EMC design pcb board

A. Overview of electromagnetic interference and compatibility

Electromagnetic interference is a major problem facing modern industrial circuit . In order to overcome interference, circuit designers have shifted walking interference sources , or trying to protect the circuit undisturbed. Its purpose is to make the circuit in accordance with the intended objectives to work - that is, to achieve electromagnetic compatibility.

Typically, the only board-level electromagnetic compatibility is not enough. Although the circuit is working at the board level , But it will be for Radiation to the rest of the system noise, resulting in a system-wide problem. In addition, the electromagnetic compatibility system-level or device-level radiation must meet certain criteria , so as not to affect the normal operation of other equipment or devices.

Many electronic devices and instruments developed countries have strict standards for electromagnetic compatibility ; in order to meet this requirement, the design must be designed from the outset to consider the board level electronic interference suppression .

1 . Composed electromagnetic environment

A simple model of electromagnetic interference consists of three parts :
Sources of electromagnetic interference
Coupling path
Receptor

Sources of electromagnetic interference

Electromagnetic interference sources comprises a microprocessor , a microcontroller, electrostatic discharge , the transmitter , the instantaneous power actuator , such as said: electromechanical relays , switching power supply, lightning . In a micro-controller system , a clock circuit is generally the largest broadband noise generator , and this noise is dispersed to the entire frequency spectrum. With the large number of high-speed semiconductor device applications, the edge transition rate is very fast , this circuit can produce up to 300MHZ harmonic interference.

Coupling path

Noise coupled into the circuit is the easiest way is to pass through the conductor . If a wire with a noise after the environment , this will receive the wire by induction noise and pass it to the rest of the circuit . Noise through the power cord into the system , is a case of such a coupling . Carried by the power line noise was spread to the entire circuit .

Coupling also occurs in sharing the load ( impedance ) of the circuit. For example, two circuit provides a power- sharing Voltage wires , and share a grounding wire. If a circuit is required to provide a burst of current , since the two circuits share a common power supply line and the same resistance , then another circuit of the power supply voltage will drop . By reducing the impact of the coupling impedance common to weaken .

Unfortunately , the impedance of the power supply within a fixed and can not be reduced , this also occurs in the grounding conductor . In a digital circuit, the return current flows to ground potential changes generated in the ground circuit to another circuit . If the ground is unstable, it will seriously degrade the performance of the op amp , ADC and analog sensors and other low-level circuits . Similarly, the electromagnetic radiation of each circuit are coupled sharing can produce . When the current is changed , it will produce electromagnetic waves. The electromagnetic energy is coupled to the vicinity of the circuit conductor and the other interfering signals .

Receiver ( acceptor )

All electronic circuits can accept the transfer of electromagnetic interference . Although part of the electromagnetic interference by radio directly accepted , but most are accepted by transient conduction . In digital circuits , the most vulnerable critical signal electronic interference. These include reset , interrupt, and control signals. Lower analog amplifiers, control circuit and power regulation circuitry is susceptible to noise.

For electromagnetic compatibility and electromagnetic compatibility design criteria , the designer needs to radiation ( leakage from the product RF energy ) is minimized , enhance its radiation ( RF energy into the product ) susceptibility and anti-jamming capability .

2 . Electromagnetic compatibility fees

The most cost-effective EMC design methods in the early stages of design fully consider the assessment of electromagnetic compatibility technical requirements.

Let the designer in the initial component selection , circuit design and PCB layout design , put the electromagnetic compatibility as a major. The design is not based on reality . However, keep in mind if designers can recommend this article , then you can reduce irrational component selection , circuit design and PCB layout situation appeared .

B. Selection and circuit design techniques element

Component selection and circuit design are the main factors affecting the performance of board-level electromagnetic compatibility . Each electronic component has its own characteristics , therefore , requires careful consideration in the design .

The following will discuss some of the commonly used to reduce or inhibit electronic components and circuit design techniques for electromagnetic compatibility .

• Components Group

There are two basic types of electronic components groups: with and without pin component pin.

