50 The PCB Design Magazine • September 2017
PLANE CAVITY RESONANCE
• When the cavity has open end boundary
conditions, resonances arise when an integral
multiple of half wavelengths can fit between
the ends of the cavity.
• When the clock or data harmonics overlap
with the cavity resonant frequencies, there is
the potential for long range coupling between
any signals that run through the cavity.
• Stitching vias between GND planes can be
placed adjacent to each signal via transition to
minimize the possibility of exciting the cavity
resonance.
• The frequency components of the voltage
noise are related to the peak impedance of the
cavity and the frequency components of the re-
turn currents.
• The goal of designing a high-performance
cavity is to push the peak frequency compo-
nents, above the bandwidth of the signals, and
to reduce the impedance peaks below the target
impedance level.
• A thin dielectric, in the plane cavity, is the
most effective way of reducing the peak ampli-
tude of the modal resonance.
• A dielectric material with a high dielectric
constant (Dk) should be selected to add more
planar capacitance.
• Effective ways of pushing up the parallel
resonant frequencies is by reducing the plane
size and by adding stitching vias between (sim-
ilar) planes of a cavity.
• Avoid square planes and simple L:W ratios
by choosing irrational numbers.
• Make the power planes slightly smaller
than the GND plane. This modifies the shape
of the fringing fields, pulling them back from
the edge.
PCBDESIGN
References
1. Barry Olney's Beyond Design columns:
Learning the Curve, Plane Crazy, Part 2
2. Principles of Power Integrity for PDN De-
sign, by Larry D. Smith, Eric Bogatin
3. What is the resonant frequency of a cav-
ity?, by Eric Bogatin
4. Advanced PCB Design and Layout for
EMC, by Keith Armstrong
5. High-Speed Digital Design, by Howard
Johnson
Barry Olney is managing director
of In-Circuit Design Pty Ltd (iCD),
Australia, a PCB design service bu-
reau that specializes in board-level
simulation. The company devel-
oped the iCD Design Integrity soft-
ware incorporating the iCD Stack-
up, PDN and CPW Planner. The software can be
downloaded from www.icd.com.au. To contact
Olney, or read past columns, click here.
Figure 4: Optimized PDN with projected EMI (Source: iCD Design Integrity).
