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How to do a good PCB board

As we all know to do is to design a good PCB board schematic to become a real PCB circuit board, Please do not look down on this process, There are many things acceptable in principle in engineering is difficult to achieve, Or someone else to achieve something while others fail to realize,So, to say to do a PCB board is not difficult, but to make a PCB board is not an easy thing.

Difficulty lies in the field of microelectronics the two high-frequency signals and weak signal processing, PCB production level in this regard are particularly important, The same principle design, the same components, different people of the PCB would have produced different results, So how can we make a good PCB board do? According to our previous experience, would like to talk about following his own views:
I. Design objectives must be clear
Receive a design task, First, to clearly identify its design objectives, Is a common PCB boards, high-frequency PCB board, small-signal processing, or both PCB boards have a small high-frequency signal processing PCB board, If it is an ordinary PCB board, As long as the reasonable and neat layout, mechanical measurements can be accurate, If the load line and the long-term, Means we must use a certain treatment, reduce the load, Long-term to strengthen the drive, Focus is to prevent long-term reflection. When the board has more than 40MHz signal line, should the signal lines of these special Example, crosstalk between lines and other issues. If the frequency is higher, the length of the wiring there is more stringent restrictions on the distribution of the parameters according to network theory, high-speed connection with the interaction between the circuit is the decisive factor in system design can not be ignored. With the gate speed improvement in the signal lines will be a corresponding increase in the opposition, crosstalk between adjacent signal lines will be proportional to the increase, often high-speed circuit power consumption and heat dissipation are also large, doing high-speed PCB should be taken seriously enough.

When the board has millivolt or microvolt levels of weak signals on these signal lines will require special attention, small-signal due to very weak, very vulnerable to interference from other strong signals, shielding measures are often necessary, or they will significantly reduced signal to noise ratio. Resulting in the desired signal submerged by noise can not be effectively extracted.

Measurement of the board of adjustment must be considered in the design stage, the physical location of test points, test points, isolation and other factors can not be ignored, because some of the small-signal and high-frequency signals can not be directly added to the probe measurements.

In addition, should also consider other relevant factors, such as board layers, using components of the package shape, mechanical strength and so on board. PCB board was doing before, to make a pretty good idea of the design goals of the design.

TwoUnderstand the functions of the components used, the requirements for placement and routing

We know that,Some special components in the layout when there are special requirements, For example LOTI and APH used in analog signal amplifier, analog signal amplifier to the power requirements to achieve a smooth, ripple small. Analog small-signal part to try to stay away from power devices. In the OTI board Small-signal amplification section also added a special shield to stray electromagnetic interference to shield out. NTOI board used GLINK the ECL chip technology, power-hungry heat worse, the heat must be carried out in the layout must be special consideration, if the use of natural heat, it is imperative GLINK chip on the air flow is relatively smooth areas , and scattered out of the calories, and yet can not pose a great impact on other chips. If the board is equipped with a horn or other high-power devices are likely to cause serious pollution to the power this had to be taken seriously enough.

Three, Components layout consideration
Components of the layout of the first to be considered a factor that electrical performance, To connect the components as close together, In particular, a number of high-speed lines, the layout should make it possible, when a short, power-signal and small signal devices should be separated. . To meet the premise of circuit performance, but also consider the components placed in neat, attractive, easy to test the mechanical board size, outlet location, etc. also need careful consideration.

High-speed system grounding and interconnection transmission line delay time is also the first time in the system design considerations. The signal line transmission time of the great influence on the overall system speed, Especially for high-speed ECL circuits, although the high speed integrated circuit block in itself, but because of the backplane interconnect line with the common (each about 30cm line with 2ns delay volume) brought about an increase in delay time, allowing system speed greatly reduced. like a shift register, synchronous counter working parts of this synchronization with a plug-in board would be better placed, because the different plug-in board of the clock signal propagation delay time is not equal, the primary production may shift register error, if not on a board, then the synchronization is the key place, from the public clock source connected to the plug-in board must be equal to the length of the clock line.
Four, Consideration of the wiring

With the OTNI and star-shaped fiber optic network design is complete, The future there will be more than 100MHz of the board with a high-speed signal lines need to design, where high-speed line will introduce some basic concepts.

