TLC2943 Datasheet, PDF (20/27 Pages) Texas Instruments – HIGH-PERFORMANCE DUAL PHASE-LOCKED BUILDING BLOCK

TLC2943
HIGH-PERFORMANCE DUAL PHASE-LOCKED BUILDING BLOCK
SLAS249 – NOVEMBER 1999
APPLICATION INFORMATION
low-pass-filter (LPF) configurations
References that include detailed design information about LPFs should be consulted for additional information.
Lag-lead filters or active filters are often used. Examples of LPFs are shown in Figure 19. When the active filter
of Figure 19(c) is used, the reference should be applied to F_IN-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 19. LPF Examples for PLL
passive filter
The transfer function for the low-pass filter shown in Figure 17(b) is;
+ ^{) VO}
1 s T2
) ) VIN 1 s (T1 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 20.
active filter
When using the active filter shown in Figure 19(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 F_IN-B terminal
and the output of the VCO divider should be applied to the input reference terminal, F_IN-A.
The transfer function for the active filter shown in Figure 19(c) is:
+ ⁾ F(s)
1 s R2 C1
s R1 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 21.
20
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