)s_@
s_@
(T1
T2
)
T2)
(2)
where
T1 + R1 _@ C1 and T2 + R2 _@ C1
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)
+
1
)
s
s @ R2
@ ^R1 @
@ C1
C1
(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.
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