Leaded components parasitic effects , especially at high frequencies . The pin forms a small inductor , which is about 1nH/mm / pin. Terminal pins can also have a small capacitive effect , about 4pF. Therefore , the length of the pin should be as short as possible . Compared to have pin components, leadless and parasitic effects of surface mount components to be smaller. Typical values ​​are: parasitic inductance 0.5nH 0.3pF of capacitance of about terminal . From the point of view of electromagnetic compatibility , surface mount components the best, followed by radial leaded components , and finally the element axially parallel pin.

1 . Resistance

Due to the characteristics of surface mount components with low parasitic parameters, therefore , surface mount resistors have a resistance is always better than the pin .

For pin resistor , carbon film resistors should be the first choice , followed by metal film resistors , and finally wound resistors. Since the relatively low frequencies (in the MHz ) , a metal film resistor is a major parasitic components, so which is suitable for high power density and high accuracy , or circuits .

Wirewound resistors have a strong inductance characteristics , and therefore it can not be used for frequency-sensitive applications. It is most suitable for use in large power processing circuit. In amplifier design, resistor selection is very important. In the high frequency environment , because the impedance of the resistor inductance resistance increases. Thus , the gain control resistor location should be as close to the amplifier circuit to reduce the inductance of the circuit board . In pullup / pulldown resistor circuit, fast switching transistors or integrated circuits increases the rise time . To reduce this effect , all of the bias resistor must be as close to the active device and his power and ground , thus reducing inductance PCB connection.

The regulator ( rectifier ) or reference circuit , the DC bias resistor should be as close as possible to relieve the active device decoupling effect should be ( ie, improved transient response time ) . In the RC filter network , the parasitic inductance of the wire wound resistors can easily cause the local oscillator , it must consider the inductive effects caused by the resistance .

2 . Capacitance

Since the capacitor wide range of different properties , not easy to choose the right capacitor . However, capacitors are used to solve many EMC problems. Several sections are the most common type of capacitor , performance and use of the method described next . Aluminum electrolytic capacitors are usually a thin layer of insulation between the spiral wound metal foil made ​​, so in unit volume within a larger capacitance values ​​obtained , but also makes the internal inductance of the part increases.

Tantalum capacitor with an insulator made ​​from a piece of candy bar and pin connection points , its internal inductance lower than electrolytic capacitors.

Ceramic capacitor structure comprises a plurality of parallel sheet metal in the ceramic insulator . The main structure of parasitic piece of sense resistant , and usually this will cause the impedance at lower MHz region .

Frequency response characteristics of different insulation materials means a type of capacitance than another more suitable for some applications. Aluminum electrolytic capacitors and tantalum electrolytic capacitors for low-frequency terminals, mainly memory and low frequency filter fields. In the mid-frequency range ( from KHz to MHz), more suitable for ceramic capacitors , commonly used in high frequency filtering and decoupling circuit . Special low-loss ( usually more expensive price ) ceramic capacitors and mica capacitors suitable for applications in the VHF and microwave circuits.

To get the best EMC characteristics , capacitors have low ESR (EquivalentSeriesResistance, equivalent series United resistance) value is important , because it will cause a large attenuation of the signal , frequencies close to the resonant frequency of the capacitor , especially in the case .

a) bypass capacitor

The main function is to produce a bypass capacitor AC shunt to remove those unwanted energy into susceptible areas. Bypass capacitor is generally as high frequency bypass to reduce the transient current demands on the power supply module. Aluminum electrolytic and tantalum capacitors are usually more suitable for bypass capacitor , whose capacitance value depends on the transient current demand on the PCB , typically in the range of 10 to 470μF . If the PCB has many integrated circuits, high-speed switching circuits and power with a long lead , you should choose a large -capacity capacitor.

b) decoupling capacitors

High-frequency switching noise generated when the switch is active device will propagate along the power line. The main function of the decoupling capacitor is Provides DC power to a local active devices to reduce switching noise propagation and noise -board guide to ground.

In fact, the bypass capacitor and decoupling capacitors should be placed as close to the power input to help filter high frequency noise. Decoupling capacitors bypass capacitor value is about 1 / 100 to 1 / 1000. To get a better EMC performance, decoupling capacitors should be placed as close to each manifold (IC), because the wiring impedance will reduce the effectiveness of the decoupling capacitors .

