74HCT9046A Datasheet, PDF (29/43 Pages) NXP Semiconductors – PLL with bandgap controlled VCO

NXP Semiconductors
74HCT9046A
PLL with band gap controlled VCO
13.1 Filter design considerations for PC1 and PC2 of the 74HCT9046A
Figure 30 shows some examples of passive and active filters to be used with the phase
comparators of the 74HCT9046A. Transfer functions of phase comparators and filters are
given in Table 9.
Table 9. Transfer functions of phase comparators and filters
Phase
comparator
Explanation
Figure
Filter type
PC1
K_PC1 = -^V---_π-^C---^C--V /r
Figure 30a passive filter without
damping
τ₁ = R3 × C2;
τ₂ = R4 × C2;
τ₃ = R4 × C3;
A = 10⁵ = DC gain
amplitude
Figure 30b passive filter with
damping
Figure 30c active filter with
damping
PC2
K_PC + ₄--⁵--_π--V /r
Figure 30d
τ₁ = R3’ × C2;
τ₂ = R4 × C2;
τ₃ = R4 × C3;
Figure 30e
R3' = R_bias/17;
R_bias = 25 kΩ to 250 kΩ
passive filter with
damping
active filter with
damping
Transfer function
F_{( jω)} = ₁-----₊-----¹-_j--_ω----_τ---₁-
F_{( jω)} = ₁-----₊----¹--_j--_ω⁺----₍--^j-_τ-^ω-₁---^τ-₊--²---_τ---₂---₎
F_{( jω)} = -₁---¹_/--_A--⁺---₊---^j--^ω-_j--_ω-^τ---²_τ---₁- ≈ ¹-----⁺-_j--_ω---^j-_τ-^ω--₁--^τ---²-
F_{( jω)} = -₁---¹-_⁄--_A--⁺----₊-^j--^ω--_j--_ω-^τ---²-_τ---₁ ≈ ¹-----⁺-_j--_ω---^j-_τ-^ω--₁--^τ---²-
A = 10⁵ = DC gain amplitude
F_{( jω)} = -₁---¹_/--_A--⁺---₊---^j--^ω-_j--_ω-^τ---²_τ---₁- ≈ ¹-----⁺-_j--_ω---^j-_τ-^ω--₁--^τ---²-
A = 10⁵ = DC gain amplitude
Table 10. General design considerations
Subject
Phase comparator
PLL locks on harmonics at
PC1
center frequency
PC2
Noise rejection at signal input PC1
PC2
AC ripple content when PLL is PC1
locked
PC2
Design consideration
yes
no
high
low
f_r = 2f_i; large ripple content at Φ_{PC_IN} = 90°
f_r = f_i; small ripple content at Φ_{PC_IN} = 0°
74HCT9046A_6
Product data sheet
Rev. 06 — 15 September 2009
© NXP B.V. 2009. All rights reserved.
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