1. To clearly design goals to accept a design task, we must first clear its design goals, is an ordinary PCB board, high-frequency PCB board, small signal processing PCB board or both high frequency and small signal processing PCB board, if Is an ordinary PCB board, as long as the layout and wiring is reasonable and neat, the mechanical size can be accurate, if there is a medium load line and long line, we must use a certain means to deal with, reduce the load, long lines should strengthen the drive, the key is to prevent long lines reflection. When there are more than 40 MHz signal lines on the board, special considerations must be taken for these signal lines, such as crosstalk between lines. If the frequency is higher, the length of the wiring will be more strictly limited. According to the network theory of distributed parameters, the interaction between the high-speed circuit and its connection is a decisive factor and cannot be neglected during system design. As the transmission speed of the gate increases, the opposition on the signal line will increase accordingly, and the crosstalk between adjacent signal lines will increase in a proportional manner. Generally, the power consumption and heat dissipation of the high-speed circuit are also large, and the high-speed PCB is made. Should be given enough attention. When the board has weak signals of millivolts or even microvolts, special care is required for these signal lines. Small signals are too weak to be easily interfered by other strong signals and shielding measures are often necessary. Greatly reduce the signal to noise ratio. As a result, useful signals are submerged by noise and cannot be effectively extracted. The commissioning of the board must also be considered during the design phase. The physical location of the test points and isolation of the test points must not be ignored because some small signals and high-frequency signals cannot be directly added to the probe for measurement. In addition, consider other related factors such as the number of board layers, the package outline of the components, and the mechanical strength of the board. Before making a PCB board, it is worthwhile to make a design goal for the design.
 

2. Understand the function of the components used in the layout and wiring requirements We know that some special components in the layout and wiring have special requirements, such as the analog signal amplifier used by LOTI and APH, an analog signal amplifier to the power supply requirements to be smooth, small ripple. The analog small-signal part should be as far away from the power device as possible. On the OTI board, the small signal amplification section is also specifically shielded to shield spurious electromagnetic interference. The GLINK chip used on the NTOI board adopts the ECL process, which consumes a lot of heat and heat. It must be specially considered when laying out the heat dissipation problem. If natural heat dissipation is used, the GLINK chip must be placed in a place where air circulation is relatively smooth. , And the heat released does not have a big effect on other chips. If the board is equipped with a horn or other high-power devices, it may cause serious pollution on the power supply should also attract sufficient attention.
 

3. Component layout considerations Component layout One of the first factors to consider is the electrical performance. Put components that are closely related to each other together as much as possible. Especially for some high-speed lines, make them as short as possible during layout. Power signals and small-signal devices should be separated. In the premise of meeting the circuit performance, it is also necessary to consider that the components are arranged neatly, beautifully, and easy to test. The mechanical dimensions of the board and the position of the socket also need to be carefully considered. TrThe transmission delay times on grthe ound and interconnect lines in high-speed systems are also the first considerations in system design. The transmission time on the signal line has a great impact on the overall system speed, especially for high-speed ECL circuits. Although the speed of the integrated circuit block itself is very high, it is due to the use of common interconnection lines (every 30cm line length The delay of 2ns brings about an increase of the delay time, which can greatly reduce the system speed. Like the shift register, the synchronous working part such as the synchronous counter is preferably placed on the same plug-in board because of the clock to different plug-in boards. The transmission delay time of the signals is not equal, which may cause the shift register to produce errors. If it cannot be placed on a board, the clock lines connected from the common clock source to the plug-in boards must be equal in length where synchronization is critical.
 

4. Consider the wiring With the OTNI and star-shaped fiber network design is completed, there will be more than 100MHz with high-speed signal lines in the future need to design the board, here will introduce some of the basic concepts of high-speed lines.
 

