A Design Close-Up: Taking a Customer to the FCC B Standard
The following article is a case study of designing a chassis to meet the FCC Part 15 Class B standard. We’ll show the steps involved in this task, and the changes performed based on simulation and measurement. One of the goals of the project was to avoid a costly card redesign or other significant components, as the customer’s boards had been released. In the end, we’ll see the EMC performance of the chassis transformed – from emitting high levels of emissions to passing FCC B well within the limits, by intelligently balancing the shielding, cooling, and performance of the chassis and only relatively minor sheet metal changes.
FCC A and FCC B
The challenges of packaging electronic hardware to ensure electromagnetic compatibility (EMC) continue to increase. In the US the governing standard for regulating the containment of emissions is given by FCC Part 15 Class A and B. FCC Class A and B establish the levels of EMI containment required for various types of electronic equipment and are given in allowable dB across a range of frequencies (Figure 1). They are meant to put a limit on the amount of radiated emissions from electronics such as radios, televisions, ATM machines, radar equipment, etc.
FCC Class A is generally meant for industrial applications, while Class B is for areas where emissions must be low, like near an airport, ATM equipment, hospitals, residential areas, and so on, and is more stringent on the level of radiated emissions allowed. In recent times EMC has taken on greater importance with the increasing use of electronics in homeland security related applications.
The Challenge
Recently, the engineers at Elma Electronic Inc. were tasked with modifying a CompactPCI system that was originally designed to meet FCC Class A, so that it would meet the more restrictive Class B requirements. EMC was a critical concern since the application was providing network security. One can imagine the importance of preventing interference in a homeland security/network security application. With too much interference, it could harm signal integrity and cause failures, resulting in a breakdown the system security. The case study illustrates the various areas of concern and the modifications employed to ensure Class B compliance.
The system under test was a 19” rack mount chassis, 7” H x 14” D, with 7 cPCI boards, power supply, CDROM, I/O cabling and fans.
The unit was made from pre-plated steal with numerous holes and seams to allow for cooling and access to components. The unit was originally designed to meet FCC Class A and the preliminary tests showed that the unit was well within these limits. However, the customer determined that their product would have greater market acceptance if Class B could be achieved. An initial scan of the unit against the Class B requirements showed offending frequencies from 120MHz to 1GHz existed.
Range of Frequencies
The wide range of frequencies exceeding the Class B limits required that a comprehensive review of the System be performed in order to eliminate all problem areas. The following is a summary list of the areas of concern:
- Source of offending frequencies (microprocessor, drive, PSU, etc.)
- Backplane grounding
- Chassis frame conductivity
- Seams, access panels and vent holes
It is instructive to understand the governing physics behind shielding effectiveness before any analysis of the chassis is made. Electronic devices and particularly microprocessors radiate electromagnetic energy and can conduct this energy into the wiring. The shielding effectiveness (SE) of a chassis is defined as 20 times the logarithm of the ratio between the incident field (Ei) and the emergent field (Eo) given in decibels (dB).
SE = 20log Ei/Eo (dB)
Shielding Effectiveness
The shielding effectiveness of the chassis material is most important at low frequency where the absorption loss due to skin-effect takes place. At higher frequency gaps, seams and holes become the bigger problem. Holes must be made as small as practical. Because seams can act as an antenna they should be avoided or protected through the use of shielding gaskets. A general guideline for determining the maximum slot length (seam) is that it should be no greater than 1/50 the wavelength.
Wavelength is calculated as:
w = c/f
c = Speed of Light (3 x 108m/s)
f = frequency (MHz)
Or given as a function of the offending frequency:
Sl = 6,000/f, where Sl = Slot Length (mm)
Thus keeping slot lengths (seam) to 6mm or less should be sufficient in most applications with frequencies up to 1GHz.
Although the sheet metal for the unit under test had been designed following the above guidelines, it is important to note that a system’s performance is based on the interaction of all of its components, cabling, and wiring. In this case the processor board’s emissions were higher than typically encountered so changes at the chassis were anticipated to be necessary to reduce emissions to FCC Class B levels.
Before modifications to the sheet metal are considered, a good place to start is by reviewing the frequency of the components and wiring that make up the system and determine if any changes can be made to improve the system’s emission levels. In this case, it was suspected that the HDD drive and the cards were the source of the offending high frequencies (500MHz –1GHz) and the PSU to the low frequency spikes. (It’s commonly known that power supplies have low switching frequencies.) The suspicion proved to be true as the cards were found to be the cause of the 600MHz and 900MHz spikes and the drive had a harmonic at 700MHz. Most switching power supplies work at frequencies around 100-200Khz and the one in the system was typical.
