An anechoic chamber (Figure 1) is a specially constructed, electromagnetically wave-free room for evaluating electronic devices. The word “anechoic” refers to something that does not refract sound. The compliance testing rooms are used for just that. Carbon-based absorbing materials (Figure 2) and ferrite tiles are used to line the walls of anechoic rooms, which completely suppresses electromagnetic radiation and reflection. Engineers can conduct precise tests on electrical components like radar systems, antennas, sensors, and more when they are shielded from outside interference. There is a wide variety of tests and measurements that can be performed in an anechoic chamber that are typically only available in certification laboratories, such as thermal noise and military specification tests.
Electronic Device Certification Requirements
RF and anechoic chamber testing is frequently required by both carriers and regulators. It is the responsibility of the manufacturer of any technological device, wireless or wired, to ensure that it complies with applicable laws and rules. Almost all goods will also have to conform to government standards for unintended emissions. In order to achieve certification or to fill out a Declaration of Conformity, devices are tested in an anechoic or semi-anechoic room at a certification laboratory. However, other calibrated rooms may be acceptable in place of the specified Open Area Test Site (OATS) in some cases. Because of the controlled atmosphere that anechoic chambers provide, they are preferable to OATS sites in these cases.
Unless exempt, the United States’ Federal Communications Commission (FCC) mandates all products with a 9kHz or higher clock to comply with FCC Part 15B. (Title 47, part 15.05 Digital Device, 15.101). Additional radio-based testing is required if the product contains a transmitter due to the spectrum the device works in and other factors associated with the product type.
A object must bear the CE mark in order to be sold in European markets. Unintentional emissions, spectrum efficiency, and immunity standards can be found in the EMC directive or Radio Equipment Directive (RED) that is part of the specifications.
What Is an Anechoic Chamber Used For?
Anechoic chambers provide a noise-free, controlled atmosphere for the testing of electronic components. Anechoic chambers are used for a variety of important experiments, including:
- RF/Antenna Performance Testing: RF/antenna performance testing is a crucial procedure carried out in an anechoic room. Certification is directly tied to the results of the assessment of RF performance. Transmitter performance can be measured in an anechoic laboratory, allowing engineers to determine whether or not antenna circuit matching or an antenna swap is necessary. Unlicensed bands like LoRa®, Wi-Fi, GPS, and BLUETOOTH®, as well as licensed bands like cellular, are all enabled for RF testing and RF performance of devices.
- Passive Antenna Pattern Testing: Passive antenna pattern testing for construction, matching, or fine-tuning is supported in anechoic chambers. Antenna gain in unlicensed certified modules is constrained by the certified module manufacturers’ limits or by limitations given by governmental bodies for the radio type and service, so testing antenna performance during design is crucial. When the antenna is built into the PCB as a trace antenna, the gain and effectiveness of the design can be determined through anechoic chamber testing.
- PTCRB/CTIA OTA Testing: In the United States, carriers like AT&T and Verizon have their own standards for over-the-air (OTA) and radiated spurious emissions (RSE), and cell phones need to get approved to satisfy those standards. Certification bodies like the PTCRB and CTIA, as well as individual cellular providers, establish the standards for over-the-air (OTA) testing.
- Other Performance or Quality Testing: Other tests that can be performed in anechoic chambers include:
- Pre-compliance testing throughout the development cycle to avoid costly re-work and redundancy in submission for final certification
- Un-intentional radiated measurement testing
- RSE testing for harmonics of the transmitter
- OTA antenna testing
- CE Immunity tests
How Is an Anechoic Chamber Constructed?
There is a wide range of sizes and types of anechoic chambers available, from compact, wall-mounted models to large, floor-standing, military-grade rooms. The size and type of container required is largely determined by the device’s mass and the frequency range being studied (i.e., measuring an automobile vs. a cellular phone). The device’s interior turntable must be strong enough to support its weight without restricting rotation. A sufficiently wide separation between the device and antenna is required for obtaining the lowest achievable frequency measurement. There are two types of anechoic chambers, completely anechoic and semi-anechoic, with the former being lined with carbon-based absorbing materials and the latter with ferrite tiles.
Fully Anechoic Chambers
The floors and walls of a full anechoic room are lined with absorbing material or ferrite material, or both. The aim is to completely absorb radiation so that the test antenna can detect only that which comes in a straight line from the product under test. Emissions testing, immunity testing, antenna pattern testing, transmitter and receiver testing (including over-the-air)—all of these can be performed in a fully anechoic room.
The walls and roof of a semi-anechoic chamber are made of absorbing material, ferrite material, or a combination of the two. Absorbing material is put between the test antenna and the product being tested in some test areas. Emissions immunity testing is conducted in a semi-anechoic room.
Electronics must go through rigorous testing to guarantee optimum performance and conformity with statutory requirements. Repeatable RF readings free of reflections and environmental noise can be taken in an anechoic chamber. Both license-free and commercial devices can be put to the test with no problems. By simulating real-world conditions, an anechoic box can reveal hidden information about a product’s functionality. It helps troubleshoot newly implemented designs, guarantees governmental compliance, and verifies that the device’s antenna performance is adequate for its intended use.