Electricity: conductors and insulators – Anti-inductive structures – Conductor transposition
Reexamination Certificate
2001-02-15
2002-09-24
Ngo, Hung V. (Department: 2831)
Electricity: conductors and insulators
Anti-inductive structures
Conductor transposition
C428S620000, C428S621000
Reexamination Certificate
active
06455770
ABSTRACT:
SPECIFICATION
1. Field of the Invention
The present invention relates in general to reduction of stray electromagnetic radiation emissions produced by electronic systems and more specifically to apparatus for shielding stray electromagnetic radiation.
2. Background of the Invention
Electronic systems (e.g., electrical circuits, etc.) which operate on alternating current (“AC”) or direct current (“DC”) voltage generate an electromagnetic field. An electromagnetic field is comprised of waves. Such fields are created because of the electrical energy being used in the devices. In most countries, agencies like the Federal Communications Commission (FCC) in the United States, Industry Canada (ICAN) in Canada, and Bundesamt Fur Post Und Telekommunikation-Zulassen Und Testen (BZT) in Germany regulate maximum amplitudes for radiated and conducted electromagnetic energy permissible at specific frequencies.
Unintentional radiators include electronic systems such as personal computers and computer networking hardware. Intentional radiators include electronic systems such as transmitters and regenerative receivers. This is by no means an exhaustive list of unintentional and intentional radiators, as various other electronic systems produce unwanted stray electromagnetic radiation emissions. Such unwanted electromagnetic radiation includes radio frequency, electrostatic and magnetic fields. As the operating speeds of these electronic systems increase, the strength of the electromagnetic fields increase. For example, present personal computers can operate at speeds above two hundred megahertz (“MHz”). These operating speeds will only increase in the future.
Stray electromagnetic fields are highly undesirable, as they can cause interference with other electronic devices, radio frequency communications systems, etc. Because of this, governmental agencies of many agencies have issued regulations which specify the maximum amplitude of stray unwanted electromagnetic radiation that an electronic system can emit and still be used in that country. If a product does not meet these specifications, it cannot be sold in that country.
Each of the government agencies has specific testing methodologies. In general, the stray electromagnetic radiation of an electronic system is measured using an antenna placed at least three meters from an electronic system on a calibrated test site. The field strength per meter of the stray fields is measured. In general, stray fields have strength with an order of magnitude in the one hundred microvolts range when the frequency is between thirty MHz and eighty-eight MHz. When recorded field strength exceeds the limit imposed by agencies such as the FCC, the electronic system must be redesigned so that it complies with those specifications. Electronic system redesign and implementation into production can delay product introduction by several months.
At present, a common method to reduce the stray electromagnetic energy produced by an electronic system is to encase the electronic system in a continuous metal enclosure. If there are no seams in an enclosure, then electromagnetic energy is contained since a Faraday Cage has essentially been built around the electromagnetic energy source. When designing such an enclosure, the amplitude and frequency of the stray electromagnetic field determines its thickness. Regardless of the thickness, however, the shielding properties of the enclosure are compromised if system being shielded has any input/output ports, data cables, displays, etc. that cannot be shielded. Because of this, additional shielding must usually be added to the enclosures, cables, displays, keyboards, etc. to assure compliance to the FCC recommendations. These additional technologies add significant product cost and development time, and detract from the cosmetics of a final product.
Summarizing, while effective, these enclosures are heavy, add cost, and reduce the appeal of the product, especially if it is intended for the consumer market. Furthermore, it is entirely possible that this shielding might not even be necessary. Thus, a product could be designed with such shielding, thereby greatly adding to its cost, when the shielding was not even necessary.
In addition, in an attempt to make products that are more attractive and/or less expensive, the electronics industry also uses plastic enclosures. If an electronic system operating inside a plastic enclosure exceeds electromagnetic interference limits allowed by the regulatory agency, a metallized surface may be applied to the plastic enclosure in attempt to transform the interior of the plastic enclosure into a metal enclosure. The cost of this technology can be prohibitive, however, because the top and bottom sections of the enclosure must exhibit electrical continuity at the seams, while maintaining a uniform thickness over the entire enclosure surface. This is difficult to achieve.
Another common method for reducing stray electromagnetic radiation is to change the location of components in an electronic system. For example, a designer might change the location of the system's crystal oscillator to place it closer to the circuit receiving the signal. This will reduce the length of the signal path (i.e., the connecting wire or printed circuit board trace) and therefore might reduce the stray electromagnetic radiation emitted. The problem with this particular method is that it requires a redesign of the electronic system. This increases the cost of the product and significantly delays the product from entering the market. In addition, there is no way of knowing if the redesign actually worked until it is retested. When it is retested, the redesigned product may still exceed the specifications for maximum stray electromagnetic radiation. Many times, a third redesign must be undertaken. This costs time and money.
Examples of the specifications for the maximum stray electromagnetic radiation that a product must satisfy to be sold in the United States include:
dB microvolt/meter
FREQUENCY (MHz)
(dB &mgr;V/m)
microvolt/meter (&mgr;V/m)
30 to 88
40.00
100
88 to 216
43.50
150
216-960
46.00
200
960-1000
54.00
500
These specifications, which have been issued by the FCC as of the filing date of this application, are measured from a position three meters from the electronic system undergoing test.
Examples of specifications that are in force in Europe include:
dB microvolt/meter
FREQUENCY (MHz)
(dB &mgr;V/m)
microvolt/meter (&mgr;V/m)
30-230
40.46
105.44
230-1000
47.46
236.05
These are also measured at a distance of three meters from the electronic system undergoing test. These three meter limits are mandated in the specification EW 55022/CISPR 22, which is entitled “Limits and Methods of Measurement of Radio Disturbance Characteristics of Information Technology Equipment.”
Furthermore, according to the newest European criteria, many products must continue to operate and be less susceptible to a field of three volts per meter from twenty-seven MHz to five hundred MHz to be allowed entry into the marketplace for sale. This is known as susceptibility. These criteria for susceptibility is best represented by the following product standards:
IEC 801-2 Electromagnetic Compatibility for Industrial-Process Measurement and Control Equipment. Part 2: Electrostatic Discharge Requirements. 1984
IEC 801-3 Electromagnetic Compatibility for Industrial Process Measurement and Control Equipment. Part 3: Radiated Electromagnetic Field Requirements. 1984
IEC 801-4 Electromagnetic Compatibility for Industrial Process Measurement and Control Equipment. Part 4: Electrical Fast Transient. 1988.
An example of an electronic system that is particularly prone to producing stray fields is a laptop computer system. Prior to being marketed for sale or being sold, a laptop computer system must be tested by a FCC registered testing laboratory. The product must comply with the Code of Federal Regulations, Title 47, Part 2 entitled Frequency Allocations and Radio Treaty Matters; General Rules and Regulations; and Part 15 entitled Radio Freque
Lyon & Lyon LLP
Ngo Hung V.
LandOfFree
Electromagnetic radiation shield for attenuating... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Electromagnetic radiation shield for attenuating..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Electromagnetic radiation shield for attenuating... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2870447