We all know that PCB is to design a good schematic diagram into a real PCB, please do not underestimate the process, there are many principles of things in the project is difficult to achieve in the project, or others can achieve Something else can not be achieved, so that it is not difficult to do a PCB board, but to do a piece of PCB is not an easy thing.
 
Microelectronics in the field of the two major difficulties lies in the processing of high-frequency signals and weak signal, in this regard PCB production level is particularly important, the same principle design, the same components, different people made PCB with different results , Then how can we make a good PCB? According to our previous experience, would like to talk about the following aspects of their views:
 
First, to clear the design goals
 
To accept a design task, first of all to clear its design goals, is the ordinary PCB, high-frequency PCB, small signal processing PCB or both high frequency and small signal processing PCB, if it is a common PCB, as long as the layout and wiring Reasonable and neat, mechanical size can be accurate, if the load line and long-term, we must use a certain means to deal with, reduce the load, long-term to strengthen the drive, the focus is to prevent long-term reflection. When the board has more than 40MHz signal line, it is necessary to consider these signal lines, such as inter-line crosstalk and other issues. If the frequency is higher, the length of the wiring there is a more stringent restrictions, according to the distributed parameters of the network theory, high-speed circuit and its connection between the connection is the decisive factor in the system design can not be ignored. As the gate transmission speed increases, the opposition in the signal line will be a corresponding increase in the adjacent signal line crosstalk will be proportional to increase, usually high-speed circuit power consumption and heat dissipation are also large, high-speed PCB Should cause enough attention.
 
When the plate has a millivolt level or even micro-level weak signal, these signal lines need special care, small signal because too weak, very vulnerable to other strong signal interference, shielding measures are often necessary, otherwise Greatly reduce the signal to noise ratio. So that the useful signal is submerged by noise, can not be effectively extracted.
 
The measurement of the board should also be considered in the design phase, the physical location of the test point, test point isolation and other factors can not be ignored, because some small signal and high frequency signal can not be directly applied to the probe to measure.
 
In addition to consider other related factors, such as the number of boards, the use of components of the package shape, the board's mechanical strength. Before doing PCB board, to make the design of the design goals.
 
Second, understand the function of the components used on the layout and wiring requirements
 
We know that some special components in the layout and wiring have special requirements, such as LOTI and APH used in analog signal amplifier, analog signal amplifier on the power requirements to be smooth, ripple small. Simulate the small signal part as far as possible away from the power device. In the OTI board, the small signal amplification part also specifically with a shield, the spurious electromagnetic interference to the shield out. NTOI board with the GLINK chip is used ECL process, power consumption big heat, the cooling problem must be in the layout of the need for special consideration, if the use of natural heat, it is necessary to GLINK chip on the air flow more smoothly , And scattered out of the heat can not form a big impact on other chips. If the board is equipped with speakers or other high-power devices, there may be serious pollution caused by the power of this point should also be given enough attention.
 
Third, consider the layout of components
 
Components of the layout of the first factor to consider is the electrical properties, the connection between the components as close as possible, especially for some high-speed lines, the layout will make it as short as possible, power signals and small signal devices To be separated. In the premise of meeting the performance of the circuit, but also consider the components placed neat, beautiful, easy to test, the board's mechanical size, the location of the socket also need to seriously consider.
 
The transmission delay on the ground and interconnects in the high-speed system is also the first factor to consider when designing the system. The transmission time on the signal line has a great effect on the overall system speed, especially for high-speed ECL circuits. Although the integrated circuit block itself is very fast, it is common to use a common interconnect (approximately 30 cm 2ns delay amount) to increase the delay time, the system speed can be greatly reduced.As the shift register, synchronous counter This synchronous work components are best placed on the same plug-in board, because the different boards on the board Signal transmission delay time is not equal, may cause the shift register master error, if not on a board, then the synchronization is the key place, from the public clock source to the plug-in board clock line length must be equal.
 
Fourth, the consideration of the wiring
 
With the OTNI and star fiber optic network design is completed, there will be more than 100MHz above the board with high-speed signal needs to be designed, here will introduce some of the basic concepts of high-speed line.
 
Transmission line:
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 reflections produced by the producers during the rise of the signal will be flooded. No longer overshoot, recoil and ring, for most of the current MOS circuit, because the rise time on the line transmission delay time ratio is much larger, so the line can be long meter without signal distortion. While for faster logic circuits, especially ultra-high-speed ECL.
 
For the integrated circuit, the length of the trace must be greatly reduced in order to maintain the integrity of the signal due to the increased speed of the edge.
There are two ways to make the high-speed circuit work on a relatively long line without serious waveform distortion. The TTL is applied to the fast descending edge using a Schottky diode clamp method so that the overshoot is clamped to a diode drop lower than the ground potential , This reduces the backflush amplitude, and the slower rising edge allows overshoot, but it is attenuated by the relatively high output impedance (50 to 80Ω) of the circuit in level "H" The In addition, due to the level of "H" state of the large degree of immunity, so that the recoil problem is not very prominent, the HCT series of devices, if the use of Schottky diode clamp and series resistance termination method, The effect will be more obvious.
 
When the fan line is out along the signal line, the TTL shaping method described above is somewhat inadequate 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, resulting in severe signal distortion and reduced immunity. Therefore, in order to solve the reflection problem, in the ECL system usually use another method: line impedance matching method. In this way, reflection can be controlled and the integrity of the signal is guaranteed.
 
Strictly, the transmission lines are not needed for conventional TTL and CMOS devices with slower edge speeds, and for high-speed ECL devices with faster edge speeds, transmission lines are not always needed. However, when using transmission lines, they have the advantage of predicting connection delay and by impedance matching to control reflection and oscillation.
 
