Electronic digital logic circuitry – Signal sensitivity or transmission integrity – Bus or line termination
Reexamination Certificate
1999-06-28
2001-08-07
Phan, Trong (Department: 2818)
Electronic digital logic circuitry
Signal sensitivity or transmission integrity
Bus or line termination
C333S182000
Reexamination Certificate
active
06271678
ABSTRACT:
FIELD OF THE INVENTION
The present invention is related to a component useable for terminating transmission lines so as to control EMI while maintaining signal integrity and in particular to a component which combines a ferrite bead with resistance, preferably in a surface mount package.
BACKGROUND OF THE INVENTION
Many electronic devices are advantageously configured to reduce or substantially eliminate electromagnetic radiation which can potentially cause interference, so called electromagnetic interference (EMI). Examples of such electronic devices include computers, network devices such as routers, bridges and the like. A major source of electromagnetic interference is signal or data transmission lines which, depending on how the lines are terminated, can effectively operate as antenna, transmitting electromagnetic signals which result in interference. Typically, the transmission lines are used for carrying signals which, for proper operation of the electronic device, must be configured to avoid distorting or changing the waveshape percent of the signal and must, in general, substantially maintain signal integrity. Accordingly, it would be useful to provide a termination device for transmission lines which is configurable to simultaneously reduce or eliminate EMI and to maintain signal integrity. In particular, resistors are commonly used as transmission line terminators, but may not be completely satisfactory, especially when both signal integrity and reduction of EMI are important. Accordingly it would be useful to improve on resistors, as transmission line terminators.
One type of transmission line commonly found in computers and other electronic devices, and believed to be responsible for a major portion of EMI are clock signal lines. Accordingly, it would be advantageous to provide a device for terminating clock signal lines which both reduces or eliminates EMI and maintains the waveshape or otherwise maintains signal integrity of clock signals.
Typically, transmission line terminators will be provided on one or more printed circuit boards (PCBs) of an electronic device. In general, many electronic devices are designed to be as small as feasible so that the circuit boards therein typically cannot be configured with additional components which are large in size and/or numerous. Accordingly, it would be useful to provide terminators for transmission lines which require relatively little space on PCBs. Preferably, the terminator components are configured so as to be relatively low in cost and relatively easy to manufacture and assemble on the PCB.
SUMMARY OF THE INVENTION
The present invention provides a terminator device which effectively reduces or eliminates EMI from a transmission line while substantially maintaining signal integrity. In one aspect, the invention is an improvement over resistors which, while effectively terminating series-terminated transmission lines in many situations, typically do not properly handle the higher frequency contents of many waveforms (e.g. typical clock signal waveforms). In this context, higher frequency content can include frequency content starting at about the “knee” frequency, 1/(&pgr;&tgr;
r
).
According to one aspect, the termination device includes a ferrite bead combined with a resistance in such a way that the device (or the equivalent circuit to the device) provides such resistance in series with the ferrite bead. Adding ferrite material (e.g. inside a surface-mount resistor package) will attenuate higher-frequency contents before they can propagated down the transmission line, thus reducing EMI. The resistance is selected, in light of the characteristics of the transmission line and the signals it will carry, to provide substantially constant (“flat”) frequency response in lower frequency ranges while the ferrite bead presents a large impedance at higher frequencies. A substantially flat frequency response in lower frequencies assures signal integrity and acts, in combination with the high frequency impedance from the ferrite beads, to effectively reduce or eliminate EMI, while preserving signal integrity. Preferably, the values of the resitance and ferrite are selected such that the impedence created by the ferrite becomes relatively significant at and beyond the “knee frequency” 1/(&pgr;&tgr;
r
).
Preferably, in order to achieve this functionality without unduly consuming surface area of the circuit board, the ferrite bead-resistance combination is provided in a surface mount package, preferably a relatively small surface mount package. Although it is possible to provide the ferrite bead and the resistance as two discrete components within the surface mount package, in one embodiment, the ferrite bead is itself configured to have a desired low frequency resistance.
According to one aspect of the invention a ferrite bead is placed in series with a terminating resistor on a transmission line such as a clock transmission line. In lower frequency ranges where the ferrite bead is substantially not effective, the transmission line will be effectively terminated by the resistor. In higher frequency ranges where the resistor is substantially no longer effective, the transmission line will be terminated by the ferrite bead. In one aspect, the resistor and ferrite bead are provided in a single surface mount package.
REFERENCES:
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patent: 4638272 (1987-01-01), Ive
patent: 4677396 (1987-06-01), Cruz et al.
patent: 4760355 (1988-07-01), Dash et al.
patent: 5095296 (1992-03-01), Parker
patent: 5654507 (1997-08-01), Hicks et al.
patent: 6020562 (2000-02-01), Burns et al.
Cisco Technology Inc.
Phan Trong
Sheridan Ross
Tran M.
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