The present invention relates to computer systems, and more particularly to signal transmitting circuit on printed circuit board and method for designing same.
Signal integrity is an important factor to be taken into account when a printed circuit board (PCB) is designed. A well-designed PCB has an elevated on-off switching speed of integrated circuits, and a high density, compact layout of components. Parameters of the components and of the PCB substrate, a layout of the components on the PCB, and a layout of high-speed signal transmission lines all have an impact on signal integrity. In turn, proper signal integrity helps the PCB and an associated computer system to achieve stable performance. Impedance matching is considered an important part of signal integrity. Therefore a characteristic impedance of a transmission line is designed to match an impedance of a load associated with the transmission line. If the characteristic impedance of the transmission line is mismatched with the impedance of the load, signals arriving at a receiving terminal are apt to be partially reflected, causing a waveform of the signals to distort, overshoot, or undershoot.
Referring to
What is needed, therefore, is a signal transmitting circuit providing greater signal integrity.
An exemplary signal transmitting circuit includes a driving circuit, a main transmission line, a resistor, a node and a plurality of receiving circuits. The driving circuit is coupled to the node via the resistor and the main transmission line. Each of the receiving circuits is coupled to the node via an offshoot transmission line. The lengths of the offshoot transmission lines are generally equal to each other.
It is of advantage that the signal transmitting circuit reduces signal reflections and maintains signal integrity.
Other advantages and novel features will become more apparent from the following detailed description of preferred embodiments when taken in conjunction with the accompanying drawings, in which:
A resistor R is connected between the north bridge chipset 20 and the node A. The resistor R is located nearer the north bridge chipset 20 than the node A. The resistance of the resistor R taken together with the internal resistance of the north bridge chipset 20 should be generally equal to the resistance of the main transmission line 28. In the above-described signal transmitting circuit of the preferred embodiment of the present invention, the resistance of the north bridge chipset 20 is about 24Ω, the resistance of the main transmission line 28 is about 56Ω, therefore the resistance of the resistor R is about 32Ω.
Generally, the length of the offshoot transmission lines 222, 242, and 262 are equal to each other but some variance is tolerable and may be calculated according to known formulas in the art.
In the above-described signal transmitting circuit of the preferred embodiment of the present invention, the north bridge 20, and the PCI-X slots 22, 24 and 26 are used as examples. In other embodiments, the signal transmission circuit with a resistor connected to the driving circuit may be used for any other signal transmission need on a PCB.
It is to be understood, however, that even though numerous characteristics and advantages of the present embodiments have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size, and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.
Number | Date | Country | Kind |
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200510036445.6 | Aug 2005 | CN | national |