AN2321 Freescale Semiconductor / Motorola, AN2321 Datasheet - Page 26

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

AN2321/D
Vias
45 ° angled tracking
Stubs
Star signal
arrangement
26
Vias or via holes are commonly used for multi-layer PCBs. On high speed
signals, a via introduces 1 to 4nH of inductance and 0.3 to 0.8pF of
capacitance to the track. Hence, vias should be kept to an absolute minimum
when laying high speed signal tracks. If layer changes are unavoidable on high-
speed parallel lines (e.g. address and data lines), make sure the number of vias
are the same on each signal line.
Similar to vias, right-angled track turns should be avoided because it can
produce a field concentration at the inner edge. This field can cause noise that
can be coupled to nearby tracks. Therefore, all orthogonal tracking should be
45 ° when making turns.
Stubs produce reflections as well as the potential of adding wavelength
divisible aerials to the circuit. Although a stub length may compute to be a non-
quarter wavelength integer of any known signal in the system, incident
radiation may resonate on a stub. Therefore, avoid producing stubs with tracks
carrying high frequency and sensitive signals.
Although star arrangements are suitable for bonding ground rails from multiple
PCBs, with a signal track this introduces multiple stubs. Therefore, the star
arrangement should be avoided for high speed and sensitive signals.
Designing for Board Level Electromagnetic Compatibility
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
Figure 25
45 °
Figure 25. Angled Tracks
Figure 26. Stub Lines
W
is the general rule for 45 ° tracking.
Stubs
Stub
> 3W
W
W
MOTOROLA