We all know the truth that to make a PCB is to make designed schematic into a real PCB board, please do not underestimate this process,many things in principle feasible projects is difficult to achieve, so making a PCB board is easy to say, but that is not an easy thing.
Two major difficulties in the field of microelectronics is the processing of high frequency signals and weak signals, in this regard PCB production level is especially important, the same principles of design, the same components on PCB made out by different people may have different results , then how to make a good PCB ? Based on our past experience, to talk about my own views on the following aspects:
1. To clear design goals
Receive a design task, we must first clarify its design goals. Is it a common PCB board, high-frequency PCB board, small signal processing PCB board or have both a high frequency small signal processing PCB board ? If an ordinary PCB board, it will be ok as long as reasonable layout wiring neat, accurate mechanical dimensions. If the load line and long-term, we must use certain means for processing, reduce the load, long-term drive to strengthen the focus on preventing long-term reflection. When the board has more than 40MHz signal line, it is necessary for these signal lines for special considerations, such as crosstalk between lines and so on. If the frequency is higher, the length of wiring have more stringent restrictions, according to the network theory of distribution parameters, the interaction between the high-speed circuit and its connection is the decisive factor in system design that can not be ignored. With the improvement of the transmission speed of the door ,the against signal line will be a corresponding increase, the crosstalk between adjacent signal lines will increase in proportion to the normally high-speed circuit power consumption and heat dissipation are also great. It should be taken seriously enough when doing high-speed PCB. When the board has millivolt or microvolt weak signals on these signal lines,then these lines need special attention, due to the small signal is too weak, very susceptible to interference from other strong signal, often shielding measures are necessary, otherwise it will greatly reduce the SNR. So that the useful signal is submerged in the noise, then it can not be effectively extracted.
Measured on the board of adjustment should be taken into account, the physical location of the test points, isolation test point factor in the design phase can not be ignored, because some small-signal and high-frequency signals can not be directly added to the probe to carry out measurements. Moreover, we should consider some other relevant factors, such as the board layers, using the package outline components, board mechanical strength and so on. Before making PCB, to make a pretty good idea of the design goals.
2. Understand the function of components for layout requirements
We know that there are special requirements when layout some special components, such as analog signal amplifier used on LOTI and APH. Small signal analog part should try to stay away from power devices. In OTI board, small signal amplification section also specifically added shield stray to block out electromagnetic interference. NTOI board with GLINK chip use ECL technology, power consumption with severe heat, heat dissipation issues must be under special consideration when layout. If using of natural cooling, it is imperative GLINK chip on the air circulation is relatively smooth places and the heat can not form a large impact on other chips. If the device is equipped with a horn or other high-power on the board, it may cause serious pollution to the power that should be taken seriously enough.
3. Consider the layout of the components
The first thing to be considered for the layout of components is the electrical properties of the components.Put the connection-close components together as possible, especially for some high-speed line, make it as short as possible when layout. The power signal and small signal devices should be separated. Under the premise of meeting circuit performance,we also need consider the components neatly, beautiful, easy to test, mechanical board size, location and other outlets also need to be carefully considered. Factors that transmission delay time in the high-speed system interconnect and ground lines in the system design is the first consideration.Transmission time of signal line has big influence on the overall system speed , particularly for high-speed ECL circuits. Although integrated circuit itself has a high speed, the base plate by a conventional interconnects (lines per length of about 30cm the amount of delay 2ns) lead to increased delay time. So the system speed can be greatly reduced. Synchronization component like a shift register, synchronization counterwork ,Preferably on the same piece plug-in board, because the signal delay time about transmission of different plug-in boards of the clock is not equal, the shift register may yield major error. If not put on the same board, for the place where synchronization is the key, the clock line from a common clock source to the length of each plug-in board must be equal.
4. Consider wiring
With OTNI and star fiber optic network design completed, there will be more boards with high-speed signal lines need to be designed, here are some basic concepts of high-speed lines.
1). Transmission line
Any "long" signal paths of a printed circuit board of can be regarded as a transmission line. If the delay time of transmission line is much shorter than the signal rise time, then the reflect made by the rise signal will be submerged.
Factors that effect whether to adopt the basic elements of the transmission line has the following five. They are:
(1) the signal alongside rate of the system,
(2) the connection distance
(3) capacitive load (fan-out number),
(4) resistive load (wire termination method);
(5) Allowed recoil and overshoot percentage (reduce the degree of immunity of exchange).
To be continue........
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