The trend of cramming more functionality into smaller and smaller packages for today’s high-speed electronic products presents a challenge if you need to meet global emission and immunity standards. You must find ways to reduce emissions plus discover an edge that will give the design team an advantage to get the product to market before the competition.
You need measurement procedures, problem solving techniques, and a collection of typical remedies blessed by experts to meet the FCC and EU EMC mandates. But what are these time-saving solutions?
It is obvious, but important, that the best thing you can do to meet FCC and— especially—EU EMC requirements is to plan ahead, said Don Sweeney, president and senior EMC engineer at D.L.S. Electronic Systems. Consider the regulations at the earliest point in the design process. Complying with these regulations may require difficult and expensive suppression techniques if you wait until you get to the EMC test lab.
Some points to consider early in the design process include the selection and implementation of the EMC standards, the interface choices, the types of cables, cabinets, chip technologies, and clock rates. Other points to ponder are the number of layers on the PCB, the internal interfacing of subsystems, and the power supply. According to Mr. Sweeney, your design process should:
Meet, as early as possible, with EMC specialists to discuss concept planning.
Keep project personnel involved and informed about the impact of EMC on the design.
Continually monitor the design process by an EMC specialist.
Test prototypes early in the process to uncover potential problems.
Test when there are extensive changes to the final model.
Continually review changes or upgrades with an EMC specialist.
If meeting the EMC specifications can be solved with something as simple as filtering, your EMC test lab should advise you accordingly, said Walter Poggi, president of Retlif Testing Laboratories. However, if the fix appears to need a redesign, the lab should recommend consultants who could respond within 24 hours. A third-party consultant will ensure there is no conflict of interest between the test lab and your design personnel.
For radiated emissions problems, the first order of business is to determine where the emissions are occurring, said Michael Violette, president of Washington Laboratories. Fortunately, this task is straightforward because energy escapes via a cable or an enclosure opening.
If the frequency is lower than 200 MHz, for example, the problem is almost certain to be cable related, said Mr. Violette. For multiple cables on the system, the first step is to reduce the system configuration to its simplest form by removing all the cables possible and rechecking emissions levels by adding one cable at a time until the culprit cable is found. Fixes may include shielding the cable, filtering the conductors, or both.
Inadequate bonding of the cable shield to the connector shell could be the problem, said Mr. Violette. The shield should form a continuous connection through a metal shell to the chassis-mounted connector. The lack of a low-impedance connection between the metal connector on the product and the chassis also is a potential problem.
If all these areas are addressed and there still is a problem, add filter capacitors or series inductance, continued Mr. Violette. A typical solution is a ferrite bead or a common-mode choke.
If the cables are shielded and filtered properly and there still are problems, examine the product case. For products of common size, such as desktop or console equipment, the case-radiation problems usually begin appearing at 150 MHz and extend to the gigahertz range, said Mr. Violette.
To locate the source, rotate the equipment to the orientation that gives the maximum reading. Case radiation usually is related to slots and apertures, and the radiation pattern normally is sharp due to the high gain of the slot radiator.
Although it may sound crude, a quick and useful diagnostic method is to tap or bang on the case while observing the spectrum analyzer or receiver, pointed out Mr. Violette. Often, this causes a make-and-break event across the seam. The emissions should reduce; and if you hit the “sweet spot,” the reduction may be 20 dB or more.
Another method uses a short piece of coaxial cable as a sniffer probe. To use this probe, the case is scanned until the leakage is found. The cable can be used for examining aperture, seam, and cable leakage and finding hot spots on PCBs.
Another simple test determines if the case shielding is adequate: the dollar bill test. For most systems, if it is possible to slip a dollar bill into a seam or aperture, the opening is too large. This quick method is effective to approximately 600 MHz.
On the circuit board, the first thing to examine is its physical makeup. For emissions problems, hold the board up to a strong light to see if the ground and power planes are intact, uncut, and uninterrupted, continued Mr. Violette. In most cases, the isolation of the ground and power planes leads to an overall increase in emissions levels.