  1. Transmission Line

Printed circuit board of any one "long" signal paths can be viewed as a transmission line. If the transmission line delay time is much shorter than the signal rise time, then the signal produced during the main increase in the reflection will be submerged. Overshoot is no longer present, recoil, and ringing on the current most of the MOS circuit, because the rise time of the line propagation delay time than the much larger, so the alignment may be long in meters without signal distortion. For faster logic circuits, especially in ultra-high-speed ECL.

Integrated circuits, because the edge of the speed increased quickly, without other measures, take the line length must be shortened in order to maintain the signal integrity.
There are two ways to make high-speed circuit at a relatively long line of serious work without waveform distortion, TTL edges of the rapid decline in use of Schottky diode clamp methods to clamp overshoot was lower than the ground level of a pressure drop on the diode, which reduces the recoil behind the range, a slower rise in the edge allows overshoot, but it is in the level "H" state, the circuit relatively high output impedance (50 ~ 80Ω) of the attenuation. In addition, Level "H" state immunity larger, so that recoil problem is not very prominent on the HCT series of devices, the use of a Schottky diode clamp and series resistance termination method of combining the improvement the results will be more apparent.

When there are fan-out along the signal line, the At a high bit rate and faster edge rates, The introduction of the TTL shaping method appears to be insufficient. Because the line there are reflected waves, they will tend to synthesis of high rates, causing a serious distortion and anti-jamming signal reduced ability. Therefore, in order to solve the reflection problem, commonly used in the ECL system, another method: the line impedance matching method. Reflection in this way can be controlled, signal integrity is assured.

Strictly speaking, For a slower edge rate of the conventional TTL and CMOS devices, the transmission line is not very much needed. To have faster edge speed, high-speed ECL devices, the transmission line is not always necessary. However, when using the transmission line, they have to predict interconnect delay and through reflection and impedance matching to control the oscillation of the advantages:

  1. Decide whether to adopt the transmission line has the following five basic factors. They are: (1) system along the signal rate, (2) connection distance (3) capacitive load (fan-out number), (4) resistive load (line termination method); (5) allows the recoil and overshoot percentage (lower level of the exchange of immunity).
  2. Several types of transmission lines

(1), coaxial cable and twisted pair cables: They are often used in the connection between systems. The characteristic impedance of coaxial cable usually 50Ω and 75Ω, twisted-pair typically 110Ω.
(2) microstrip line on a printed circuit board
Microstrip line is a guide strip (signal line). And the ground plane with a dielectric between the isolate. If the line thickness, width, and with the distance between the ground plane can be controlled, then it is also a characteristic impedance can be controlled. Microstrip line characteristic impedance Z0 as follows:Where: Er as a printed circuit board dielectric material of relative permittivity
6 for the thickness of dielectric layers
W for the line width of the
t is the thickness of the line
Unit length of microstrip transmission line delay time, just depends on the dielectric constant with the line width or spacing has nothing to do.
(3) The stripline PCB
Strip line is placed in a plane between two layers of conductive dielectric copper belt center line. If the line thickness and width of the dielectric constant of the medium, as well as the distance between two conductive plane is controllable, then the line characteristic impedance is controllable, the characteristic impedance stripline B as follows:Where:
b is the distance between the two ground plates
W for the line width of the
t is the thickness of the line
Similarly, the unit length of the stripline transmission delay time and the line width or spacing is not relevant; only depend on the relative dielectric constant of the medium.
3, transmission line termination
In the receiving end of a line with a characteristic impedance and line termination resistor equal to, claimed that the transmission line for the parallel side wiring. It is mainly in order to obtain the best electrical properties, including the driver and used the distribution of the load.
Sometimes, in order to save power consumption of the termination resistor and then cascaded to form a capacitor 104 terminated the exchange of circuits, it can effectively reduce the DC losses.
In the drive and the transmission line between the series with a resistor, while the line termination resistor terminal is no longer then, this termination method is called series termination. A long line overshoot and ringing can be damped in series or series termination technology for the control. Series damping is the use of a gate output and drive a small series resistance (typically 10 ~ 75Ω) to achieve. This method is suitable for damping with the characteristic impedance to the line under control associated with (for example, backplane wiring, no ground plane around the circuit board and most of the wiring and so on.
Series termination resistor value when the series circuit (driver door) and is equal to the output impedance of the transmission line characteristic impedance. Tandem Alliance side wiring exists only in the end-use of lumped load and transmission delay time longer shortcomings. However, this can be terminated through the use of redundant series transmission line method to overcome them.
4, non-terminated transmission line
If the line delay time is much shorter than the signal rise time, you can not series-terminated or parallel terminated transmission lines of the circumstances under which, if a non-side wiring of the two-way delay (signal transmission line from time to time) than the pulse the signal rise time is short, then due to non-termination caused by the recoil is about 15% of the logic swing. Maximum length of open-line approximation as follows:
Lmax<tr/2tpd
Where: tr is rise time
tpd unit length of the transmission delay time
5, several termination methods comparison
Parallel and series wiring terminal-side wiring have their own strengths, how to use which one, or both are used, depending on the designer's preferences and the system request. The main advantage of parallel side terminal is the system speed and signal integrity of on-line transmission distortion. Long-term load on both the long-term will not affect the drive of the driving gate propagation delay time, but will not affect its signal edge speeds, but will permit the transmission of signals along the long-term increase in delay time. When driving large fanout, the load can be distributed through the branches along the short-term, rather than as series termination, as must be the load in the lumped-line terminals.

