Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1999-02-02
2002-10-01
Pascal, Leslie (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200, C361S752000, C361S816000, C361S818000, C361S799000, C361S800000, C024S563000, C439S607070, C439S616000, C174S034000, C174S051000, C385S088000, C385S092000
Reexamination Certificate
active
06459517
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an enhanced electromagnetic interference shield, and in particular, to an electromagnetic interference shield that significantly reduces electromagnetic interference emissions from an optical transceiver, for example.
2. Background Information
Many electrical devices, when operated, generate emissions that include electromagnetic radiation. When this electromagnetic radiation influences the proper functioning of another device, the result is known as electromagnetic interference (also known as EMI).
In order to ensure that electromagnetic radiation is emitted only at acceptable levels, i.e., to prevent electromagnetic interference, various standards have been developed. For example, both the United States and Canada have determined acceptable electromagnetic radiation emission limits for electrical devices operating at set frequencies. If the electrical device exceeds the determined acceptable emission limits, the sale or use of the electrical device may be prohibited.
Various shield devices are known that can be used to reduce emitted electromagnetic radiation. The conventional shields typically cover a substantial portion of the associated electrical device, and are usually formed of a metal that, when grounded, will attenuate the electromagnetic radiation. However, shields will typically have one or more apertures (openings) formed therein, to allow for the passage of electrical or fiber-optic cables, for example, which are used to couple the shielded electrical device to other associated components. These apertures may allow for the passage of emitted electromagnetic radiation (in the form of electromagnetic radiation waves), thus raising the possibility of electrical magnetic interference.
However, it is known that an aperture will attenuate electromagnetic radiation waves when the aperture is less than ½ of the wavelength &lgr; to be attenuated (i.e., length of aperture <½&lgr;). Moreover, the smaller the aperture, the greater the attenuation of the electromagnetic radiation waves. The attenuation of electromagnetic radiation waves due to passage through an aperture can be determined using the following formula:
S=20 log (&lgr;/2L), where
S=the shielding effectiveness of the aperture (in decibels);
&lgr;=the wavelength of the electromagnetic radiation; and
L=the maximum linear length of the aperture (in meters).
Moreover, the wavelength &lgr; can be determined by dividing the velocity of the electromagnetic radiation wave (i.e., the wave speed, which is approximately 3×10
8
m/sec
2
) by the frequency of the electromagnetic radiation emissions.
Thus, as the operational frequency (and hence, speed) of an electrical device increases, the associated wavelengths become smaller, thus requiring smaller apertures. However, the apertures are typically limited in minimum size to allow for the passage of associated cables, for example. If the size of the apertures are reduced too much, then passage of the associated cables therethrough may be prohibited. Therefore, it is clear that the size of the aperture, which typically has a minimum size, may limit the speed (i.e., bandwidth) of the associated electrical device.
As an example, optical transceivers use pulses of light to carry signals and transmit and receive data at very high speeds. Typically, the light pulses are converted into, or converted from, associated electrical signals using known circuitry. Such optical transceivers are often used in devices, such as computers, in which data must be transmitted at high rates of speed.
Optical transceivers may use light emitting diodes (LEDs) or lasers to transmit the light pulses, and a photodiode to receive light pulses. Typically, the photodiode is located adjacent to the LED or laser, to form a so-called duplex optical transceiver. Fiber-optic cables are then coupled to the respective LED or laser, and the photodiode, so that the light pulses can be transmitted to and from other optical transceivers, for example.
Further, optical transceivers typically have standard sizes and shapes, so that they may be readily incorporated into a computer system, for example, without modification. For instance, a typical duplex optical transceiver has a length of 39.12 millimeters, a width of 25.40 millimeters, and a thickness of 10.35 millimeters.
The optical transceivers are normally located on either input/output cards or port cards that are connected to an input/output card. In order to facilitate the connection of the fiber-optic cable to the optical transceiver, the transceiver is usually located on a periphery of the card.
Moreover, in a computer system, for example, the input/output card (with the optical transceiver attached thereto) is typically connected to a circuit board, for example a mother board. The assembly may then be positioned within a chassis, which is a frame fixed within a computer housing. The chassis serves to hold the assembly within the computer housing.
The LED or laser, and the photodiode are typically operated at a very rapid speed in order to transmit and receive data. For example, a typical high-speed optical transceiver may transmit or receive up to 622 megabits/second. In order to transmit, for example, data at this speed, the LED or laser must be repeatedly activated and deactivated 622 million times per second. This rapid switching action generates large amounts of electromagnetic radiation. Likewise, the rapid operation of the photodiode will generate a large amount of electromagnetic radiation.
As is known, it is only at harmonics of an operational frequency that potentially harmful electromagnetic radiation emissions occur. For example, the aforementioned high-speed optical transceiver, when transmitting or receiving 622 megabits per second, operates at a frequency of about 622 MHz. This operational frequency does not generally generate problematic electromagnetic interference. However, harmful emissions may be generated at the harmonics of this frequency, i.e., at 1.244 GHz, 1.866 GHz, 2.488 GHz, etc. Moreover, the third harmonic frequency is generally one of the more difficult frequencies to attenuate (i.e., at 2.488 GHz in the aforementioned scenario).
If the frequencies of the emitted electromagnetic radiation are not sufficiently attenuated, electromagnetic interference may affect the performance of other components, for example other input/output components, that are either located on the same card as the optical transceiver or that are located on other cards. The electromagnetic interference may cause other input/output components, for example, to either transmit or receive faulty data, thus affecting the operation of the overall system.
To prevent electromagnetic interference from having an adverse affect on other components, it is known to keep optical transceivers adequately spaced apart from other components. However, this disadvantageously limits the number of components that can be placed within a predetermined area. Moreover, more area is needed to accommodate a set number of components with a predetermined spacing therebetween. However, as is readily known, the trend in computer systems, for example, is to provide more functionality in less space. Thus, keeping the optical transceivers separated from other components may not be a practical solution.
Alternatively, it is also known to use a chassis of a computer system, for example, which has the card containing the optical transceiver mounted thereto, as a shield to block the electromagnetic radiation from permeating to other devices. However, the chassis typically only holds the periphery of the card, and will not prevent electromagnetic interference from affecting other components that may be located on the same card as the optical transceiver.
Moreover, because the chassis does not completely encapsulate the card containing the optical transceiver, the typical chassis is only able to serve as a shield when the electromagnetic radiation emissions are re
Duncan Timothy Paul
Maas John Stephen
Peacock James Larry
Thorvilson Scott Marvin
Berdo, Jr. Robert H.
International Business Machines - Corporation
Pascal Leslie
Phan Hanh
Rabin & Berdo PC
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