AN2536 Freescale Semiconductor / Motorola, AN2536 Datasheet - Page 2

• Manufacturer Part Number: AN2536
• Description: MC9328MX1 and MC9328MXL High Speed Layout Design Guidelines
• Manufacturer: Freescale Semiconductor / Motorola

Design Consideration
Where Q
of dielectric (F/m). Each PCB substrate has a different relative dielectric constant. The dielectric constant
is permittivity of a relative to that of empty space, see Equation 2.
Where
dielectric constant compares the effect of an insulator on the capacitance of a conductor pair, to the
capacitance of the conductor pair in a vacuum. The dielectric constant affects the impedance of a
transmission line, and signals can propagate faster in materials that have a lower
A high-frequency signal that propagates through a long line on the PCB from driver to receiver is severely
affected by the loss tangent of the dielectric material. A large loss tangent means higher dielectric
absorption. Material with a high loss tangent value affects the high frequency signal on a long line.
Dielectric absorption increases attenuation at higher frequencies. Table 1 shows the loss tangent value for
FR-4 and GETEK materials.
The most widely used dielectric material for PCBs is FR-4, a glass laminate with epoxy resin that meets a
wide variety of processing conditions. The
that can be used in high speed boards. GETEK is composed of epoxy and resin (polyhenylene oxide) and
has an
Another factor to affect signal performance and noise separation is transmission line effect and modeling.
Transmission line is a trace, and has a distributed mixture of resistance (R), inductance (L), and
capacitance (C). There are two types of transmission line layouts: Microstrip and Stripline:
Any circuit trace on the PCB has characteristic impedance associated with it. This impedance is dependent
on width (W) of the trace, thickness (T) of the trace, dielectric constant (
height (H) between the trace and reference plane.
Microstrip Impedance
A circuit trace routed on an outside layer of the PCB with a reference plane (i.e., GND or Vcc) below it,
constitutes a microstrip layout. Use Equation 3 to calculate the impedance of a microstrip trace layout.
2
ε
ε
r
r
1
between 3.6 and 4.2.
is dielectric constant,
and Q
Microstrip Transmission Line Layout
GE Electromaterials
Isola Laminate Systems
2
are charges, r is distance between the charges (m), F as force (N) and
Table 1. Loss Tangent Value of FR4 and GETEK Materials
Manufacturer
Freescale Semiconductor, Inc.
For More Information On This Product,
ε
Figure 1. Transmission Line Layout
o
MC9328MX1/MXL Application Note
is permittivity of empty space (F/m), and
Go to: www.freescale.com
ε
ε
r
r
W = width of trace, T = thickness of trace, and H = height between trace and reference plane
for FR-4 is between 4.1 and 4.5. GETEK is another material
=
ε
/
ε
o
Material
GETEK
FR-4
Stripline Transmission Line Layout
Loss Tangent Value
ε
r
0.019 @ 1MHz
) of the material used, and
0.010 @ 1MHz
ε
is permittivity (F/m). The
ε
r
.
ε
is permittivity
MOTOROLA
Eqn. 2