Ceramic decoupling capacitor is often used , its value is determined by the fastest signal rise time and fall time . For example, a33MHz clock signal , and can be used to 100nF 4.7nF capacitor ; against a 100MHz clock signal can be used 10nF capacitor.

When selecting decoupling capacitors , in addition to considering the capacitance value addition , ESR values ​​also affect the ability of decoupling . For decoupling , should choose.

Optional values ​​below 1 ohm ESR capacitor.

c) the resonant capacitor

The following briefly discusses how to choose the value of the bypass capacitor and decoupling capacitors according to the resonant frequency. Capacitance capacitive presented below resonant frequency , then , because the capacitor will lead lengths and wiring inductance presented sensibility.

Another factor is the effectiveness of the decoupling capacitor dielectric material ( dielectric ) . Decoupling capacitors often used in the manufacture of
Barium titanate ceramics (Z5U) and strontium titanate (NPO) of the two materials . Z5U having a large dielectric constant, the resonance frequency between 1MHz to 20MHz. NPO has a lower dielectric constant, but the resonance frequency is high ( greater than 10MHz). So Z5U more suitable for low frequency decoupling , and NPO used as decoupling frequencies above 50MHz .

Common practice is to use two decoupling capacitors in parallel . This can reduce the power distribution network in the wider spectrum generating switching noise. Multiple decoupling capacitors connected in parallel to provide 6dB gain to suppress the active device switch caused by RF current.

Multiple decoupling capacitors not only provide a wider spectrum , and offer a wider wiring to reduce lead inductance , and therefore can be more effective in improving the decoupling capacity. Two capacitance values ​​should be two orders of magnitude in order to provide more effective decoupling ( eg 0.1 μF + 0.001μF parallel ) .

Note that the decoupling of the digital circuit , a low ESR values ​​is more important than the resonance frequency because the low ESR values ​​can provide a lower impedance path to ground , so that when more than emotional resonance frequency capacitance presents still provide adequate decoupling capability.

3 . Inductance

The inductor is a magnetic and electric fields can be linked to the element , its inherent role of each of the magnetic field may be able to force to potentially more sensitive than other components. And capacitance Similarly , cleverly using inductive EMC can solve many problems .

Here are two basic types of inductors : open and closed loop . They differ in that the magnetic field inside the ring . In the open-loop design meter , a closed magnetic field through the air ; the closed loop design , the magnetic field through the core to complete the magnetic circuit .

One advantage compared to the capacitor and the inductor or resistor is that it is no parasitic inductance , and thus the surface mount type lead. There is no difference between the types. Open- loop inductive magnetic field through the air , which causes radiation and generate electromagnetic interference (EMI) problems. When selecting an open loop inductance, winding shaft rod or solenoid than better , because it will be controlled magnetic field in the core ( ie, the local magnetic field magnetic body ) .

Closed loop inductance, the magnetic core is completely controlled, and therefore the circuit design of this type of inductor better, of course, they are also relatively expensive. One of the advantages of the closed-loop inductance of the spiral ring: it will not only control the magnetic core can also eliminate all alien with its own field of radiation.

Inductor core materials are mainly of two types: iron and ferrite. Iron core inductors for low frequency applications (tens of KHz), and ferrite core inductors for high frequency applications (to MHz). Therefore more suitable ferrite core inductors in EMC applications. In particular the use of two special types of inductors

EMC applications: Ferrite Beads and ferrite clip.

Iron can be used for inductors and ferrite core skeleton. Core inductors often used in low frequency applications (tens of KHz), and ferrite core inductors often used in high frequency applications (MHz). So ferrite core inductor is more suitable for EMC applications. In EMC's special applications, there are two special inductor: ferrite and ferrite bead clip.

Ferrite bead is a single loop inductance, typically a single strand of wire through the ferrite material to form a single ring. This high-frequency devices attenuation range is 10dB, and the amount of attenuation at DC. Similar ferrite beads, ferrite clip up to MHz within the frequency range of common mode (CM) and differential mode (DM) can reach 10dB attenuation to 20dB.

In the DC-DC conversion, the inductor saturation current must be able to withstand high and low radiation. Bobbin inductors have to meet characteristics required for the application. Between low impedance and high impedance power digital circuit LC filter needed to ensure impedance matching of the power supply circuit.

