Usually made from a copy board or PCB board schematics, technical requirements needed is not high , so fast PCB board is very simple, but the actual operation need to be clear goal , of course, any course is focused on understanding the functional layout with components wiring requirements , reasonable good layout and PCB board layout components , will be able to make a high-quality PCB board
PCB copy board that is in the case of existing PCB circuit board circuit board cloning , export, schematics, etc. , it is a document produced for . Also be carried out on the basis of secondary development based on user needs , re- design, implementation, upgrade and expand the product's features . So how do you become a real PCB board PCB copy board papers from the schematics of it ?
Perhaps we all know , oh , is to make the PCB design / clone becomes a real good schematics of PCB circuit board , please do not underestimate this process , there are a lot of things in principle feasible but difficult project achieve , or others can achieve something others do not realize , and hence make a PCB board is not difficult , but to make a PCB board is not an easy thing .
Difficulty lies in two areas of microelectronics processing high-frequency signals and weak signals in this regard PCB production level is especially important , the same diagram, the same components , different people making out with different results on PCB ? , then how can it make a good PCB board based on our past experience , to talk about their views on the following aspects :
First, to clear objectives
Received a design / copy board task , we must first define its objectives. Is a common PCB, high frequency PCB, PCB board or small signal processing both high- frequency signal processing have small PCB board , if only ordinary PCB board design , layout reasonable as long as the wiring neat , precise mechanical dimensions you can correct , if the load line and the long-term , we must adopt some means for processing , reducing the load , drive to strengthen the long-term focus is to prevent long-term reflection . When the board has more than 40MHz signal line , it is necessary for these signal lines for special considerations, such as crosstalk between lines and other issues . If the frequency is higher , the length of wiring have more stringent restrictions , in accordance with the parameters of the distribution network theory , the interaction between high-speed circuit and its connection is a decisive factor in system design can not be ignored. With the improvement of transmission speed doors , opposition will be a corresponding increase in the signal line , crosstalk between adjacent signal lines will increase in proportion to the usually high-speed circuit power and heat dissipation are also great at doing high-speed PCB should be taken seriously enough .
When the board has millivolt microvolt even weak signals of these signal lines need special attention , due to the small signal is too weak, very susceptible to interference from other strong signals , shielding measures are often necessary, otherwise the greatly reduced noise ratio. So that the useful signal is submerged in the noise , can not be effectively extracted.
Measured tone of the board should be taken into account , the physical location of test points , test points isolation factor in the design phase can not be ignored , because some small signal and high frequency signal is not directly coupled to the probe conducted measurements.
Moreover, you should consider some other relevant factors , such as board layers , using the package outline components , the board of mechanical strength. PCB board to do before , to make a pretty good idea of the design of the design goals .
Second, understand the function of the components layout requirements
We know that some specific components in the layout have special requirements, such as LOTI and APH used analog signal amplifier , analog signal amplifier power requirements to have a smooth , ripple . A small portion of the analog signal to be kept away from the power devices. In OTI board , a small part of a special signal amplification plus a shield , the stray electromagnetic interference to block out . NTOI board with the GLINK chip is ECL technology, high power consumption severe heat , cooling problems in the layout must be special consideration , the use of natural cooling , we should GLINK chip placed where the air flow is relatively smooth , and comes out of the heat can not form a large impact on other chips . If the speakers or other devices equipped with high-power on the board , it may cause serious pollution to the power that should be taken seriously enough .
Third, consider the layout of the components
Layout components first thing to consider is that the electrical properties of a factor , closely related components to connect together as much as possible , especially for some of the high-speed line , when the layout is necessary to make it as short as possible , the power signal and small signal devices to separate. Under the premise of meeting the circuit performance , but also consider the components neatly, appearance, ease of testing, mechanical board size, location, etc. sockets also need careful consideration.
Factors transmission delay time and high-speed system interconnect line is also grounded in the system design must first be considered. Time signal transmission line effects on the overall system speed is large, especially for high-speed ECL circuit , although a high speed integrated circuit itself , but on the base plate with the common interconnection line ( the line length of about 30cm per 2ns delay of ) brings increased delay time , can greatly reduce the system speed . elephant shift register , synchronous counter this synchronization working parts with a plug-in board is best placed , because the clock is a different plug-in board transmission delay time of the signal is not equal to the shift register may be the primary production of an error , if not on a board , it is critical to place the synchronization , from a common clock source connected to the length of the plug-in board must be the same clock line .