Specific measurement procedures are determined by the Code of Federal Regulation 47 (CFR 47) according to which part is applicable to the product in question, said Rick Linford, facility manager and regulatory engineer at DNB Engineering. For example, Information Technology Equipment falls under Part 15 Radio Frequency Devices, paragraph 15.31 (6), which calls out ANSI C63.4-1992 for the procedures to be used. The limits specified in Part 15 Subpart B unintentional radiators are listed in Table 1 and 2:
Applying Class A or B limits is based on the type of product being tested, said Mr. Linford. Only digital devices can apply the less stringent Class A limit.
Class A digital devices are not intended to be used in the home. The FCC determines this by the price, how it is marketed, and if home application is practical. For example, personal computers that can be used for home or office must meet the more severe limits of Class B. The less severe limits of Class A apply to mainframe computers not used in the home.
In Europe, electrical or electronic equipment must comply with EMC Directive 89/336/EEC and have the CE Marking. Compliance to the directive is similar to the FCC except you also must meet requirements for immunity.
The procedures and limits for testing are determined by the type of product and where it is used. Some families of products have specific standards. Some examples are the EN 60601-1-2 which applies to medical devices and covers emissions and immunity testing. EN 55022 is for Information Technology Equipment and covers only emissions.
If a product family does not have a specific standard, then the manufacturer must apply the generic emissions standard, EN 50081, and the EN 50082 generic immunity standard. Both standards have two parts. Part 1 of both is for residential, commercial, and light industry, part 2 is for industrial. The appropriate part is determined by where the product is used and if the AC power is shared with residential customers.
Passing EMC tests on the first attempt should be one of your goals, and it will help you save on test costs, said Michael Loerzer, vice president at Euro EMC Service. To get you through the obstacles quickly, the safety and EMC prechecks should be done first, followed by the ESD immunity tests.
The EMC precheck should begin with the easiest test procedures, such as RF current-clamp and spectrum-analyzer measurements, which give a direct correlation to the electromagnetic field emission. For example, if the RF current on the signal, communications, and power ports is less than 18 to 20 dBµA , the product probably will pass the radiated emissions tests such as CISPR 22 and FCC Part 15.
The second pretest is the ESD immunity test. According to Mr. Loerzer, this series combines pulse-disturbance characteristics and RF characteristics, as outlined in IEC 1000-4-2 with trise = 0.7 ns Þ fmax = 455 MHz. By combining the results from the EMC and ESD tests with the conducted emissions test data, you qualify a product to EN 55022 and EN 50082-1 requirements.
Meeting the essential requirements of the EMC Directive is difficult if you do not have a test plan that accounts for the electromagnetic environment where the apparatus is intended to work, said John Corda, senior EMC engineer at Wyle Laboratories. For example, many electronic equipment manufacturers measure emissions using one of the product-category standards such as EN 55011 or EN 55022, and then test for immunity using the generic standard EN 50082-1.
Any declaration of conformity should be supported with testing specific to the certified equipment and its intended operation, said Mr. Corda. Reports should contain details such as procedures unique to the equipment under test and the actual levels of susceptibility.
This approach is more cost-effective because it minimizes testing that may be required by future revisions of the EN harmonized standards. Such compliance data also is more valuable to the manufacturer in designing equipment to meet the protection requirements in the EMC directive.
A good starting point in the design stage is to determine the suitability of the appropriate power supply. Because most products use a switching power supply design that can create significant unintended electrical signals, be careful when reviewing the possibilities. Without proper attention, you may end up with the unenviable job of suppressing unwanted signals. The signals would radiate in the interior of the product and couple onto every component and wire, which then would re-radiate the signals, said Paul Bishop, EMC/EMI group leader at Dayton T. Brown.
Any mechanical opening, such as vent holes, would become a window for these signals to radiate out into the surrounding environment. Attempts to design an enclosure to suppress these signals would be costly.
