Every circuit contains some electromagnetic (EM), and radio frequency (RF) waves which interact with other elements, leading to circuit failure. Although there are several shielding options to block the EM or RF interferences, not all of them are equally efficient. Various tests are performed to evaluate the effectiveness of different circuit board shielding solutions. Below are two such techniques which can determine how efficient a shield is.
Nested Chamber theory
A nested chamber consists of two halves. The larger one encloses the smaller reverberation chamber (RC) and both of them employ paddle wheels for stirring modes within each cavity. There is a window in the smaller chamber using which the material to be tested is mounted over the aperture. This chamber also encloses a receiver antenna for measuring the field inside. In the large chamber, on the flipside, a transmission antenna is placed which generates the reference field.
To measure the shielding property of a material, at first, the aperture is kept open, and the received power in the smaller chamber is measured. Then, the window is kept covered with the material and again the power is measured. The ratio of these two measurements is considered as shielding effectiveness (SE) of the material of interest.
While measuring the SE of a material, it is assumed that the two chambers are coupled tightly with the open aperture. This system works when the window is larger than the small chamber which has a high-quality factor. If the chambers are loosely coupled, the aperture is assumed to provide some inherent isolation. Another important consideration for this test is, the difference in loading of the large chamber with and without the smaller chamber will be negligible.
Each element of the nested chamber plays a special role in the SE measurement. For instance, the aperture which can be any hole, imperfect joint or ventilation grid, is one of the major routes for EM radiation penetration, and this factor may affect the continuity of the shield. The incidence angle together with polarization of incident wave determines the transmission cross-section. There are several techniques based on intermediate level tools, full numerical models, and small hole theory for handling aperture penetration.
The positioning of the aperture is an important aspect of getting the accurate results. The EM field distribution depends upon the antenna placement. So, the aperture should be in the area which is exposed to the strongest field. Such a positioning would excite the aperture the most, and the material of interest will receive the maximum power leaks when covering the aperture. An extensive mathematical calculation s needed to identify the strongest field.
Sometimes, a double wings stirrer is used to reduce the influence of aperture location on SE measurement test. In this case, the angular position of stirrer determines how the maximum current and associated field will move.
Dual TEM Cell Measurement
A TEM (Transverse Electromagnetic) cell, a linear transducer, can convert RF signals into EM field. Due to the bidirectional nature, a dual TEM cell, which is a system consisting of two TEM cells coupled through a common aperture, can measure EM field.
How to measure SE using Dual TEM Cell?
In a Dual TEM cell system, the test-object is mounted on the aperture. The upper cell or the driving cell acts as power transmitter whereas the lower cell works as the receiver. This system stimulates near-field conditions. For any input signal, the output can be measured in either the forward or backward port while a 50 Ohm load is used to terminate the other port. As energy is coupled asymmetrically at two output ports, the electric and magnetic coupling of the sample can be determined by adding or subtracting the two output signals, using a hybrid junction device or a vector network analyzer.
Both the nested chamber and the dual TEM cell techniques help measure the effectiveness of circuit board shielding accurately. Although the type of facility and equipment used in the test sometimes affect the findings of the RC method, its other advantages, namely its robustness, repeatability and better prediction of uncertainties make it a popular choice. The TEM cell method is preferred for its lower cost and smaller size.