 Electromagnetic Compatibility: Demystifying the Specifications
So just what is a “Robust Product?” There is no clear definition, but if your product functionality needs to perform outside of the “standard”, then you may need a “Robust Product”. Sure, you can go to engineering web sites and compare “EMC” (Electromagnetic Compatibility) and other specifications for various products, but how do you know what is just passable and what will really make your system robust? This paper will review some EMC specifications and give you a better understanding of what you should consider paying extra for and what are simply “to-be-expected.”
There are four general categories to be considered:
- EMC – this includes FCC/CE tests such as ESD and EFT
- Mechanical – shock, vibration, drop testing
- Safety – UL, CSA, TUV, Class 1 Division 2
- Miscellaneous – Isolation, RFI Suppression, Enclosure ratings
This paper will focus on EMC requirments. Future technical papers from B&B Electronics will focus on the other specifications.
EMC – whether your system includes a radio transceiver such as WiFi, 802.11 or Zigbee; or a “non-RF”
device such as an Ethernet switch, EMC specifications are critical to your overall system robustness.
Emissions: This is RFI (Radio Frequency Interference) the product generates between 30 MHz and 1000 MHz. In the United States, the Federal Communications Commission requires that all electronic equipment be certified to comply with either Class A or Class B requirements.
- Class A - This is a general industrial standard and is the least stringent.
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Class B - This standard certifies that the product will not interfere with residential and commerical equipment and is tougher to obtain. Therefore, FCC Class B devices have significantly lower emissions (about 10 dB RF). If you are assembling a system that could be sensitive to emissions, choose the Class B device.
The following statements are included on the product label or within the product documentation:
FCC Class A Notice:
This device complies with Part 15 of the FCC Rules. Operation is subject to the following two conditions:
- This device may not cause harmful interference.
- This device must accept any interference received, including interference that may cause undesired operation.
Note: This equipment has been tested and found to comply with the limits for a Class A digital device, pursuant to Part 15 of the FCC Rules. These limits are designed to provide a reasonable protection against harmful interference when the equipment is operated in a commercial environment. This equipment generates, uses, and can radiate radio frequency energy and, if not installed and used in accordance with the instruction manual, may cause harmful interference to radio communications. Operation of this equipment in a residential area is likely to cause harmful interference in which case the user will be required to correct the interference as his own expense.
FCC Class B:
This device complies with part 15 of the FCC Rules. Operation is subject to the following two conditions:
- This device may not cause harmful interference.
- This device must accept any interference received, including interference that may cause undesired operation.
Note: This equipment has been tested and found to comply with the limits for a Class B digital device, pursuant to Part 15 of the FCC Rules. These limits are designed to provide reasonable protection against harmful interference in a residential installation. This equipment generates, uses and can radiate radio frequency energy and, if not installed and used in accordance with the instructions, may cause harmful interference to radio communications. However, there is no guarantee that interference will not occur in a particular installation. If this equipment does cause harmful interference to radio or television reception, which can be determined by turning the equipment off and on, the user is encouraged to try to correct the interference by on or more of the following measures:
- Reorient or relocate the receiving antenna.
- Increase the separation between the equipment and receiver.
- Connect the equipment into and outlet on a circuit different from that to which the receiver is connected.
- Consult the dealer or an experienced radio /television technician for help.
Immunities: This is a class of specifications that, strictly speaking, is only required if a product is to be used in Europe. However, over time they have become synonymous with “Robust Products.” Immunity specs include ESD, Conducted Immunity, Fast Transient Burst, among others.
In order to discuss these specifications, the Performance Criteria must be understood:
Performance Criteria A - The equipment must continue to operate as intended during and after the test. No degradation of performance or loss of functionality is allowed. This implies that no data errors are allowed during the testing.
Performance Criteria B - The equipment will continue to operate as intended after the test. This implies that data errors are acceptable, but that when the test stimulus is removed, the equipment needs to recover and function normally.
In both cases, operator intervention to re-establish normal operation is not allowed.
EN 61000-4-2: ESD (Electrostatic Discharge) – This can be compared to the “zap” you get after walking across carpet on a dry winter day. The test is performed using a piece of equipment called and ESD Gun. Two tests are required, a Contact Test and an Air Discharge Test. Performance criteria B is required.
Contact Test: The ESD Gun comes in direct contact with the device.
Air Discharge Test: The ESD Gun does not make contact with the device. However, the air discharge causes an arch to jump from the ESD Gun to the device under test.
Over the past several years the level of testing on many devices has increased to an air discharge level of 15KV. A device passing this level is more robust that a similar device that only passes 8KV or less.
EN 61000-4-3: Radiated Immunity – This involves bombarding the device with RF (Radio Frequency) energy from 80 to 1000 MHz. The standard test level is at 3 volts per meter (v/m), although in some cases 10 v/m may be used. This test must be performed in a semi-anechoic chamber (usually at an EMC test lab), since the transmitted RF causes interference across the frequency spectrum. Performance criteria A is required.
EN 61000-4-4: Electrical Fast Transient and Conducted Immunity –
Electrical Fast Transient: Transient bursts, such as those caused by large loads (i.e. a motor), are induced on each cable connecting to the device, with typical test levels of +/-500V to +/‑1000V. Performance Criterion B required.
Conducted Immunity: Intense levels of steady RF are induced on each cable connected to the device. Typical test levels of 3Vrms or 10Vrms are injected into the induction test coil. The test frequency is swept slowly from 150 KHz to 80 MHz, with frequency steps of 1% and a 3-second dwell time at each frequency. Performance criteria A is required.
Other tests are required if the equipment is connected to the AC line:
AC Voltage Surges, test standard EN 61000-4-5
AC Power Frequency Magnetic Fields, test standard EN 61000-4-8;
Voltage Dips and Interruptions, test standard EN 61000-11.
EMC testing is highly significant in selecting robust devices for your system or product. It pays to compare EMC specs on the products you are considering, and to choose these with the best EMC specs.
This paper has discussed EMC specs at a high level, and it is best to become more educated with detailed reading when appropriate.
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