1. Transmission Lines Any "long" signal path on a printed circuit board can be considered as a transmission line. If the transmission delay time of the line is much shorter than the signal rise time, the reflection of the main product produced during signal rise will be submerged. Overshoot, recoil, and ringing are no longer present. For most modern MOS circuits, the ratio of rise time to line delay time is much greater, so traces can be measured in meters without signal distortion. For the faster logic circuits, especially the ultra-high-speed ECL integrated circuits, the edge length must be greatly shortened to maintain the signal integrity due to the increased edge speed. There are two ways to make a high-speed circuit work on a relatively long line without serious waveform distortion. TTL uses a Schottky diode clamp method for a fast falling edge, and the overshoot is clamped at a diode drop below ground potential. At the level, this reduces the subsequent kickback amplitude. The slower rising edge allows overshoot, but it is attenuated by the circuit's relatively high output impedance (50-80Ω) in the “H” state. . In addition, due to the large degree of immunity of the level “H” state, the problem of backlash is not very prominent. For the devices of the HCT series, the combination of the Schottky diode clamp and the series resistance termination method improves the combination. The effect will be more pronounced. When there is fanout along the signal line, the above-mentioned TTL shaping method appears to be insufficient at higher bit rates and faster edge rates. Because there are reflected waves in the lines, they tend to be synthesized at high bit rates, causing severe signal distortion and reduced immunity to interference. Therefore, in order to solve the reflection problem, another method is usually used in the ECL system: line impedance matching method. In this way, the reflection can be controlled and the signal integrity can be guaranteed. Strictly speaking, the transmission line is not very necessary for conventional TTL and CMOS devices with slower edge speeds. For high-speed ECL devices with faster edge speeds, transmission lines are not always required. But when using transmission lines, they have the advantage of predicting the connection delay and controlling the reflection and oscillation through impedance matching. There are five basic factors that determine whether or not to use a transmission line. They are: (1) Edge rate of the system signal, (2) Connection distance (3) Capacitive load (how much fan-out), (4) Resistive load (line termination method); (5) Permissible Backlash and overshoot percentage (degree of reduction in AC immunity).
 

2. Several types of transmission lines (1) Coaxial cable and twisted pair: They are often used in the connection between the system and the system. The characteristic impedance of the coaxial cable is usually 50Ω and 75Ω, and the twisted pair is usually 110Ω. (2) Microstrip line on the printed board The microstrip line is a strip conductor (signal line). Separated from the ground plane with a dielectric. If the line thickness, width, and distance from the ground plane are controllable, its characteristic impedance is also controllable. The characteristic impedance Z0 of the microstrip line is: where: [Er is the relative permittivity of the dielectric material of the printed board 6 is the thickness of the dielectric layer W is the width of the line t is the thickness of the line unit length of the microstrip line transmission The delay time depends only on the dielectric constant and is independent of the width or interval of the line. (3) Stripline in printed circuit strip The stripline is a copper strip line placed in the middle of the dielectric between two conductive planes. If the thickness and width of the wire, the dielectric constant of the medium, and the distance between the two conductive planes are controllable, then the characteristic impedance of the wire is also controllable. The characteristic impedance of the stripline B is: where b is two The distance between the ground boards is the width of the wire and the thickness of the wire is the thickness of the wire. Similarly, the transmission delay time per unit length of the stripline is independent of the width or pitch of the wire; it depends only on the relative permittivity of the medium used.
 

3. Terminating Transmission Lines When a receiving end of a line is terminated with a resistor equal to the line's characteristic impedance, the transmission line is said to be connected in parallel. It is mainly used to obtain the best electrical performance, including driving distribution loads. Sometimes in order to save power consumption, a 104 capacitor is connected in series with the terminated resistor to form an AC termination circuit, which can effectively reduce the DC loss. A resistor is serially connected between the driver and the transmission line, and the terminal of the line is no longer connected to the termination resistor. This termination method is called serial termination. Overshoot and ringing on longer lines can be controlled by series damping or series termination techniques. Series damping is achieved using a small resistor (typically 10 to 75 Ω) in series with the output of the drive gate. This damping method is suitable Used in conjunction with a controlled impedance of characteristic impedance (such as backplane routing, circuit boards without ground planes, and most of the windings, etc.) The sum of the series resistor value and the output impedance of the circuit (driver gate) is equal to the transmission line. Characteristic impedance. There is a disadvantage that the series connection can only use the lumped load and the transmission delay time at the terminal. However, this can be overcome by using an extra series termination of the transmission line.
 

