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Keyword: Circuit board, PCB design, PCB

PCB design is based on the graph is based on circuit principle, realize the function of the circuit designer need. PCB design mainly refers to the layout design, need to consider the layout of the external connections. Optimize the layout of the internal electronic components. Metal wire and hole layout optimization. Electromagnetic protection. Various factors such as heat dissipation. Good landscape design can save production costs and achieve good circuit performance and heat dissipation performance. Simple layout design can be implemented by hand and complex layout design be achieved with the aid of computer aided design (CAD).

In the design of high-speed, controllable impedance characteristics of the plate and the circuit impedance is one of the most important and the most common problems. First look at the definition of a transmission line, transmission line is composed of two conductors of a certain length, a conductor used for sending signals, another is used to receive signals (remember "loop" to replace the concept of "land"). In a multilayer board, every line is part of the transmission line, adjacent to the reference plane can be used as the second line or loop. A route as "good performance" is the key to make it the characteristic impedance of a transmission line remained constant throughout the line.

Circuit board become "controllable impedance board" is the key to satisfy all line characteristic impedance of a specified value, usually between 25 ohms and 70 ohms. In multilayer PCB, good performance is the key to make it a transmission line characteristic impedance of the constant in the whole line.

But, what is the characteristic impedance? To understand the simplest method is to look at the signal characteristic impedance met what in transit. When moving along a cross section has the same transmission line, the microwave transmission of similar as shown in figure 1. Assume that add 1 kv voltage staircase to the transmission lines, such as 1 volt battery connected to the front end of a transmission line (it is located in between transmitting circuits and circuit), once connected, the voltage wave signal along the line at the speed of light, its speed is usually about 6 inches/nanoseconds. Of course, this signal is sent to the line and circuit, the voltage difference between any part of it can be sent from the line and loop neighboring points to measure. Figure 2 is the voltage signal transmission schematic diagram.

Zen approach is to "signal" first, and then along the transmission line at the speed of 6 inches/nanoseconds. The first 0.01 nanoseconds 0.06 inch forward, then send the line with excess positive charge, and circuit has extra negative charge, it is precisely these two kinds of charge difference maintained 1 v voltage difference between the two conductors, and the two conductors of a capacitor.

In the next 0.01 nanoseconds, and ye shall offer a 0.06 -inch transmission line voltage from 0 to 1 v, it must add some positive charge to send the line, and add some negative charge to receive circuit. Each mobile 0.06 inches, it is necessary to add more positive charge to send line, and add more negative to the circuit. Every 0.01 seconds, another section of transmission lines must be charging, then the signal to start along that part of the transmission. From the front of a transmission line battery charge, when moving along the line, give the continuous part of a transmission line charging, and thus formed between the sending circuit and circuit the voltage difference of one volt. Every 0.01 seconds, forward to get some out of the battery charge (+ Q), constant time interval (+ t) inside the battery from the constant power (plus or minus Q) is a kind of constant current. Negative current flowing into the loop actually equals the positive current flows, and as it happens, in the front of the signal wave, consisting of alternating current through the upper and lower line capacitance, over the entire cycle.

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