EMI, EMC, and Shielding Advanced EMC Topics Informational

What is the resonant frequency of a shielded compartment on a PCB and how do I avoid problems at that frequency?

Q: How do I know if the cavity resonance is a problem?

The resonance is a problem if: the resonant frequency falls within the operating frequency band of any circuit inside the compartment, the cavity Q is high enough to cause significant field amplification (Q > 10), and the circuit inside has gain at the resonant frequency (potentially causing oscillation). Symptoms: unexplained gain peaks, oscillation, or degraded isolation at specific frequencies that correspond to the calculated cavity modes.

A shielded compartment (shield can or frame) on a PCB forms a rectangular cavity resonator with resonant frequencies determined by its internal dimensions. At resonance, electromagnetic fields are amplified inside the compartment, which can: increase coupling between circuits within the compartment (defeating the purpose of the shield), create feedback paths that cause oscillation in amplifiers, and reduce the shielding effectiveness at the resonant frequency. The resonant frequencies of a rectangular cavity are: f_mnp = (c / (2 sqrt(Er))) x sqrt((m/a)^2 + (n/b)^2 + (p/d)^2), where a, b, d are the internal dimensions of the cavity (width, length, height), m, n, p are the mode indices (integers, at least two must be non-zero), Er is the dielectric constant of the substrate filling part of the cavity, and c is the speed of light. The lowest resonant mode (TE101 or TE110 depending on the dimensions) has the lowest frequency. For a typical PCB shield can (20 x 15 x 3 mm on FR4, Er = 4.4): the lowest resonant frequency is approximately f_101 = c / (2 x sqrt(4.4)) x sqrt((1/0.02)^2 + 0 + (1/0.003)^2) approximately 24 GHz. This is well above most digital and RF circuit frequencies. However, larger compartments (> 30 mm) or higher-frequency circuits (> 10 GHz) can have resonances within the operating band.

Category: EMI, EMC, and Shielding

Updated: April 2026

Product Tie-In: Shielding, Gaskets, Absorbers, Filters

PCB Shield Can Cavity Resonance

Shield cans are widely used on PCBs to isolate sensitive RF circuits, protect receivers from transmitter leakage, and contain emissions from noisy digital circuits. Understanding and managing cavity resonance is essential for shield can design, especially at mmW frequencies where even small compartments can resonate.

Resonance Management

Keep dimensions small: The lowest resonant frequency is approximately c/(2 sqrt(Er) × L_max) where L_max is the largest internal dimension. For the lowest resonance to be above 10 GHz on FR4: L_max < 7 mm. This is practical for small MMIC compartments but difficult for larger circuit blocks
Add absorber material: Placing microwave absorber sheets (ferrite-loaded foam or silicone) inside the compartment dampens the resonance by increasing the cavity loss. This reduces the Q of the cavity mode from hundreds to single digits, eliminating the resonance peak
Add internal ground vias: A row of ground vias within the compartment divides it into smaller sub-cavities, each with a higher resonant frequency. Effective for pushing the resonance above the operating frequency

PCB Shield Can Resonance Calculations

Cavity resonance: f_mnp = c/(2√Er) × √((m/a)² + (n/b)² + (p/d)²)
Lowest mode (TE101): f_101 = c/(2√Er) × √((1/a)² + (1/d)²)
For 20mm × 15mm × 3mm on FR4 (Er=4.4):
f_101 = 3e8/(2×2.1) × √(2500 + 111111) = 24 GHz
For 50mm × 40mm × 5mm: f_101 = 5.0 GHz (may be problematic)

Common Questions

Frequently Asked Questions

How do I know if the cavity resonance is a problem?

The resonance is a problem if: the resonant frequency falls within the operating frequency band of any circuit inside the compartment, the cavity Q is high enough to cause significant field amplification (Q > 10), and the circuit inside has gain at the resonant frequency (potentially causing oscillation). Symptoms: unexplained gain peaks, oscillation, or degraded isolation at specific frequencies that correspond to the calculated cavity modes.

What absorber thickness do I need?

For effective damping: the absorber should occupy a significant fraction of the cavity volume or be placed at the location of maximum magnetic field (near the walls for TE modes). Typical absorber sheets are 0.5-2 mm thick and reduce the cavity Q by a factor of 10-100. Even a thin (0.5 mm) ferrite-loaded sheet can reduce the Q from several hundred to less than 10, which is sufficient to prevent resonance problems.

Do vias in the ground plane help?

Ground vias from the top ground plane to the bottom ground plane (stitching vias) are essential around the perimeter of the shield can to ensure a continuous ground connection and prevent slot mode propagation. Additional stitching vias inside the compartment create internal walls that subdivide the cavity. Via spacing should be < lambda/10 at the highest problem frequency. For 10 GHz operation: via spacing < 1.5 mm in FR4.

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