4. Non-Terminated Transmission Lines If the line delay time is much shorter than the signal rise time, the transmission line can be used without series termination or parallel termination if there is a two-way delay for a non-terminal connection (the signal goes back and forth once on the transmission line. The time) is shorter than the rise time of the pulse signal, so the backlash caused by non-termination is about 15% of the logical swing. The maximum open route length is approximately: Lmax
 

5. Comparison of several termination methods Both parallel-connection and series-connection have their own advantages. Which one is used, or both, depends on the designer's preferences and system requirements. The main advantage of parallel-connected terminals is that the system speed is fast and the signal transmission on the line is complete without distortion. The load on the long line will neither affect the transmission delay time of the drive gate driving the long line nor affect its signal edge speed, but will increase the transmission delay time of the signal along the long line. When driving a large fan-out, the load may be distributed along the branch stubs rather than the terminals that must assemble the load online as in the series termination. The series termination method allows the circuit to have the ability to drive several parallel load lines. The delay in the series connection due to capacitive loading is about twice as large as the corresponding parallel connection, while the short line is due to capacitive loading. Slower speeds and longer drive gate delays, but the crosstalk of the series-connected terminals is smaller than that of the parallel-connected terminals. The main reason is that the amplitude of the signal transmitted along the series-connected terminals is only one-half of the logical swing. The switch current is also only half of the switch current that is connected in parallel, and the small signal energy crosstalk is also small.
 

5. PCB board wiring technology When doing PCB, it is to choose whether to use double-sided board or multi-layer board. It depends on the maximum operating frequency and the complexity of the circuit system and the requirements on the assembly density. The multi-layer board is preferred when the clock frequency exceeds 200 MHz. If the operating frequency exceeds 350MHz, it is best to use a printed circuit board with PTFE as the dielectric layer because its high-frequency attenuation is smaller, the parasitic capacitance is smaller, the transmission speed is faster, and the Z0 ratio is higher. Larger and less power consumption, the following principles are required for the traces of the printed circuit board. (1) Larger intervals should be left between all parallel signal lines to reduce crosstalk. If there are two signal lines that are closer together, it is better to take a ground line between the two lines. This can provide shielding. (2) When designing signal transmission lines, sharp bends must be avoided to prevent reflections of the characteristic impedance of the transmission lines from causing reflections. It should be designed as a uniform circular arc with a certain size. The width of the printed line can be calculated based on the above-mentioned formula of the characteristic impedance of the microstrip line and the strip line. The characteristic impedance of the microstrip line on the printed circuit board is generally between 50 and 120Ω. To get a large characteristic impedance, the line width must be made very narrow. But thin lines are not easy to make. Considering various factors, it is generally appropriate to select a resistance value of about 68Ω. Since the characteristic impedance of 68Ω is selected, the best balance between delay time and power consumption can be achieved. A 50Ω transmission line will consume more power; a larger impedance can actually reduce power consumption, but it will increase transmission delay time. Because the negative line capacitance will cause the transmission delay time increases and the characteristic impedance decreases. However, the intrinsic capacitance per unit length of a segment with a very low characteristic impedance is relatively large, so the transmission delay time and the characteristic impedance are less affected by the load capacitance. An important feature of a properly terminated transmission line is that the branch stub should have no effect on the line delay time. When Z0 is 50Ω. The length of short branches must be limited to 2.5cm. In order to avoid a great ringing. (4) For double panels (or four layers in a six-layer board). The lines on both sides of the board should be perpendicular to each other so as to prevent mutual crosstalk between producers and producers. (5) If high-current devices, such as relays, indicator lights, and horns, are installed on the printed circuit board, their ground lines should preferably be separated and moved separately to reduce ground noise. The ground lines of these high-current devices should be Connect to a separate ground bus on the board and backplane, and these separate grounds should also be connected to the ground point of the entire system. (6) If there is a small signal amplifier on the board, the weak signal line before amplification should be far away from the strong signal line, and the trace should be as short as possible. If possible, it must be shielded with a ground line.