R31UH0005EU0100 Rev.1.0
Page 7
Jun 3, 2021
8V19N49x Hardware Design Guide
Figure 7. Typical 3
rd
Order Loop Filter
The Rs, Cs, and Cp can be the actual value used in the 2
nd
order loop filter. The following equation helps
determine the 3
rd
order loop filter R3 and C3.
1. Pick an R3 value. Suggest R3 ~ 1.5xRs
Where,
γ is ratio between the 1
st
pole frequency and the 2
nd
pole frequency. Suggest γ greater than 3.
2. The Timing Commander software tool is provided to calculate the loop filter component values. To use the
spread sheet, user can enters the follow parameters:
fc, Fpd, fvco, α and β
The software tool will provide the component values, Rs, Cs, and Cp as result. The tool will also calculate
maximum phase margin for verification.
The 3
rd
order loop filter, R3 and C3, are also calculated using the actual 2
nd
order loop filter components values.
3.3
Loop Filter Calculation Examples
3.3.1
Loop Filter for VCXO PLL
3.3.1.1
Second Order Loop Filter for the VCXO PLL
This section provides calculation examples for the VCXO PLL loop filter value. The 8V19N490 VCXO phase lock
loop block diagram is displayed in
. A 2
nd
order loop filter for VCXO is shown in
. In this example,
the reference CLK input frequency = 30.72MHz and a VCXO with output frequency of 122.88MHz is used.
Figure 8. 8V19N490 VCXO PLL Block Diagram
Rs
C3
Cs
R3
Charge Pump Driv er
VCO Control Input
Loop Filter Return
Cp
