Effective Impedance And Tolerance/Variation; Power/Reference Planes, Pcb Stackup, And High Frequency Decoupling - Intel VC820 - Desktop Board Motherboard Design Manual

Advanced System Bus Design
Symbols for
•
S is the speed of the signal on an unloaded PCB in ns/ft. This is referred to as the board
0
propagation constant.
•
S
0 MICROSTRIP
stripline trace on the PCB in ns/ft.
is the intrinsic impedance of the line in Ω and is a function of the dielectric constant (ε
•
Z
0
line width, line height and line space from the plane(s). The equations for Z
in this document. See the MECL System Design Handbook by William R. Blood, Jr. for these
equations.
•
C is the distributed trace capacitance of the network in pF/ft.
0
•
L is the distributed trace inductance of the network in nH/ft.
0
•
C is the sum of the capacitance of all devices and stubs divided by the length of the network's
D
trunk, not including the portion connecting the end agents to the termination resistors in pF/ft.
•
S
EFF
board is "loaded" with the components.
3.4.2

Effective Impedance and Tolerance/Variation

The impedance of the PCB needs to be controlled when the PCB is fabricated. The method of
specifying control of the impedance needs to be determined to best suit each situation. Using
stripline transmission lines (where the trace is between two reference planes) is likely to give better
results than microstrip (where the trace is on an external layer using an adjacent plane for reference
with solder mask and air on the other side of the trace). This is in part due to the difficulty of
precise control of the dielectric constant of the solder mask, and the difficulty in limiting the plated
thickness of microstrip conductors, which can substantially increase cross-talk.
The effective line impedance (Z
Equation
3.4.3 Power/Reference Planes, PCB Stackup, and High
Frequency Decoupling
3.4.3.1 Power Distribution
Designs using the Pentium III processor require several different voltages. The following
paragraphs describe some of the impact of two common methods used to distribute the required
voltages. Refer to the Flexible Motherboard Power Distribution Guidelines for more information
on power distribution.
The most conservative method of distributing these voltages is for each of them to have a dedicated
plane. If any of these planes are used as an "AC ground" reference for traces to control trace
impedance on the board, then the plane needs to be AC coupled to the system ground plane. This
method may require more total layers in the PCB than other methods. A 1-ounce/ft
recommended for all power and reference planes.
A second method of power distribution is to use partial planes in the immediate area needing the
power, and to place these planes on a routing layer on an as-needed basis. These planes still need to
be decoupled to ground to ensure stable voltages for the components being supplied. This method
has the disadvantage of reducing area that can be used to route traces. These partial planes may also
3-20
Equation 3-5 through Equation
and S
0 STRIPLINE
and Z are the effective propagation constant and impedance of the PCB when the
EFF
EFF
3-10.
3-12:
refer to the speed of the signal on an unloaded microstrip or
) is recommended to be 60 Ω ±15%, where Z
are not included
0
is defined by
EFF
2
thick copper is
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Intel 820 Chipset Design Guide
), the
r