Do PCB board is to design the schematic into a real PCB circuit board, please do not underestimate this process, there are a lot of things that work in principle in the project is difficult to achieve, or others can achieve things other people can not achieve, so it is not difficult to do a PCB board, but to do a PCB board is not an easy thing.
The two major difficulties in the field of microelectronics are high-frequency signal and weak signal processing, in this respect PCB production level is particularly important, the same principle design, the same components, different people produced PCB has different results, then how to make a good PCB board? Based on our past experience, I would like to share my views on the following aspects:
1. to clear the design objectives
To accept a design task, first of all to clarify its design objectives, is the ordinary PCB board, high frequency PCB board, small signal processing PCB board or both high frequency and small signal processing PCB board, if it is an ordinary PCB board, as long as the layout is reasonable and neat, the mechanical size is accurate, if there is a medium load line and long line, It is necessary to use certain means for processing, reduce the load, and strengthen the drive in the long line, focusing on preventing long-term reflection. When there are more than 40MHz signal lines on the board, special considerations must be taken into account for these signal lines, such as cross-talk between lines. If the frequency is higher, there are more stringent restrictions on the length of the wiring, according to the network theory of distributed parameters, the interaction between the high-speed circuit and its wire is a decisive factor, which can not be ignored in the system design. With the increase of the transmission speed of the door, the opposition on the signal line will increase accordingly, the crosstalk between the adjacent signal lines will increase in direct proportion, usually the power consumption and heat dissipation of high-speed circuits are also large, and sufficient attention should be paid to doing high-speed PCB.
When there is a weak signal of millivolt level or even microvolt level on the board, special care is needed for these signal lines, small signals are too weak, very susceptible to interference from other strong signals, shielding measures are often necessary, otherwise it will greatly reduce the signal-to-noise ratio. So that useful signals are drowned out by noise and cannot be extracted effectively.
The commissioning of the board should also be considered in the design stage, the physical location of the test point, the isolation of the test point and other factors can not be ignored, because some small signals and high-frequency signals can not be directly added to the probe for measurement.
In addition, some other relevant factors should be considered, such as the number of sub-layers of the board, the package shape of the components used, the mechanical strength of the board, etc. Before making a PCB board, it is necessary to have a clear idea of the design goal of the design.
2. Understand the requirements of layout and wiring for the functions of the components used
We know that some special components have special requirements in the layout and wiring, such as the analog signal amplifier used by LOTI and APH, and the analog signal amplifier requires stable power supply and 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 part is also specially equipped with a shielding cover to shield the stray electromagnetic interference. The GLINK chip used on the NTOI board uses the ECL process, the power consumption is large and the heat is severe, the heat dissipation problem must be special consideration in the layout, if the natural heat dissipation is used, the GLINK chip must be placed in a place where the air circulation is relatively smooth, and the heat emitted can not have a big impact on other chips. If the board is equipped with a horn or other high-power devices, it is possible to cause serious pollution to the power supply, which should also cause enough attention.
3. consideration of component layout
One of the first factors to consider in the layout of components is electrical performance, put the components that are closely connected together as far as possible, especially for some high-speed lines, the layout should make it as short as possible, and the power signal and small signal devices should be separated. Under the premise of meeting the circuit performance, it is also necessary to consider that the components are neatly placed, beautiful, easy to test, the mechanical size of the board, the location of the socket, etc., also need to be seriously considered.
Transmission delay times on ground and interconnect lines in high-speed systems are also the first factors to be considered in system design. The transmission time on the signal line has a great impact on the total system speed, especially for high-speed ECL circuits, although the integrated circuit block itself is very fast, but because of the increase in the delay time brought by the ordinary interconnect on the bottom plate (about 2ns delay per 30cm line length), the system speed can be greatly reduced. Like the shift register, the synchronization counter this synchronization working part is best placed on the same plug-in board, because the transmission delay time of the clock signal on the different plug-in board is not equal, which may make the shift register produce the main error, if it can not be placed on a board, then the length of the clock line from the common clock source to the plug-in board must be equal in the synchronization is critical.