It can get worse if the power source cannot meet the EU and FCC requirements because undesired signals will be placed on the power line. This will prevent the product from meeting the requirements for conducted susceptibility.
Problem-solving techniques are not only limited to reducing the signal strength at the source, said Daniel Signore, president of Radiation Sciences (RSI). Other methods include probing the product for emissions, circuit isolation, grounding, shielding, decoupling, and filtering.
For example, a recent customer of RSI failed the radiated emissions test. The unit was probed, and energy was found escaping from a small LCD panel. The source of the emissions was associated with the CPU clock circuitry so suppression components were suggested to fix the problem. However, because the design was frozen and already in production, the customer requested an alternate fix. A small conductively coated strip of mylar fabricated to fit over the LCD solved the problem without affecting the design.
Telecom and Medical Issues
The familiar saying “the devil is in the details” aptly describes the dilemma of trying to prove conformance to the many directives and specifications, said Washington Lab’s Mr. Violette. The critical first step in addressing compliance for any industry, including the telecom and medical fields, is to ensure that a complete, up-to-date copy of the standards is on hand.
The next step is to understand and apply the standards properly to the product. under examination. It is inevitable, due to the arcane structure of the standards, that an expert will have to be consulted regarding application and interpretation of specific portions of the standard. This statement applies more to the product safety requirements which contain a convoluted list of requirements for construction and design as opposed to the EMC standards that address the system more as a “black box” to which the tests are applied.
Clearly, you need to stay current on all related standards, including those in place as well as those being developed, agreed Mr. Poggi of Retlif Testing Laboratories. And if you are working with a test lab, ask them to address as many of the applicable standards as possible in one test program.
The regulatory commissions often issue a confusing snarl of rules and requirements which creates difficulty when trying to address specific subjects, observed Dean Ghizzone, president of Northwest EMC. But, you need to know what the current regulations are.
The reason is simple: An informed designer can take countless measures early in the process to eliminate potential test and compliance problems. The key is to foster and maintain a design group that is informed about current regulations and their future direction as well as the steps to ensure product compliance when the testing phase is reached.
For the U.S. telecom industry, products such as cellular phones must be certified by the FCC. The manufacturer or a third-party lab must test and evaluate the product to the relevant technical standards and submit the data in an application to the FCC. The FCC will approve the application, direct that more testing be performed, or request samples to confirm that testing was done.
Telecom testing in the EU depends on the product and specific standard, said Mr. Linford of DNB. For some products, the tests are performed by a single Notified Body for acceptance by all EU member nations, while other products require testing performed by a Notified Body from each of the countries where they will be marketed.
Medical devices in the United States must be approved by the Food and Drug Administration (FDA). Although the FDA does not require that electro-medical equipment meet EMC standards, it would greatly accelerate the application process, said Mr. Linford. Data that meets IEC 601-1-2 currently is acceptable to the FDA. This standard covers emissions and immunity testing that must be completed at a qualified laboratory.
If a product uses intentional RF radiators to perform a function, such as cutting or burning tissue, it also must comply with FCC Part 18, continued Mr. Linford. Otherwise, it must meet FCC Part 15 subpart B.
Medical-device testing has a slightly different approach but applies the same basic standard IEC 601-1-2, said Mr. Linford. In the EU, the EN 60601-1-2 is the approved version. Some devices may have an additional requirement. For example, muscle stimulators also must comply with EN 60601-2-10, Medical Electrical Equipment Part 2: Particular Requirements for Safety of Nerve and Muscle Stimulators, which includes particular requirements for the safety of these products.
The approval route depends on the function of the device. A Class I device, not to be confused with Class A or B of EMC levels, can be self-declared if the manufacturer has a quality system in place for engineering and manufacturing the product. A Class II device will require a Notified Body for test and evaluation. One of the most important actions is to get a quality system approval from an EU accrediting body to the ISO 9001 requirements or equivalent agency requirements.
