An EMC test is known as Electromagnetic Compatibility, a certification for electronic devices to maintain their limitation of electromagnetic waves. As discussed in the previous article, there are two types of EMC tests: emission (EMI) and immunity (EMS). EMI (Electromagnetic Interference) tests measure the magnetic waves emitted by the device, and EMS (Electromagnetic Susceptibility) tests are performed to test the device's emissions handling immunity.
This article covers some of the most common EMI tests performed in electronic device EMC testing laboratories. Not all tests are suitable for all devices and additional testing may be required depending on the device application.
Fig.1: Conducted and radiated emission
Emission types (EMI)
We have already talked about the types of emissions in the previous article. The image above is a diagrammatic representation of these emissions.
Electronic devices can produce any magnetic interference in the environment. Every device emits EMI, but the country's EMC standards regulate this emission. Regulations may vary according to different countries. In the USA, EMC standards are defined by the FCC (Federal Communications Commission). In Canada, the EMC standard is regulated by Industry Canada.
An EMI test is necessary to bring a new product to market. This test ensures that the device does not emit any harmful electromagnetic fields or damage any other device due to its interference.
Below are some of the most common tests performed by EMC laboratory on devices.
- Radiated emission
- Conducted emission
- Flicker
- Harmonic test
Radiated emission
This test involves measuring airborne EMI which is an emission unintentionally generated by the device under test. Because this emission travels through the air, it is called radiated emission.
Fig. 2: Radiation emission
This is the most common EMC test performed by EMC laboratories around the world. Depending on the type of industry, there are certain radiated emission limitations on the market. Below are some of the different radiated emission testing facilities used by testing laboratories.
➢ Type of radiated emissions test sites
The main purpose of the radiation emission test site is to measure the radiation coming from the product and ensure that the radiated emission is within the limit. There are two types of test sites used to measure radiated emission. Both test locations are explained below.
- Open Area Test Site (OATS)
Fig 3: Test location in open area
The open area test site is the most common radiated emission test site, also known as OATS.
The distance between the Equipment Under Test (EUT) and the antenna is 3m, 10m or 30m. The measuring distance is crucial because it is a Far Field test, ensuring that measuring distance of the field strength in the far field is opposite to the Near Field. In the near field, the distance between the antenna and the EUT is less than the wavelength. In the far field, the distance of the EUT and the antenna is greater than the wavelength.
Laboratories do not use near-field testing because the measurement oscillates between magnetic and electrical fields. The electric field is not stable enough to take accurate measurements in the near field.
The electromagnetic field varies with distance, so some standards require a specific distance separation and some other standards allow the use of two or more lengths to measure radiation emission. Limits are recalculated for each distance.
- Semi Anechoic Chamber (SAC)
Fig. 4: Semi Anechoic Chamber
The semi-anechoic chamber (SAC) is the same as an open area test site, but is housed in a shielded room. The semi-anechoic housing is made of RF shielded material and the inner wall is made of absorbent material. An RF shielded enclosure is used to block external interference, noise or radio signals. An absorbent material is used to prevent echo in the signals so that the RF signals do not reflect too much off the walls. If RF absorbing material is not present in the room, the antenna will receive an unquantifiable signal contribution from wall and ceiling reflections, making the measurement wildly inaccurate.
Compared to an open area test site, there is no need to constantly adjust background noise.
Laboratories use this method mainly because it is a good solution for measuring EMI in noisy environments.
Radiated Emission Test Setup
Fig. 5: Radiated emission measurement setup
No product produces an electromagnetic waveform in a spherical pattern. The electromagnetic wave tends to be quite directional. Therefore, testing laboratories place a product or EUT on a wooden rotating table to measure emission in all directions from the product by rotating the table; Furthermore, the height of the antenna varies between one and four meters for all possible readings.
The ground plane must be covered with an electromagnetic reflective surface (aluminum, steel, wire mesh, etc.), and also the ground plane must be flat, so that the antenna can pick up RF signals directly from the EUT and also measure reflection from the ground To increase accuracy in EMI measurement.
Radiated emission limits
Limits for radiated emission depend on two aspects of country standards and the specific application of the device. If the device is manufactured for specific industries such as military, automotive or medical, etc., the emission limits will be much stricter and more difficult to pass the test.
A sample of the radiated emission is provided below, the blue line is for the FCC and the green line is for Europe. They are very similar, but there is a subtle difference between them. It is possible to pass the FCC, but may fail the EC emission limits. This graph is from OATS because there are several larger signals on the FM and cellular bands.
Fig.6: Radiated emission limits
Some of the standards have a fixed measurement frequency, but the FCC varies the measurement frequency range according to the frequency of the device.
Table 1.
Measuring antennas for radiated emission
Various antenna types are used to measure EMI in the laboratory. Each antenna has a different gain profile in different frequency bands. Below we show images with different frequency ranges.
loop antenna
Frequency: – 10kHz to 30Mhz
Fig.7: Loop Antenna
Biconical Antenna
Frequency: – 30 MHz to 300 MHz
Figure 8: Biconical Antenna
Register Periodic Antenna
Frequency: – 300 MHz to 1 Ghz
Fig.9: Periodic Recording Antenna
Horn antenna
Frequency: – 1Ghz to 25Ghz
Fig.10: Horn Antenna
Conducted emission
Mains power interference can affect multiple devices connected to the same mains power. The device generates electromagnetic energy or noise conducted through a power cord, interfering with the power supply. This is called conducted emission. Testing laboratories measure these emissions in the frequency range of 150 Hz to 30 MHz to verify that the conducted emission is within the limit.
The emissions tests conducted are mainly performed on the device connected to the AC power supply. Some standards have limits for devices that run on DC power.
Fig.11: Conducted emission
Emission test setup performed
Fig.12: Conducted emission configuration
As shown in the diagram above, there is a LISN (Line Impedance Stabilization Network) device.
The LISN device provides a standardized impedance at the EUT measurement points. Couples the EUT measurement point to the receiver. Furthermore, it prevents unwanted interference signals from reaching the other device.
The receiver is a spectrum analyzer that measures the RF signal passing through the LISN device. The LISN and EUT device is placed on the grounded plane.
Conducted emission limits
The FCC sets conducted limits for Class A and Class B devices. The Class B limit is for the home environment, so it is more stringent. The product must not interfere with other connected household devices.
Fig.13: Emission limits driven by the FCC
Flicker and harmonics test
Harmonics are the distortion of the typical electrical waveform. They are usually transmitted by switched or other non-linear power connectors, such as motors, transformers, and lamps.
If there is an arc at the load contact point that causes non-linear current consumption, it will induce voltage variations that will affect nearby lamps. This is called voltage fluctuation.
Flicker and Harmonics Testing Equipment
Flicker and harmonics can be measured with a low-cost equipment solution.
TTI (Thurlby Thandar Instruments) can analyze oscillations and harmonics associated with a low-distortion AC power supply.
Fig.14: TTI for network and harmonic analyzer
The circuit below measures the voltage fluctuation at the power supply terminal. This is the simplest block diagram. There is a clean power supply with known impedance used for power. Any fluctuation or oscillation at the EUT terminal can be measured at the EUT terminal.
Fig.15: Scintillation measurement block diagram
The image below shows the simplest block diagram for measuring harmonics in the EUT. Similar to the diagram above, a clean power supply is used as the ESE source, but there is a sense resistor in series with the ESE to measure the harmonic current.
Fig.16: Harmonic current measurement block diagram
