TLC2942 Datasheet, PDF (19/28 Pages) Texas Instruments – HIGH-PERFORMANCE DUAL PHASE-LOCKED LOOP BUILDING BLOCK

TLC2942
HIGH-PERFORMANCE DUAL PHASE-LOCKED LOOP BUILDING BLOCK
SLAS146B – NOVEMBER 1996 – REVISED JUNE 1997
APPLICATION INFORMATION
low-pass-filter (LPF) configurations
Many excellent references are available that include detailed design information about LPFs and they should
be consulted for additional information. Lag-lead filters or active filters are often used. Examples of LPFs are
shown in Figure 25. When the active filter of Figure 25(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 25. LPF Examples for PLL
the passive filter
The transfer function for the low-pass filter shown in Figure 25(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 26.
the active filter
When using the active filter shown in Figure 25(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 25(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 27.
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