高速设计精彩比喻文章

在”high speed signal propagation“上看到的，强啊，利用铁轨做传输线，传差分信号用于控制和通讯，绝对有创造力！！
In general the telegrapher's equations may be successfully applied to any transmission media having the following characteristics:

At least two conductors, insulated from each other,

Having a uniform cross section along the entire length of the structure,

With a cross-sectional geometry small compared to the wavelength of the signals conveyed, and

A length long compared to the spacing between the conductors.

Any such structure, if properly terminated, may be used to convey high-speed data. One example of an unusual transmission structure is a railroad track. This works only in dry areas of the country where the tracks are not shorted together by ground moisture. The tracks have to be designed to maintain electrical continuity along their entire length.

A fascinating example of such a track is the Bay Area Rapid Transit (BART) system built in the early 1980's in the San Francisco region. This system uses light-gauge railroad tracks held off the ground on concrete pylons to avoid ground moisture. In the early versions of the system, digital control data was transmitted through the rails in a differential mode from the master control center to each train. The trains picked up and responded to the control signals through their metal wheels. The axles on each car were of course insulated from side to side to prevent shorting out the tracks. This system worked reasonably well until, after a few months of operation, the rust buildup on the tracks began to interfere with the rail-to-wheel electrical contact, producing intermittent behavior that foiled the system.

在 high speed digital design里面，也用这个方形管子里面的水流来做比喻，真是很形象的。
At high frequencies, when the edge rate (rise and fall times) of the digital signal is small compared to the propagation delay of an electrical signal traveling down the PCB trace, the signal will be greatly affected by transmission line effects. The electrical signal will travel down the transmission line in the way that water travels through a long square pipe. This is known as electrical wave propagation. Just as the waterfront will travel as a wave down the pipe, an electrical signal will travel as a wave down a transmission line. Additionally, just as the water will travel the length of the pipe in a finite amount of time, the electrical signal will travel the length of the transmission line in a finite amount of time. To take this simple analogy one step further, the voltage on a transmission line can be compared to the height
of the water in the pipe, and the flow of the water can be compared to the current.

更多的讨论，请去我们中国PCB技术网论坛SI交流区参与。