How to avoid high-speed transmission line effects in PCB Assembly?-2
3, the rational planning of the topology of the alignment
The other transmission line effects solution is to choose the right path and terminal wiring topology. Routing topology is the order of a network cable wiring and cabling structure. When using high-speed logic devices in PCB Assembly, unless the alignment to maintain very short branch length, or the edge of a rapidly changing signal will be the backbone traces of the branches of the signal traces are distorted. Under normal circumstances, PCB assembly traces in two basic topology, the daisy chain (Daisy Chain) cabling and star (Star) distribution.
For the daisy-chain cabling, wiring from the driver side started, and then click to reach the receiver. If you use a series resistor to change the signal characteristics, the location of the series resistance should be close to the driver side.china pcb assembly Traces in the control of high order harmonic interference, the daisy-chain the best alignment. However, this alignment method routability minimum 100% of cloth is not easy to pass. The actual design, we are to make daisy chain wiring length as short as possible in the branch, the length of the value of security should be: Stub Delay <= Trt * 0.1.
For example, high-speed TTL side branch of the circuit length should be less than 1.5 inches. The wiring topology takes up less space and can be a single resistor matching end. However, this structure makes the alignment signal receiver in a different signal reception is not synchronized.
Star topology can effectively avoid the problem of clock signals are not synchronized, but in high density PCB assembly board layout by hand is very difficult. Automatic router to complete the star-wiring of the best. Each branch requires termination resistors. Termination resistor and the connection should match the characteristic impedance. This can be calculated by hand, CAD tools can also be calculated through the characteristic impedance and termination resistance.
In the above example uses a simple two terminal resistance, the actual option of using more complex matching terminal. The first option is to match the RC terminal. RC matching terminal can reduce power consumption, but only the work used in the signal more stable situation. This approach is best suited to match the clock line signal processing. The disadvantage is that the terminal capacitance RC match signals may affect the shape and spread.
Match the terminal series resistance does not produce additional power consumption, but will slow down the signal.china pcb assembly This approach has little effect for the time delay of bus driver circuit. The advantage of matching the terminal series resistance is also used to reduce the number of on-board devices and connection density.
Finally a way to match the terminal for the separation, matching components in this way be placed in the vicinity of the receiver. The advantage is not low signal, and can be good to avoid the noise.China pcb assembly. A typical input signal for the TTL (ACT, HCT, FAST).
In addition, termination resistor package types and mounting must also be considered. SMD Surface mount resistors are usually more than through-hole components with low inductance, so the SMD components as the preferred package. If you choose, there are two common line resistance installation options: horizontally and vertically.