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How does cavity resonance affect EMI/RFI shielding?

May 14, 2020

Electronic devices and the circuit boards that control them are trending smaller and faster all the time. High-speed applications operate at increasing frequencies within the radio and microwave ranges of the electromagnetic (EM) spectrum. The result is slick consumer and industrial electronics but the challenge for designers lies in controlling EM radiation emissions and the impact they can have on performance.

Protecting circuits with EMI/RFI shielding

EM radiation can affect signals from the same or other devices as it travels between circuits or devices, disrupting signals or making it so that data sent or received is incomplete or incorrect. This interference affects performance and operation of circuits, and in turn components like sensors, signal receivers, and transmitters in vehicles, appliances, medical devices, and more.

Metal shields can control interference by reflecting or absorbing EM waves. Shields are used to create a physical barrier between a circuit and its neighbors. Most are board-mounted, that is, attached directly to a printed circuit board (PCB). They can be simple five-sided rectangular boxes or may contain additional internal partitions to separate critical components.

Cavity resonance

One thing that can affect shielding effectiveness (SE) is cavity resonance. This phenomenon happens when wavelengths of surrounding EM frequencies (from other devices or circuits) correspond to the dimensions of the shield.

In shield design, it’s important to know the minimum frequency that will cause the shield (i.e. cavity) to resonate. Generally speaking, when shield dimensions approach 1/2 the wavelength of a given frequency, the cavity will resonate. This formula is used to calculate the lowest frequency (in MHz) at which a rectangular shield will resonate:

see equation 2 on this page:

To avoid this effect, designers need to know the longest wavelength of the resonant frequency range and plan shield dimensions accordingly.

Cavity resonance and shield effectiveness

When a wave at or above the minimum resonant frequency enters the cavity, as Eric Bogatin explains, “each time the wave reflects from the front end [of the cavity], the new reflections are coincident with and add to the old waves. The net wave will build to higher and higher amplitude, limited by the energy leaking into the cavity and the losses of the signal while in the cavity.”

The resonance creates a strong magnetic field inside the shield, which can cause any apertures (e.g. holes or gaps) to leak radiation. That can interfere with other circuits and create or add to EM emissions leaks. Resonance can also create a secondary coupling path between circuits. The overall result is more “noise” within the device, which can impact data transmission or reception between circuits and potentially cause devices to malfunction or fail.

A factor that can amplify the effect of cavity resonance is the Q or “quality” factor of the shield. A high Q factor slows energy loss in the shield and keeps it vibrating (i.e. resonating) longer. The Q factor can be lowered by adding an absorber inside the metal shield. These materials, including foams and elastomer sheets, have the effect of damping cavity resonance by speeding up energy loss and slowing vibrations. The size and placement of an absorber within a shield is unique to each application.

Reducing the dimensions of the shield box or adding partitions to further divide circuits is another way to reduce cavity resonance; however, ”making the shielding box smaller is not an unlimited option … as the shield interface on the PCB takes up PCB area where it has to be soldered down. So adding more and more walls to make the shield sections smaller and smaller eventually ends up with a big PCB where all the space is taken up by the shielding tracks with no room for circuitry,” according to author Steve Hageman.

All of this points to the importance of custom metal shielding based on the application and its environment – design a shield for your application to best balance speed, performance, and emissions control. At CEP Technologies, we partner with you to achieve optimal components, progressive stamped to spec every time.

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