AN2321 Freescale Semiconductor / Motorola, AN2321 Datasheet - Page 27

• Manufacturer Part Number: AN2321
• Description: Designing for Board Level Electromagnetic Compatibility
• Manufacturer: Freescale Semiconductor / Motorola

Radiating signal
arrangement
Constant track width
Hole and via
concentrations
Split apertures
Ground metallized
patterns
Minimize loop areas
MOTOROLA
A radiating signal arrangement is usually the shortest tracking and causes
minimum delay from source to all receivers, but this can also cause multiple
reflections and radiated interference. Again, this should be avoided for high
speed and sensitive signals.
The width of a signal track should be constant from driver to load. Varying track
width creates changes in track impedance (resistance, inductance, and
capacitance) and, consequently, can cause reflections and line impedance
imbalances. It is better to keep a track thin rather than vary its width.
A concentration of via holes that pass through the power and ground planes
produce a localized impedance difference near the holes. The area not only
becomes a “hot spot” of signal activity, but the supply planes are high
impedance at this point and less effective as RF sinks.
This is the same with hole and via concentrations, split apertures (i.e. long
holes or wide vias) in power and ground planes create an area of non-
uniformity within the planes and reduce their effectiveness as shields, as well
as locally increasing the impedance of the power and ground planes.
All metallized patterns should be connected to ground, otherwise these large
metal areas can act as radiating aerials.
Keeping signal tracks and its ground return close together will help to minimize
the ground loop, thus, avoiding a potential aerial loops. With high speed single-
ended signals, sometimes the ground return may also have to be tracked
alongside the signal if the signal does not track over a low impedance ground
plane (see
Designing for Board Level Electromagnetic Compatibility
Freescale Semiconductor, Inc.
For More Information On This Product,
Re-routing the ground
to reduce ground loop
Figure
Go to: www.freescale.com
27).
Figure 27. Ground Return Loops
PART 3: PRINTED CIRCUIT BOARD LAYOUT TECHNIQUES
GND
Critical signal
return
loop
AN2321/D
27