The speed of light is just too slow. Commonplace, modern, volume-manufactured digital designs require control of timings down to the picosecond range. The amount of time it takes light from your nose to reach your eye is about 100 picoseconds (in 100 ps, light travels about 1.2 in.). This level of timing must not only be maintained at the silicon level, but also at the physically much larger level of the system board, such as a computer motherboard. These systems operate at high frequencies at which conductors no longer behave as simple wires, but instead exhibit high-frequency effects and behave as transmission lines that are used to transmit or receive electrical signals to or from neighboring components. If these transmission lines are not handled properly, they can unintentionally ruin system timing. Digital design has acquired the complexity of the analog world and more. However, it has not always been this way. Digital technology is a remarkable story of technological evolution. It is a continuing story of paradigm shifts, industrial revolution, and rapid change that is unparalleled. Indeed, it is a common creed in marketing departments of technology companies that "by the time a market survey tells you the public wants something, it is already too late." As the reader undoubtedly knows, the basic idea in digital design is to communicate information with signals representing 1s or 0s. In order to maximize the speed of operation of a digital system, the timing uncertainty of a transition through the threshold region must be minimized. This means that the rise or fall time of the digital signal must be as fast as possible. Ideally, an infinitely fast edge rate would be used, although there are many practical problems that prevent this. Realistically, edge rates of a few hundred picoseconds can be encountered. The reader can verify with Fourier analysis that the quicker the edge rate, the higher the frequencies that will be found in the spectrum of the signal.
Every conductor has a capacitance, inductance, and frequency-dependent resistance. At a high enough frequency, none of these things is negligible. Thus a wire is no longer a wire but a distributed parasitic element that will have delay and a transient impedance profile that can cause distortions and glitches to manifest themselves on the waveform propagating from the driving chip to the receiving chip. The wire is now an element that is coupled to everything around it, including power and ground structures and other traces. The signal is not contained entirely in the conductor itself but is a combination of all the local electric and magnetic fields around the conductor. The signals on one interconnect will affect and be affected by the signals on another. Furthermore, at high frequencies, complex interactions occur between the different parts of the same interconnect, such as the packages, connectors, vias, and bends. All these high-speed effects tend to produce strange, distorted waveforms that will indeed give the designer a completely different view of high-speed logic signals.
In today's high-speed digital systems, it is necessary to treat the printed circuit board (PCB) or multichip module (MCM) traces as transmission lines. It is no longer possible to model interconnects as lumped capacitors or simple delay lines, as could be done on slower designs. This is because the timing issues associated with the transmission lines are becoming a significant percentage of the total timing margin. Great attention must be given to the construction of the PCB so that the electrical characteristics of the transmission lines are controlled and predictable.(eCAE Internal Education: System R&D, SEIN R&D Center)
No comments:
Post a Comment