Preface: usually from copy board or schematic diagram to make PCB, need technical requirement is not high, do fast PCB is very simple, but in practice need to clear objectives, first, of course, the key still is to understand the function of components used for the requirements of the layout, reasonable make the layout of the components and PCB wiring, must can do a high quality PCB board
PCB copy board is under the condition of the existing PCB, the principle diagram of the circuit board for cloning, export, etc., it is a kind of to produce the file. Can also be based on the secondary development according to user requirements, to design and realize the function of the product upgrading and expansion. So how do you from PCB copy board file schematic diagram into a real PCB?
Maybe everyone knows well, do is the PCB design/good clone principle diagram became a real piece of PCB, please don't look down upon this process, there are a lot of things on the principle of practicable in engineering has difficult to achieve, or something that others can realize others not achieve them, so it is not difficult to make a PCB, but get a PCB board is not an easy thing.
Two major difficulties in the field of microelectronics is the high frequency signal and weak signal processing, PCB production level is particularly important in this respect, the same principle diagram, the same components, different people made PCB will have different results, so how to make a good PCB board? According to past experience, we want to talk about their views on the following:
A, need to have a clear goal
Received a design task/copy board, the first thing to clarify its goals. Are ordinary PCB board, high-frequency PCB, small signal processing PCB or there are both high frequency and small signal processing of the PCB, if only ordinary PCB design, as long as do layout reasonable, accurate mechanical size, load line and in the long term, if any, will be handled by a certain means, lighten the load, long term to strengthen the drive, the key is to prevent the long reflection. When the board has more than 40 MHZ signal lines, is about to consider these signal lines are special, such as crosstalk between line and other issues. If frequency is higher, have more restrictions on the length of the wiring, according to the distribution parameters of network theory, the interaction between high speed circuit and its attachment is the decisive factor, the system can't be ignored in design. With the improvement of the door transmission speed, on line against will increase, the crosstalk between adjacent signal lines will be proportional to the increase in, usually high-speed power consumption and heat dissipation of the circuit is very big, should cause enough attention when doing high-speed PCB.
When board have millivolt level even microvolt level when the weak signal, the signal will need special care, small signal is too weak, very vulnerable to other strong signal interference, shielding measures is often necessary, otherwise will greatly reduce the signal-to-noise ratio. So that the useful signals submerged by noise, cannot be effectively extracted.
On board the measure also should be taken into consideration during the design phase, the physical location of test points, test point of isolation factors cannot be ignored, because some of the small signal and high frequency signal is not directly add probe to measure.
In addition to consider some other related factors, such as the board layer, USES the components of the packaging appearance, mechanical strength of the board, etc. Before taking a PCB board, to make the design of the design goals in mind.
B, understand the functions of the components used to layout and wiring requirements
As we know, there are some special components in the layout have special requirements, such as LOTI and APH used analog signal amplifier, analog signal amplifier for power requirement for smooth, small ripple. As far as possible away from power device simulation of small signal part. On the OTI plate, small signal amplifier section also specifically with shielding cover, give to block the stray electromagnetic interference. NTOI plate with GLINK chip USES is ECL technology, power consumption big fever, to the problem of heat dissipation must be conducted when the layout must be special consideration, if the natural cooling, will put the GLINK chip in air flow is smooth, and out of the heat is not a big impact to other chips. As we know, there are some special components in the layout have special requirements, such as LOTI and APH used analog signal amplifier, analog signal amplifier for power requirement for smooth, small ripple. As far as possible away from power device simulation of small signal part. On the OTI plate, small signal amplifier section also specifically with shielding cover, give to block the stray electromagnetic interference. NTOI plate with GLINK chip USES is ECL technology, power consumption big fever, to the problem of heat dissipation must be conducted when the layout must be special consideration, if the natural cooling, will put the GLINK chip in air flow is smooth, and out of the heat is not a big impact to other chips.
C, components layout consideration
The layout of the components first one factor to consider is the performance, closely related to the connection of components together as far as possible, especially for some high speed line, the layout is to make it as short as possible to separate power signal and small signal device. On the premise of meet the circuit performance, consider components neat, beautiful, easy to test, mechanical size of the board, the position of the socket, etc. Also need to seriously consider.