Because the customer’s boards had been released, it was clear that modifications to the cards would not be possible. However, there was some flexibility with the power supply and drive. The drive originally tested in the unit did not have FCC approval so a drive was sourced that met FCC Class A. To address the power supply a torrid was added to the DC output wires of the PSU (similar to the small cartridges found on most laptop PSU DC cables). A torrid is comprised of ferrous material that neutralizes low frequency, electromagnetic waves and prevents the power cables from acting as an antenna.
Low Frequency Design Modifications
After ensuring that everything possible had been done at the component level to reduce or eliminate the offending frequencies, containment became the target. Continuing to focus on the low frequency spikes, Elma technicians reviewed the chassis frame conductivity and the system’s grounding. Initially, the backplane was grounded to the chassis at only a few locations. Although typically sufficient, it was suggested that additional grounding contacts be made in order to eliminate any possible ground loops that might magnify the low frequencies. This was done by adding more than double the original ground locations. It was further suspected that the chassis frame was not properly conducting between the side plates, removable covers and front bezel. Elma employs a patented EMI spring technology that stamps grounding contacts into the sheet metal at 50mm spacing. The concept was to develop a robust enclosure consisting of few parts made from the same base material. By integrating the EMC contact points into the parts the use of additional gaskets and dissimilar materials was avoided. This also keeps the use of screws to a minimum. Using an milli-ohmmeter, the chassis conductivity was checked at multiple locations and the resistance was found to be at acceptable limits between all parts of the chassis frame (<50 milli-ohms). Even so, to eliminate any possible area of concern it was agreed that the distance between the shielding springs that made contact between the side plate and top would be reduced to 25mm. By the formula provided above, one can calculate that 25mm will give protection up to 240 MHz, which is within the range limits. The result of the efforts mentioned above was great improvement in EMC levels. (See figure 4).
High Frequency Shielding
With the offending low frequency under control, attention focused on the high frequencies. This meant eliminating or reducing all apertures in the units, approaching a perfect Faraday cage. For EMC purposes the ideal enclosure would have no seams or openings, preventing radiated emissions and interference with other devices. Of course, this is not practical sense electronic equipment requires cooling ventilation, and component access. The challenge is to provide these apertures without compromising the EMC of the systems.
In the case of the unit being reviewed, the Engineers focused on three areas surrounding the front bezel. Although the frame of the chassis had been shown to me make a good conductive connection to the front bezel, the bezel itself had the following apertures:
· Card cage access seams
· Drive access opening
· Airflow perforations
In order to allow the installation of 6U x 4HP CPCI cards, a 10.5” W x 5.6” H window is present in the front bezel. The left and right sides of this opening are closed by vertical extrusions that hold the cards in place. Also, the upper and lower edge of the bezel created a seam with the chassis. It was believed that these vertical slots could be acting as an antenna. To solve this problem copper beryllium gaskets were placed between the front bezel and the contact points top/bottom and between the sides and the vertical extrusions
The second area of concern was the opening for accessing the CDROM. As discussed above, a CDROM drive had been sourced that met FCC Class A requirements, but one tested to FCC Class B was not available. One option considered was the use of a wave-guide. The opening through which the energy is flowing can be considered a wave-guide. If the wavelength of this energy is too long compared with the lateral dimensions of the wave- guide, little energy will pass through. If a length to diameter ratio of 1 can be achieved, an attenuation of approximately 30dB is to be expected (and within FCC B limits). Creating a wave-guide to encompass the CDROM would have added unwanted cost and complexity to the unit so another solution was sought.
Because access to the CDROM was only required for infrequent software upgrades it was decided that the problems could simply be solved by recessing the drive and adding a removable access door with metal impregnated rubber casketing material. Thus, the CDROM could be completely sealed inside the chassis.
The remaining concern was the air ventilation holes located on the front-left side of the unit. At first these holes were not considered to be a cause for concern because of their 6mm size. As discussed above, 6mm apertures should be sufficiently small to eliminate any offending frequency in the range being encountered. Although it would be a simple matter to reduce the size of the holes to achieve less radiated emissions, there was a concern that this would negatively impact the system’s cooling for a fix that might not be necessary. A cooling analysis showed that by reducing the hole size to 3mm, but increasing the area of perforation, adequate airflow could be achieved.
After making a prototype chassis with the above changes the unit was again sent to test.
In the end, the unit passed FCC Class B with relatively minor changes to the sheet metal and without requiring any costly card redesign or other components within the chassis. Although some of the changes to the chassis may not have been required to meet the minimum of Class B at the various frequencies, they were made to ensure a 5 to 6 dB margin below the limits. Understanding the complex issues of EMC is critical in designing systems that are both functional and cost effective. Balancing all the different elements of design requires knowledge and a little creativity to arrive at compliant solutions.
Shan Morgan
VP of Sales & Marketing
Elma Electronic Inc.
www.elma.com
510-656-3400
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