Executive Summary
Electronic devices need to be certified for compliance with Electromagnetic Compatibility (EMC) standards, to ensure that they will not cause interference to other devices and withstand external interference. Simulation is increasingly used to assess EMC performance during design and to replicate certification with virtual testing. This requires a virtual twin of the device under test (DUT) and surrounding that accurately represent the test set-up.
A reverberation chamber is crucial equipment for testing the susceptibility of a device to external electromagnetic fields. The walls and stirrer of a reverberation chamber reflect electromagnetic waves and distribute them into a statistically uniform field to expose the device to interference from all directions. In this blog post, we demonstrate how a reverberation chamber can be modeled and simulated in SIMULIA CST Studio Suite.
Simulating the reverberation chamber allows test engineers to design the appropriate chamber set-up and find the best configuration for their DUT. A virtual test chamber also allows desginers to analyze susceptibility at the earliest stages of development, and resolve any problems before committing to constructing prototypes. This saves development and reduces the cost of reworking and building physical prototypes, helping manufacturers bring products to market quicker.
Background: What is EMC Simulation?
For any product to be sold, the manufacturer must be able to prove compliance with EMC standards. There are many EMC standards to meet, varying in different countries and markets. Ensuring that the product meets every relevant standard can require significant testing. Simulation reduces the time and cost associated with EMC testing in several ways:
- Many of the physical tests traditionally performed during product development can be replaced by virtual tests. Simulations can be carried out rapidly, and no physical prototype needs to be constructed.
- Simulation can reveal EMC issues early in development, before the design is locked in. If problems are identified, they can be quickly resolved with less rework.
- Regulators increasingly accept simulation results as part of the data used to support the certification process. This reduces the number of physical tests needed.
EMC considerations can be broadly divided into a few groups. Particularly important is the distinction between emissions – the potential interference due to electromagnetic fields and currents generated by the device– and susceptibility – the vulnerability of the device to interference from other devices/sources. These can be further divided into radiated and conducted, depending on the path of interference into or out of the device. Each of these requires a distinct test set-up. For radiated susceptibility, the test lab often takes the form of a reverberation chamber.
What is a Reverberation Chamber?
Reverberation chambers consist of a shielded room with walls that are reflective at the relevant frequencies. Inside the reverberation chamber is a transmitting antenna to create the electromagnetic field. The chamber acts as a resonant cavity, resulting in fields distributed throughout the test chamber. The chamber will also include a stirrer – an irregular metal surface intended to prevent standing waves and modes from forming, ensuring homogeneity and creating a consistent random field known as a statistical uniform field.
The statistical uniform field impinging on the DUT replicates real-world interference scenarios both artificial – for example, wi-fi or cellular networks – and natural – lightning and other environmental electromagnetic effects (E3).
The performance of the reverberation chamber is limited by the homogeneity of the field and the lowest usable frequency (LUF) – typically, the smaller the chamber is, the higher the LUF will be, limiting its usefulness. Test chamber engineers aim to optimize the placement and design of the antenna and stirrer in order to produce a homogenous field for a wide range of frequencies, while also maintaining a compact footprint to minimize construction costs and space use.
The Benefits of MODSIM for Reverberation Chamber Design
Integrated Modeling and Simulation (MODSIM) allows test chamber engineers to calculate the field at every point in the chamber at every frequency without a single measurement. The field homogeneity and LUF can be analyzed at the design stage, before construction begins, reducing the risk of issues emerging later.
There are many important design parameters for a reverberation chamber. These include the dimensions of the room, the geometry of the stirrer, and the position of both the stirrer and the antenna. Standards such as IEC 41000-6-21 specify a number of different stirrer positions – in the frequency range 200 – 3000 MHz, there are 50 specified stirrer positions required. An automatic parameter sweep or optimization can find a combination of values that satisfy the design requirements, and help designers to minimize the footprint of the stirrer (or the number of stirrers needed) and the total volume of the chamber.
Workflow for Modeling and Simulating a Reverberation Chamber
Detailed Chamber Model
The reverberation chamber shown above was designed and optimized in SIMULIA CST Studio Suite. This is an industry-leading electromagnetic simulation tool, which includes a number of workflows and features tailored to the specific requirements of EMC simulation. There are several solvers, which are appropriate for different aspects of simulation.CST Studio Suite includes a powerful 3D TLM solver, which is suitable for many EMC applications. The TLM solver supports octree meshing, which can reduce the mesh cell density significantly in empty space. This reduces the computational requirements for simulating the empty space of the chamber while still capturing the crucial small details of the geometry such apertrures in the enclosure, PCB details etc.
A disadvantage of TLM for reverberation chambers is that it is a time-domain method, and a high Q-factor reverberation chamber can take many microseconds to lose energy. This means that an accurate calculation of fields can require relatively long simulation time. In this case, the Frequency Domain solver (F solver) may be more appropriate. This uses adaptive tetrahedral elements to mesh the geometry and calculates the fields at each specified frequency point, to cover a desired frequency range.
Picture of FEM mesh of chamber
Once the reverberation chamber has been designed and modeled, it is ready for use. For the virtual testing of products, the precise geometry of the test chamber does not usually matter – it has already been demonstrated that the reverberation chamber produces a statistical uniform field.
Plane Wave Superposition Based Chamber Model
To accelerate virtual testing, we have developed a plane wave superposition tool in CST Studio Suite. This produces sets of plane wave excitations with random phase and polarization and uses these to simulate the electric and magnetic fields in the working volume.
A Monte Carlo method is used to iterate the sets of randomized waves to ensure that the resulting peak fields meet the uniformity requirements of the IEC standards.
To illustrate the benefit of the plane wave superposition methods, we performed an EMC test simulation of a wireless router, using the TLM solver. Instead of simulating the whole chamber, we only had to simulate a 1 cubic meter working volume. Despite simulating a wide range of frequencies (100 MHz to 7.25 GHz), we were able to simulate the EMC performance of the router efficiently. The simulation revealed coupling paths through seams in the enclosure that an engineer could rework to reduce radiated susceptibility, without having to construct and test a physical prototype.
Conclusion
Reverberation chambers are widely used in EMC testing to produce a uniform field in order to analyze susceptibility to interference. Virtual EMC testing can reduce and replace physical tests – this can cut analysis time and reduce costs as well as allowing designers to analyze EMC at any stage of development, reducing risk. A virtual reverberation chamber can be constructed either in full 3D or replicated using superimposed plane waves. Both approaches allow engineers to test the EMC performance of their product using a virtual twin.
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Stephen is an Advocacy Offer Marketing Specialist in Global Marketing.