Shields are your friend, except when… article tells that engineers just love to put shields on circuits, mostly as a defensive measure against signals on the outside getting into and disturbing our circuits, but they also keep signals inside from getting out and this really makes the folks responsible for EMI compliance happy. Even on low-frequency circuits, shielding can take care of drift due to air currents and AC power mains pickup. At higher frequencies shielding can take care of emissions and pickup.
So what could go wrong when you add shielding? Well 10 years ago, not much. We were still working at frequencies up to 2.5 GHz for almost all the standard wireless applications. And shields generally only helped the situation. Then we considered “commonplace RF” to be anything below 3 GHz.
But today we consider “commonplace” to be below 6 GHz, and we may routinely need to lay out circuits that operate to 6 GHz even in consumer circuits. Things start getting different at those higher frequencies: When we place a shield on a circuit board we are creating a conductive electrical cavity and this cavity likes certain frequencies or various TEM (Transverse ElectroMagnetic) modes. In other words it becomes resonant at certain frequencies.
It is possible to predict the frequencies where this starts to happen by using simple math shown in Shields are your friend, except when… article. This equation from article gives the lowest frequency (TEM101 Mode) that the shield cavity can resonate at (W and L are biggest box dimensions in meters,).
Shields are your friend, except when… (Part 2) article looks at some solutions for those resonant cavity problems.
First off we can make the shielding box smaller in the longest directions. This will increase the resonant frequencies inside the box, hopefully outside you circuit operating frequency. The box itself can shrink or we can add walls and sub divide the box.
Various combinations of absorbers can also be used to control resonances. These absorbing materials can be placed on the circuits themselves or to the sides of boxes. While adding absorber directly to circuit board can sometimes fix misbehaving circuits, but it is not recommended in general because placing these materials on carefully designed impedance controlled traces will change the impedance of the circuits.