Fourth, consider wiring
With OTNI and star optical network design is complete , I will have more than 100MHz board with high-speed signal lines need to design, here will introduce some basic concepts of high-speed lines .
1. Transmission line
A printed circuit board of any "long" signal path can be regarded as a transmission line. If the transmission line delay time is much shorter than the signal rise time , then the reflected signal produced during the rising of the Lord will be submerged. No longer present overshoot, recoil and ringing , most of the existing MOS circuits, due to the rise time of the line is much larger than the transmission delay time , so traces can grow meters without signal distortion. For fast logic circuitry, particularly the ultra-high speed ECL
Integrated circuits , because of the faster edge speeds , the absence of other measures , trace lengths must be shortened in order to maintain the integrity of the signal
Sex .
There are two ways to make high-speed circuit to work in a relatively long line without serious waveform distortion , TTL fast Schottky diode clamp on the falling edge method that is clamped overshoot than one diode drop below ground potential on the level , which reduces the amplitude of the back of the recoil , allows a slower rising edge overshoot , but the relatively high output impedance which is at level "H" state of the circuit (50 ~ 80Ω) attenuated . In addition, due to the larger immunity level "H" state , so that the problem is not very prominent recoil of HCT series devices , the use of a Schottky diode clamp and series resistor termination method combined with the improvement the effect will be more pronounced.
When the fan- out in the signal line , the higher the bit rate and a faster edge rates , TTL shaping method described above appears to be insufficient . Because there is a line in the reflected waves , they will tend to be synthesized at a high rate , causing serious signal distortion and noise immunity is reduced. Therefore, to solve reflection problems , the system generally used in ECL another method : the line impedance method. This method can be used to control the reflection , the signal integrity is guaranteed .
Strict , he said, there is a slower edge speeds for conventional TTL and CMOS devices, the transmission line is not very needed. Edge for a faster speed , high-speed ECL devices , the transmission line is not always needed. However, when using a transmission line , which is an advantage and the interconnect delay can be predicted by the impedance matching for controlling the reflection and oscillation .
1. Decide whether to adopt the fundamentals of the transmission line has the following five . They are:
( 1 ) the rate of the system along the signal,
( 2 ) the connection distance
( 3 ) capacitive load ( number of fan-out ) , ( 4 ) resistive load ( Line Termination ) ; ( 5 ) allowed recoil and overshoot percentage ( lower level of the exchange of immunity ) .
2. types of transmission lines
( 1 ) a 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, twisted pair is usually 110Ω.
( 2 ) on the PCB microstrip lines
Microstrip line is a strip guide ( signal line ) . Between the ground plane by a dielectric isolated. If the distance between the line thickness, the width and the ground plane can be controlled , it is also possible to control the characteristic impedance .
( 3 ) of the stripline printed circuit board
Stripline dielectric is placed in an intermediate plane between two conductive copper wire . If the distance between the conductive layers and the dielectric constant of the thickness and width of the flat cable , the medium is controlled between , the characteristic impedance of the cable is also controllable .
Similarly , the length of the stripline transmission unit delay time and the line width or spacing is irrelevant ; only depends on the relative dielectric constant of the medium used .
3. terminated transmission line
At the receiving end of a line with a characteristic impedance of the cable termination resistor equal , we say that the end of the transmission line is parallel wiring . It is mainly in order to obtain the best electrical properties , including the driver and the use of load distribution .
Sometimes in order to save power consumption, and then on the termination resistor capacitor connected in series to form a 104 AC termination circuit , it can effectively reduce the DC loss.
Between the drive and the transmission line in series with a resistor , while the line is no longer connected to the terminating resistor terminals , such termination method called tandem termination. A long line of overshoot and ringing are available in series or series termination damping technology to control tandem damping is the use of a gate drive output of the series with a small resistor ( typically 10 ~ 75Ω) to achieve. This method is suitable damping characteristic impedance line and associated with a controlled (e.g., backplane wiring , no ground plane around the board and most of the wiring and the like.