4. the consideration of wiring
With the completion of the design of OTNI and star fiber network, there will be more boards with high-speed signal lines above 100MHz to be designed in the future, and some basic concepts of high-speed lines will be introduced here.
Any "long" signal path on the printed circuit board can be regarded as a transmission line. If the transmission delay time of the line is much shorter than the rise time of the signal, then the reflection generated during the rise of the signal will be drowned. Overshoot, recoil and ringing are no longer present, and for most current MOS circuits, due to the much larger ratio of rise time to line transmission delay time, the line can be long in meters without signal distortion. And for faster logic circuits, especially ultra-high speed ECL.
For integrated circuits, due to the increase in edge speed, if no other measures, the length of the line must be greatly shortened to maintain the integrity of the signal.
There are two ways to make high-speed circuits work on relatively long lines without serious waveform distortion. TTL adopts the Schottky diode clamping method for fast falling edges, so that the overimpulse is clamped at a level one diode voltage drop below the ground potential, which reduces the backkick amplitude, and the slow rising edge allows for overshoot. However, it is attenuated by the relatively high output impedance (50-80 Ω) of the circuit in the level "H" state. In addition, due to the high immunity of the level "H" state, the recoil problem is not very prominent. For HCT series devices, if Schottky diode clamping and series resistance end method are combined, the improvement effect will be more obvious.
When there is a fan out along the signal line, the TTL shaping method introduced above is somewhat insufficient under the condition of higher bit rate and faster edge rate. Because there are reflected waves in the line, they tend to synthesize at high speed, which causes serious distortion of the signal and decreases the anti-interference ability. Therefore, in order to solve the reflection problem, another method is usually used in ECL systems: line impedance matching method. In this way, the reflection can be controlled and the integrity of the signal can be guaranteed.
Strictly, he says, transmission lines are not needed for conventional TTL and CMOS devices with slower edge speeds. For high-speed ECL devices with faster edge speeds, transmission lines are not always needed. But when using transmission lines, they have the advantage of being able to predict connection delay and control reflection and oscillation through impedance matching.
1. There are five basic factors that determine whether to use a transmission line. They are:
a. Speed along the system signal, b. Connection distance, c. Capacitive load (how much fan out), d. Resistive load (how the line is terminated), e. Allowable percentage of recoil and overshoot (how much AC immunity is reduced).
2. several types of transmission lines
2.1. coaxial cable and twisted pair: They are often used in the connection between systems. The characteristic impedance of coaxial cable is usually 50Ω and 75Ω, and twisted pair is usually 110Ω.
2.2. Microstrip line on printed board
A microstrip line is a strip guide (signal line). It is separated from the ground plane by a dielectric. If the line's thickness, width, and distance from the ground plane are controllable, then its characteristic impedance can also be controlled. The characteristic impedance Z0 of the microstrip line is:
2.3. Ribbon line in printed board
A strip line is a copper strip line placed in the middle of a dielectric between two conductive planes. If the thickness and width of the line, the dielectric constant of the medium, and the distance between the two conductive planes are controllable, then the characteristic impedance of the line is also controllable, and the characteristic impedance of the strip line is:
3. End the transmission line
When the receiving end of a line is terminated with a resistance equal to the characteristic impedance of the line, the transmission line is called a parallel terminal connection. It is mainly used to obtain the best electrical performance, including driving distributed loads.
Sometimes in order to save power consumption, a 104 capacitor is connected to the opposing resistor in series to form an AC termination circuit, which can effectively reduce DC loss.
A resistance is connected in series between the driver and the transmission line, and the end of the line is no longer connected to the end resistance, this termination method is called series termination. Overshoot and ringing on longer lines can be controlled by series damping or series terminating techniques. Series damping is achieved by using a small resistance (generally 10-75ω) in series with the output of the drive gate. This damping method is suitable for use in connection with lines whose characteristic impedance is controlled (such as baseboard wiring, ground-less circuit boards, and most wound wires).
The sum of the value of the series resistance and the output impedance of the circuit (drive gate) at the series termination is equal to the characteristic impedance of the transmission line. The series-connected terminals have the disadvantages of only lumped load at the terminal and long transmission delay time. However, this can be overcome by using redundant series terminating transmission lines.