In the high-speed system earthing and interconnection line transmission delay time is the first thing to consider when system design factors. Signal on the transmission time had a great influence on the overall system speed, especially for high-speed ECL circuits, although high speed integrated circuit block itself, but as a result of on the floor with common interconnect (every 30 cm long, about the delay of the 2 ns) bring the increase of the delay time, can make the system speed is greatly reduced. Like a shift register, synchronous counter this synchronization working parts on the same piece of card, best because of the different transmission delay time of the clock signal on the board is not equal, could lead to a shift register main production errors, if not on a plate, where synchronization is the key, from public clock source connected to the clock line must be equal to the length of the board.
D, considering the wiring
As OTNI and star optical fiber network design is completed, there will be more after more than 100 MHZ board needs to be designed with high speed signal lines, introduces some basic concepts of high-speed line here.
1. The transmission line
printed circuit boards of any of the "long" signaling pathways can be regarded as a kind of transmission line. If the line transmission delay time is much shorter than the signal rise time, then the signal rise during the master from the reflection will be submerged. No longer appear overshoot, recoil and ringing, for at the moment, most of the MOS circuit, because of the rise time of line transmission delay time is much bigger, so it can be in metres long and no signal distortion. For fast logic circuit, especially the high-speed ECL
Integrated circuit, due to the edge of fast, without other measures, must shorten the length of the line go, to maintain signal integrity
There are two ways to make the high-speed circuit in a relatively long line work without serious distortion, is used for the rapid decline in edge TTL schottky diode clamping method, make impulse be restraint in a diode is lower than the ground potential pressure drop on the level of reducing the recoil at the back of the amplitude, the slower the rising edge of the overshoot is allowed, but it is level "H" in a state of relatively high output impedance of the circuit (50 ~ 80 Ω) attenuation. In addition, due to the level of "H" state immunity, larger recoil problem is not very outstanding, device of HCT series, if using schottky diode clamping and series resistance side connect the method, the improved effect will be more obvious.
When along the line has a fan out in high bit rate and faster edge rate, the introduction of TTL plastic methods seem to be some shortcomings. Because the line of the reflection wave, they will tend to synthesis under high speed, causing serious signal distortion and anti-interference ability. Therefore, in order to solve the problem of reflection, in ECL systems usually use another method: line impedance matching method. In this way can make the reflection under control, signal integrity is guaranteed.
Strict, he says, for a conventional with slower edge speed TTL and CMOS device, a transmission line is not very need. On some edge rapidly speed of high speed ECL device, a transmission line is not always needed. But when using a transmission line, they can predict interconnect delay and control through the impedance matching reflect advantages and oscillation.
Decide whether to adopt has the following five basic factors of a transmission line. They are: (1) the system signal along the rate, (2) the attachment distance (3) the capacitive load (how much fan out), (4) resistive load (line termination); (5) the percentage of recoil and overshoot allowing lower degree of immunity (communication).
2. Several types of a transmission line
(1) coaxial cable and twisted pair: they are often used in the connection between the system and the system. Characteristic impedance of the coaxial cable is usually Ω 50 and 75 Ω, twisted pair is usually 110 Ω.
(2) the microstrip line on the PCB
Microstrip line is a ribbon guide (signal). Between the ground plane and move away in a dielectric. If the line thickness, width and the distance between the ground plane is controlled, its characteristic impedance is also can be controlled.
(3) the stripline in PCB
Stripline is one between the two layers of conductive plane dielectric copper strip in the middle of the line. If the line thickness and width, medium dielectric constant, and the distance between two layers of conductive plane can be controlled, so line characteristic impedance is controllable.
Also, the unit length of the stripline transmission delay time with the line width or spacing is irrelevant; Only depends on the relative dielectric constant of the medium used.
3. Termination of transmission line
In a line at the receiving end of a line with a characteristic impedance equal to the resistance of the termination, is called the transmission lines in parallel connection. It is mainly in order to obtain the best performance, including the driving load distribution and use.
Sometimes in order to save power consumption, the termination on the resistance of the concatenated another 104 capacitance forming ac termination circuit, it can effectively reduce the loss of dc.