The Streamlined FCC
In August 1996, the FCC approved a Declaration of Conformity (DoC) route to certify Class B (home and office use) personal computers and peripherals in the United States. The old system held Class B products from the market until the submittal was processed by the FCC. The certification route takes approximately eight weeks and has an application fee of $895.
Under the DoC procedure, products can be marketed the same day they pass EMC testing at an accredited lab. It has no application fee and requires only the time it takes to assemble the technical file.
Computer peripherals must be tested with or inside a host computer to qualify for the DoC procedure, said Todd Robinson, at CKC Laboratories. The test setup must be in accordance with ANSI C63.4 Standard Methods of Measurement of Radio-Noise Emissions from Low-Voltage Electrical and Electronics Equipment in the Range of 9 kHz to 40 GHz and have RF emissions levels in accordance to FCC Part 15 Class B limits. A DoC Statement of Compliance must be packaged with each device needing the qualification. The DoC is helpful for products that depend on timely market entry.
Motherboards and power supplies also must be tested with or inside a host computer, added Mr. Robinson. If you want to use the DoC route for compliance, test the motherboard and power supply independent of the enclosure or within a sealed enclosure.
Testing the motherboard and power supply independent of the enclosure requires that the top and two sides of the enclosure be removed, said Mr. Robinson. The test setup must be done in accordance to C63.4 and the RF emissions in accordance to FCC Part 15, Class B limits, with a 6-dB margin of error. When tested within a sealed enclosure, the representative host, motherboard, and power are considered to be one entity to be covered by one DoC Statement of Compliance.
All CPU boards had to comply with DoC procedure by Sept. 17,1997. Compliance also can be demonstrated by a Grant of Certification from the FCC.
The streamlined procedure has had a positive impact on turnaround time for manufacturers, agreed DNB’s Mr. Linford. It used to take 40 days or more and had a fee. For products with a life span of 180 days, the 40 extra days represented a large loss of revenue.
The DoC route allows you to begin shipping when the DoC technical file is complete and signed by the responsible party. However, to use the DoC procedure, you must use a laboratory accredited by the American Association for Laboratory Accreditation or National Voluntary Laboratory Accreditation Program.
European manufacturers wishing to export their products to the United States also would have to use an accredited lab if they wanted to use the FCC’s streamlined procedure. However, EU countries can and still do business the old way by filing with the FCC, waiting for the paperwork to be processed, and paying the filing fee.
The major challenge with the new streamlined procedure is the requirement that the FCC must consistently monitor the process, said Mr. Poggi of Retlif Testing. If the system is to work properly, manufacturers and laboratories need to know the FCC will sample their products and work for conformity.
Acknowledgments
These test labs provided information for this article:
Chomerics/Parker Hannifin 617-935-4850
CKC Laboratories 209-966-5240
Dayton T. Brown 516-589-6300
D.L.S. Electronic Systems 847-537-6400
DNB Engineering 801-336-4433
Euro EMC Service…………………………………………………………………. 011 493328430141
Garwood Laboratories 562-949-2727
Hermon Laboratories…………………………………………………………….. 011 972-6-6288001
Instrument Specialties 717-424-8510
National Technical Systems 800-723-2687
Northwest EMC 503-537-0728
Radiation Sciences 215-256-4133
Retlif Testing Laboratories 516-737-1500
Washington Laboratories 301-417-0220
Wyle Laboratories 205-837-4411
Radiated Limits
Class B Class A
MHz dBµV/m dBµV/m
30 to 88 30.0 39.1
88 to 216 33.5 43.5
216 to 960 36.0 46.0
Above 960 47.0 49.5
Conducted Limits
Class B Class A
MHz dBµV/m dBµV/m
0.45 to 1.705 48.0 60.0
1.705 to 30 48.0 69.5
Copyright 1997 Nelson Publishing Inc.
November 1997