One of the most widely used for the AC filter inductor.

4. Diode

A semiconductor diode is the simplest device. Because of its unique characteristics, some diodes and help solve and prevent EMC-related problems.

Many applications of diode circuit inductive load, under the action of high-speed switching currents produce transient peak current system. The diode is most effective voltage spike suppression device noise sources.

In automotive control applications, regardless of brush or brushless motor, when the motor is running, will produce noise or change the brush the noise. Hence the need noise suppression diodes, in order to improve noise suppression diode should be as close to the motor contacts. The power input circuit, it is necessary to use a high voltage varistor TVS or noise suppression. One of EMI problems signal connection interface is electrostatic discharge (ESD).

Shielded cables and connectors for protection from outside interference static. Another method is to use a varistor or TVS protection signal line.

IC Modern digital integrated circuit (IC) is mainly manufactured using CMOS technology. Static power consumption of CMOS devices is very low, but in the case of high-speed switching, CMOS devices require power to provide instantaneous power than similar devices bipolar dynamic power requirements of high-speed CMOS devices. Therefore, these devices must add decoupling capacitors to meet the instantaneous power requirements.

1. IC packaging

There are a variety of integrated circuit packaging structure, the separation element, the pin, the smaller the EMI problem. Because paste devices have a smaller mounting area and lower mounting position, it is better EMC performance, and therefore should be preferred surface mount devices. Even die installed directly on the PCB.

Pinout IC can also affect the EMC performance. Power cord from the module is connected to the center pin IC shorter, it's the equivalent electrical the less sense. So decoupling capacitor between VCC and GND closer more effective.

Both the IC, PCB board or the entire system, the clock circuit is a major factor in EMC performance impact. Integrated most circuits are associated with noise and many times the clock frequency harmonics related. Therefore, both the circuit design or PCB design should consider the clock circuit to reduce noise.

Reasonable ground, proper decoupling and bypass capacitors can reduce radiation. High impedance for clock distribution buffers also help to reduce the clock signal reflections and ringing.

For mixing TTL and CMOS logic circuit devices, due to their different switch / hold time, will produce harmonic clock, the useful signal and power. To avoid these potential problems, it is preferable to use the same family of logic devices. Due to wide threshold CMOS devices, most designers now choose CMOS devices. Since the manufacturing process is a CMOS process, and therefore also the interface circuit is preferably a microprocessor such a device. Special attention is needed, the unused CMOS pin should be grounded cord or power. In the MCU circuit, the noise from not connect / input terminals, as well as MCU execution error code.

It is also the preferred design microcontroller interface logic products, these microcontrollers also based on CMOS technology. About CMOS devices, an important aspect is its unused input pins to be floating or grounded. In MCU Circuit, the noise may cause confusion to run these inputs, but also cause the MCU to run garbled.

2. Voltage calibration

For a typical calibration circuit, the appropriate decoupling capacitors should be placed as close as possible calibration circuit output position, because in the tracking process, the distance between the calibration of the output inductor and the load will have impact and cause calibration circuit internal vibration. A typical example of the input and output calibration circuit, coupled with 0.1uF decoupling capacitor to prevent possible internal vibrations and filter high frequency noise. In addition, in order to reduce output ripple, to add a relatively large bypass capacitor (10uF / A).

3. Line Terminal

When the circuit is operating at high speed, the impedance matching between the source and destination is important. Because the error will cause the signal to match the feedback and ringing. Excessive radiation or RF energy will affect other parts of the circuit, causing EMI (electromagnetic compatibility) problems.

Termination signals help reduce these non-expected results.

Not only can reduce the signal termination in impedance matching between the source and purpose of the feedback signal and ringing, but can also slow down the signal edge of fast rise and fall. There are a variety of signal termination method, each method has its pros and cons.