Between the drive and transmission line stringing a resistor, and line terminal is no longer answer the termination resistor, this termination method is called series termination. More long-term overshoot and ringing on the available series damping or tandem technology to control. Series damping is to use a and gate drive output series of small resistance (generally is 10 ~ 75 Ω). This method is suitable for damping is combined with line characteristic impedance control to use (such as the backplane wiring, circuit board without ground plane and most round wire, etc.
Series termination when the value of series resistance and the circuit (drive door) output impedance is equal to the sum of the characteristic impedance of a transmission line. The series coupling end connection there can only be used in the terminal set total load and transmission delay time longer shortcomings. However, this can be achieved by using redundant serial termination of a transmission line method to overcome it.
4. The termination of transmission line
If line delay time is much shorter than the signal rise time, can need not series termination or parallel termination using transmission lines, if a non end connection of the round-trip delay in a back and forth on a transmission line (signal) than the short rise time of pulse signal, then caused by the termination of the recoil is about 15% of logic swing. The largest open route length approximation for: lMAX Type: the tr as the rise time TPD line long transmission delay time for the unit.
5. Comparing with several methods of termination
Parallel connection and serial port connection has its advantages, how to use which one, or both, it depends on the designer's interest and the requirements of the system. The main advantage of the parallel port connection is the system speed and signal transmission complete online without distortion. Long-term load will not affect driving on long drive transmission delay time of the door, and will not affect its velocity signal edge, but will make the signal along the long transmission delay time increases. When driving big fan out, load by branch distribution along the short term, and not, as in series termination must put load lumped online terminal.
Series termination methods make the circuit has the ability to drive several parallel load, the series end connection due to the capacitive load caused by the delay time increment about is twice as large as corresponding end in parallel connection, while short-term for capacitive load to slowing down of the edge and drive door delay time increases, but the series end connection of crosstalk is smaller than end in parallel connection, its main reason is that the series end connection of transmitted signal amplitude is only half the logic of the swing, and the switch current is half parallel termination of the switch of electric current, and small crosstalk signal energy is small.
E, the PCB wiring technique
Do is choose double panel or a multilayer PCB, depends on the highest working frequency and the complexity of the circuit system and the requirement for packing density. When more than 200 MHZ clock frequency had better choose a multilayer. If more than 350 MHZ operating frequency, had better choose to ptfe as dielectric layer of the printed circuit board, because of its high frequency attenuation to smaller, smaller parasitic capacitance, and transmission speed faster, also to save power consumption, because of its high Z0 walk line has the following principle requirements for printed circuit board.
(1) all try to leave big intervals between parallel signal lines, in order to reduce crosstalk. If there are two signal lines close apart, should take a grounding line between the two line, shielding effect.
(2) the design of signal transmission lines to avoid sharp turns, in case the reflection of the transmission line characteristic impedance discontinuity, try to design a uniform circular arc line has a certain size.
The width of the PCB according to the characteristics of the microstrip line and ribbon line impedance calculation formula, the surface characteristic impedance of the microstrip line in the printed circuit board between 50 ~ 120 Ω commonly. To get a big characteristic impedance, line width must be done very narrow. But it's very thin line and not easy to make. Comprehensive considering various factors, generally choose 68 Ω impedance value is more appropriate, because choose 68 Ω characteristic impedance, can achieve the best balance between the time delay and power consumption. A 50 Ω transmission will consume more power; Larger impedance is can reduce power consumption, but will make the transmission delay time abhorred. Due to the negative line capacitance will cause the increase of transmission delay time and low characteristic impedance. But low characteristic impedance line segment intrinsic capacitance per unit length is bigger, so the transmission delay time and the characteristic impedance is affected by the load capacitance is small. With proper termination is an important feature of a transmission line, branches short delay time should be no impact to the line. When Z0 50 Ω. Branches short length must limit within 2.5 cm. Lest appear a lot of ringing.
(4) for double panel (or six layer board of four layers), circuit board to both sides of the line perpendicular to each other, in order to prevent the mutual induction to produce the main crosstalk.
(5) on the PCB if equipped with a large current device, such as relay, lights, horn, etc., the ground had better go to separately, in order to reduce the noise on the ground, the ground of large current device should be connected to the board and an independent on the backplane bus, and the independent ground should also be connected to the ground point of the whole system.
(6) if there is a small signal amplifier board, amplified in front of the weak signal line to stay away from the strong signal lines, and to get the line as short as possible so that if possible even land line on the screen.