Series termination method makes several parallel circuit drives the ability to load line, series capacitive load-side wiring due to the delay caused by time increment of approximately parallel side than the corresponding wiring doubled in size, while the short-term due to the capacitive load so that the edge slowed down and the driver gate delay time increases, but the series end crosstalk wiring wiring than the parallel side is smaller, mainly due to end in series along the cable transmission signal amplitude is only half of the logic swing, which switches Current parallel termination only half the switching current, the signal energy of a small cross-talk also small.

2, PCB board layout techniques
PCB to do when a choice of two panels, or plywood, depending on the maximum operating frequency and the complexity of electrical systems as well as the density of the assembly requirements to decide. 200MHZ clock frequencies exceeding the best choice when the multilayer. If the operating frequency of over 350MHz, the best choice to PTFE as a dielectric layer printed circuit board, because of its high-frequency attenuation to be smaller, parasitic capacitance should be smaller, the transmission speed to be faster, but also because Z0 more province of large power consumption, for the alignment of printed circuit board has the following principle requirements:

  1. All the parallel between the signal lines as far as possible leave large intervals in order to reduce crosstalk. If there are two signal lines are in close proximity, it is best to go between the two lines a ground wire, so you can play a role in shielding.
  2. The design of the signal transmission line when turning to avoid anxious to prevent the transmission line characteristic impedance resulting from mutations in reflection, as far as possible designed to have a certain uniformity of size of the arc line.
  3. Printed line width according to the above-mentioned microstrip line and stripline characteristic impedance calculation formula, the printed circuit board microstrip line characteristic impedance is generally between 50 ~ 120Ω. To get a large characteristic impedance, line width must be done very narrow indeed. But it is not easy to make fine lines. Integrated a variety of factors to consider, the general choice of about 68Ω impedance more suitable, because the choice of 68Ω characteristic impedance, you can delay time and achieve the best balance between power consumption. A 50Ω transmission line will consume more power; large while resistance can reduce power consumption, but would hate large transmission delay time. As the negative-line capacitance will cause the increase of transmission delay time and the characteristic impedance lower. But the characteristic impedance of the segment is very low intrinsic capacitance per unit length of the relatively large, so that transmission delay time and the characteristic impedance of less affected by the load capacitance. With the appropriate transmission line termination is an important feature of the branched short-term delay of the line should have no impact on them. When Z0 is 50Ω time. Short length of the branch must be limited to less than 2.5cm. So as to avoid significant ringing.
  4. For the pairs of panels (or the six-board to go four lines). Circuit board on both sides of line is to be perpendicular to each other in order to prevent crosstalk between the main sensor output.
  5. If the PCB is equipped with high current devices such as relays, lights, speakers, etc., and their ground is best separately to go to in order to reduce to line noise, these high-current devices to be connected to the ground plug on the board and the backplane bus, an independent ground up, and these independent ground with the entire system should also be connected to the access site.
  6. If you board a small-signal amplifier, the amplification of weak signals before the line is to be away from the strong signal lines, but also take the line is to be as brief as possible, if possible, but also shield them land lines.
 
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Address: Building 60#, Zhongwu Industrial Estate, Xixiang, Baoan District, Shenzhen.
Tel:+86-0755-29702195 Fax:+86-0755-29702200 E-mail:zjxpcb@163.com Web:http://www.zjxpcb.com