4. unterminated transmission line
If the line delay time is much shorter than the signal rise time, the transmission line can be used without series terminating or parallel terminating, if the double delay of an unterminated wire (the time for the signal to travel once on the transmission line) is shorter than the rise time of the pulse signal, then the recoil due to the non-terminating is about 15% of the logical swing. The maximum open route length is approximately:
Lmax < tr / 2tpd (tr is the rise time,tpd is the transmission delay time per line length)
5. Comparison of several termination methods
Parallel terminal and series terminal have their own advantages, which one to use, or both, depends on the designer's preferences and the requirements of the system. The main advantages of the parallel terminal connection are high system speed and complete signal transmission on line without distortion. The load on the long line will not affect the transmission delay time of the driving gate driving the long line, nor will it affect its signal edge speed, but will increase the transmission delay time of the signal along the long line. When the large fan is driven out, the load can be distributed along the branch short lines, rather than the terminals where the load must be lumped online as in series terminations.
The series terminal method enables the circuit to drive several parallel load lines. The delay time increment caused by the series terminal line due to the capacitive load is about twice that of the corresponding parallel terminal line, while the edge speed is slowed down and the driving gate delay time is increased due to the capacitive load of the short line. However, the crosstalk of the series terminal line is smaller than that of the parallel terminal line. The main reason is that the signal amplitude transmitted along the series terminal is only half the logical swing, so the switching current is only half the switching current of the parallel terminal, and the signal energy is small crosstalk is small.
Whether to choose dual panels or multi-layer boards when doing PCB depends on the highest operating frequency and the complexity of the circuit system and the requirements for assembly density. Multi-layer board is best used when the clock frequency exceeds 200MHZ. If the operating frequency exceeds 350MHz, it is best to choose a printed circuit board with PTFE as a dielectric layer, because its high frequency decay is smaller, the parasitic capacitance is smaller, the transmission speed is faster, and because the Z0 is larger and saves power, the wiring of the printed circuit board has the following principles:
5.1. all parallel signal lines should leave a large interval as far as possible to reduce crosstalk. If there are two signal lines that are close together, it is best to take a ground line between the two lines, which can play a shielding role.
5.2. the design of the signal transmission line to avoid sharp bends, in order to prevent the transmission line characteristic impedance mutation and reflection, to try to design into a uniform arc line with a certain size.
5.3. the width of the printed line can be calculated according to the above microstrip line and strip line characteristic impedance calculation formula, the printed circuit board microstrip line characteristic impedance is generally between 50-120Ω. To obtain a large characteristic impedance, the line width must be made very narrow. But very thin lines are not easy to make. Considering various factors, it is generally appropriate to choose an impedance value of about 68Ω, because the choice of 68Ω characteristic impedance can achieve the best balance between delay time and power consumption. A 50Ω transmission line will consume more power; Although the larger impedance can reduce the power consumption, the transmission delay time will be large. Due to the negative line capacitance, the transmission delay time increases and the characteristic impedance decreases. However, the intrinsic capacitance per unit length of line segment with low characteristic impedance is relatively large, so the transmission delay time and characteristic impedance are less affected by the load capacitance. An important feature of a transmission line with proper terminations is that the branching stub should have little effect on the line delay time. When Z0 is 50Ω. The length of branching short lines must be limited to 2. Within 5cm. To avoid loud ringing.
5.4. for double panels (or six layers of the board to walk four layers of line). The lines on both sides of the board should be perpendicular to each other to prevent cross-talk induced by each other.
5.5. if the printed board is equipped with high-current devices, such as relays, indicators, horns, etc., their ground wires are best to go separately to reduce the noise on the ground wire, the ground wire of these high-current devices should be connected to an independent ground bus on the plug-in board and backplane, and these independent ground wires should also be connected to the ground point of the entire system.
5.6. 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 line should be as short as possible, if possible, but also the ground line to shield it.
These are the conditions we need to design a good PCB board according to our experience, and grasp these points in PCB design to get twice the result with half the effort.
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