1. There are five basic factors that determine whether or not to use a transmission line. They are:
(1) the rate along the system signal, (2) the distance (3) the capacitive load (fan out of the number), (4) resistive load (line termination); (5) Percentage of overshoot (degree of reduction in AC immunity).
 
2. Several types of transmission lines
(1) coaxial cable and twisted pair: they are often used in the system and the connection between the system. Coaxial cable characteristic impedance is usually 50Ω and 75Ω, twisted pair is usually 110Ω.
(2) microstrip lines on printed circuit boards
The microstrip line is a strip lead (signal line). And the ground plane with a dielectric isolation. If the thickness of the line, the width, and the distance from the ground plane are controllable, its characteristic impedance is also controllable.
(3) stripline in printed circuit boards
The stripline is a copper strip line in the middle of the dielectric placed between the two conductive planes. The characteristic impedance of the line is also controllable if the thickness and width of the line, the dielectric constant of the medium, and the distance between the two conductive planes are controllable.
 
3. Termination transmission line
In the end of a line with a line impedance with the same resistance of the termination, then the transmission line that the parallel wiring. It is mainly used to obtain the best electrical performance, including the use of driving the distribution load.
Sometimes in order to save power consumption, the termination of the resistor on a series of 104 capacitors connected to the formation of AC termination circuit, it can effectively reduce the DC loss.
A resistor is connected in series between the driver and the transmission line, and the terminal of the wire is no longer connected to the resistor. This termination method is called a series termination. Overshoot and ringing on longer lines can be controlled by tandem damping or tandem termination. Tandem damping is achieved with a small resistor (typically 10 to 75Ω) in series with the drive gate output. This damping method is suitable for And the characteristic impedance to control the line associated with (such as the floor wiring, groundless circuit board and most of the wiring and so on.
The sum of the series resistance and the output impedance of the circuit (drive gate) is equal to the characteristic impedance of the transmission line when the series termination is terminated, and there is a drawback that only the total load and transmission delay time can be used at the terminal. Can be overcome by using redundant serial termination of the transmission line.
 
4. Non-terminated transmission line
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 a non-terminated wiring is delayed (the time the signal travels once on the transmission line) The rise time of the signal is short, then the recoil caused by the non-termination is about 15% of the logical swing. The maximum open line length is approximately: Lmax <tr / 2tpd. Where: tr is the rise time tpd is the transmission delay time per unit length
 
5. Comparison of several termination methods
Parallel terminal wiring and serial terminal wiring have their own advantages, what kind of use, or both are used, depending on the designer's preferences and system requirements. The main advantage of the parallel termination wiring is that the system is fast and the signal is completely distorted on-line transmission. The load on the long line will not affect the transmission delay time of the drive gate driving the long line without affecting 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 can be distributed along the branch line along the line, rather than the series termination as the load must be the main assembly line.
 
The series termination method allows the circuit to drive several parallel load lines with the capacity of the series-side wiring due to the capacitive load, which is about twice as large as the corresponding parallel termination line, and the short- The slowdown of the drive and the delay time of the drive gate are increased, but the crosstalk of the series connection is smaller than that of the parallel terminal. The main reason is that the amplitude of the signal transmitted along the serial terminal is only one half of the logical swing Switching current is only half of the parallel termination of the switching current, the signal energy is also a small crosstalk small.
 
PCB is used when the double-panel or multi-layer board, depending on the maximum operating frequency and the complexity of the circuit system and the requirements of the assembly density to decide. In the clock frequency of more than 200MHZ when the best choice of multi-layer board. If the operating frequency of more than 350MHz, the best choice to Teflon as a dielectric layer of the printed circuit board, because of its high frequency attenuation to be smaller, parasitic capacitance to be smaller, faster transmission, but also because Z0 Large and provincial power consumption, the printed circuit board alignment has the following principles:
(1) between all parallel signal lines to try to leave a large interval to reduce crosstalk. If there are two lines closer to the signal line, it is best to take a ground wire between the two lines, so you can play a shielding effect.
(2) to design the signal transmission line to avoid sharp turn, to prevent the transmission line characteristic impedance of the mutation and produce reflection, to try to have a certain size of the uniform arc.
(3) The width of the printed line can be calculated according to the characteristic impedance calculation formula of the microstrip line and the stripline. 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 very narrow But very fine lines are not easy to make. Considering a variety of factors, the general choice of 68Ω impedance value is more appropriate, because the choice of 68Ω characteristic impedance, you can delay time and power consumption to achieve the best balance between. A 50Ω transmission line will consume more power; the larger impedance can reduce the power consumption, but will delay the transmission delay. As the negative line capacitance will cause the transmission delay time increases and the characteristic impedance decreases. But the characteristic impedance is very low line unit length of the intrinsic capacitance is relatively large, so the transmission delay time and characteristic impedance by the load capacitance of the smaller. 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 the branch stub must be limited to less than 2.5cm. To avoid a big ringing.
(4) for double-sided (or six-layer board to go four lines). The lines on both sides of the circuit board should be perpendicular to each other to prevent mutual induction of the main crosstalk.
(5) If the printed circuit board is equipped with high current devices, such as relays, lights, speakers, etc., their ground is best to separate separately to reduce the ground noise, the high current device ground Connected to a separate bus on the board and backplane, and these separate ground wires should also be connected to the grounding point of the entire system.
(6) If there is a small signal amplifier on the board, the weak signal line before amplification is far away from the strong signal line, and the trace should be as short as possible, if possible, also use the ground to shield it.