When the series termination resistor in series with the circuit value (driving gate ) and output impedance equal to the characteristic impedance of the transmission line. Tandem -linked terminal wiring shortcomings exist only in the end-use load and propagation delay lumped a long time , but this can be overcome by using a redundant series terminated transmission line approach.
4. non-terminated transmission line
If the line delay time is much shorter than the rise time of the signal , the transmission line can be used in the case of termination without series or parallel termination , if a non-terminal junction of two-way delay ( round trip signal transmission line in time ) than the pulse signal rise time is short, then the recoil due to the non- termination caused about 15% of the logic swing . The maximum path length is approximately the opening :
Lmax
In which : tr is the rise time
tpd unit length of the transmission delay time
Comparison 5 . Several ways termination
End end parallel and series wiring wiring has advantages , about which , or both are used , depending on the preferences and requirements of the system designer may be. The main advantage of the parallel end of the cable is on the system speed and signal transmission line complete without distortion . Load on both the long-term will not affect the long-term transmission drives the gate driver delay time , but will not affect its signal edge speeds , but will make the signal propagation delay along the long- time increases. When driving large fan-out , the load can be distributed through branches along the short-term , rather than as a series of such termination must be lumped load line terminals.
Series termination method allows the circuit has several parallel load line drive capacity , delay time increment series terminal wiring caused due to the capacitive load than the corresponding parallel about twice as large end of the cable , while the short-term due to capacitive loads make the edge slow down and drive the gate delay time increases , however , the series end crosstalk ratio parallel wiring to the small end of the cable , the main reason is the signal amplitude along the transmission end of the cable series is only half of the logic swing , thus switching current parallel termination only half the switching current , small crosstalk signal energy also small .
Five , PCB board layout techniques
PCB is the choice when making double-sided or multilayer, the maximum operating frequency and complexity of the system depends on the circuit and density requirements for the assembly to decide. In the best selection of plywood clock frequency exceeds 200MHZ. If the frequency of more than 350MHz, the best choice as the dielectric layer PTFE printed circuit board, because of its high frequency attenuation to be smaller , the parasitic capacitance to be smaller , the transmission speed is faster , but also because than Z0 large and provincial power, on the printed circuit board traces following principles require
To try to leave between
( 1 ) All signal lines parallel to a larger interval to reduce crosstalk. If there are two signal lines are in close proximity , it is best to take a ground wire between the two lines , so you can play the shielding effect.
( 2 ) when the signal transmission line designed to avoid sharp turn to avoid the characteristic impedance of the transmission line mutations of reflection , to try to design a uniform arc line has a certain size.
The width of the printed circuit board according to the characteristic impedance of microstrip and stripline above calculation formula, the printed circuit board microstrip characteristic impedance is typically between 50 ~ 120Ω. To get a big characteristic impedance must be made very narrow linewidth . But it is not easy to produce fine lines . A combination of factors to consider , the general choice of about 68Ω impedance value is appropriate , 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 ; larger impedance can certainly reduce power consumption , but would hate large transmission delay time . Since the negative result in reduced transmission line capacitance increases and the delay time characteristic impedance . However, the characteristic impedance of the line low intrinsic capacitance per unit length is relatively large , so the characteristic impedance of the transmission delay time and less affected by the load capacitance. An important feature with the proper termination of the transmission line is short on branch line delay time should have little impact . When Z0 is 50Ω time . Short branch lengths must be limited to less than 2.5cm. Avoid a lot of ringing.
( 4 ) For double-sided ( or six plates go four lines ) , both sides of the line perpendicular to the circuit board to prevent mutual inductive crosstalk primary production .
( 5 ) If the PCB is equipped with high-current devices , such as relays , lights , horn , etc., they ground it is best to separate walk alone , to reduce noise in the line , these large ground current of the device should be be connected to a separate plug-in board and backplane bus up, and these should also be independently connected to ground and the ground of the system.
( 6 ) If the board has a small -signal amplifier, the amplification of weak signals before the line is to stay away from strong signal line , and traces as short as possible , but also land line if possible be shielded....