Micro-controller circuit

Nowadays, many IC manufacturers continue to reduce the size of the microcontroller to achieve more parts in the unit of silicon purposes. Reducing the size of transistors will usually faster. Thus, although the MCU clock rate can not be increased, but the rise and fall times will increase, thereby making the frequency harmonics rise. In many cases, reduce the size of the micro-controller can not be notified to the user, so the initial circuit MCU is normal, but later in the product life cycle of a time EMC problems may occur. The best solution to this is to design the circuit at the beginning on the design of a more robust circuits. Many applications require high-speed real-time MCU, the designer must take seriously its circuit design and PCB layout to reduce potential EMC problems.

With Power MCU needs to increase its processing power increased. Let supply circuit (such as calibration circuit) close to the microcontroller is not difficult to do, and then a separate capacitor can reduce the impact of the DC power to other circuits.

MCU usually have an on-chip oscillator, which use their own crystal or resonator is connected, thus avoiding the use of other clock .A clock driver circuit. This independent clock can better prevent noise radiation generated by other parts of the system. In terms of clock frequency, MCU is usually the most demanding of the power equipment, so let the clock near the MCU can guarantee only the minimum clock frequency drive demand.

1. I / O port pins

For most MCU, pins are usually high impedance input or hybrid input / output. High-impedance input pins are susceptible to noise affected, and in the end will lead to a non-normal register latch error level. A non-internal terminal needs to have a high impedance input pin (eg 4.7KΩ or 10KΩ) to connect each pin to ground or to power level in order to ensure a known logic state. Input pin is normally unconnected floating around the median level of power, or because there is an internal leak paths while floating in an uncertain voltage.

For IRQ or reset pin (input pin), its end is more important than the general I / O port pin. If the noise cause false triggering these two pins, it will have a huge impact on the behavior of the entire circuit. When the input pin is not connected, while input latches half open half closed, it will cause the IC internal current leakage, then you can usually see the high current consumption, especially in CMOS devices. Therefore, in a high-impedance input pin terminal connections can reduce the supply current, like other benefits as electromagnetic compatibility.

2. IRQ pin port

Due to the interruption of the MCU operation, so it is one of the most sensitive elements of the pin. From the remote device to PCB MCU board, even in the plug-in adapter or subsystem cards, IRQ can be queried. Therefore, to ensure that any connection with the interrupt request pin has an instantaneous electrostatic discharge protection is very important. For electrostatic discharge, in the IRQ there bidirectional diode, transorbs or metal oxide varistor terminals on the connection is usually sufficient, but they can not produce a large line under load conditions to help reduce overshoot and damping shocks. Even for price-sensitive applications, IRQ line termination resistor is also indispensable.

3. Reset pin

Improper reset will cause many problems because of the use of different applications and different conditions MCU start of power. Reset the most basic function ensures Once powered, MCU began with a controlled manner to execute code. On power-up, power up to the MCU operating voltage before the crystal stability requires some time. Therefore, the reset pin to have the time delay. The easiest is to delay resistance - capacitance (RC) network, when the current through the resistor capacitor start charging until the level reaches the value of the MCU can be reset circuit logic 1 state when the detected so far. Not strictly required resistor and capacitor size Ideally, but there are other considerations. The reset pin ministry of leakage current is typically not exceed specified 1μA (for Motorola HC08 MCU), which means a maximum resistance of 100kΩ, the capacitor can be electrolytic capacitors, in order to maintain the current minimum stop. Ceramic capacitors are recommended because it compromises the low price, low leakage, high-frequency response performance advantages. Reset pin capacitance is very small (Motorola HC08MCU below 5pF). For the minimum impedance value is also limited, because the maximum pull-up current is about 5mA, 1V (VOL). Plus an external capacitor low-impedance voltage source, determines the minimum value of the pull-up resistor is 2KΩ. A diode to clamp reset pin voltage is also a recommended practice to prevent excessive supply voltage, and can quickly discharge the capacitor off season.

4. Oscillator

Many MCU synthesized inverting amplifier, used to form the knot Pierce oscillator with an external crystal or ceramic resonator structure. The following discusses the external components used in conjunction with a special get together oscillation amplifier minimum gain (transconductance).

C. printed circuit board layout techniques

In addition to the selection and design of the circuit components, a good printed circuit board (PCB) wiring Electromagnetic Compatibility is also a very important factor. Since it is an inherent component of the system PCB, PCB layout enhanced electromagnetic compatibility in the final product will not be completed to bring additional charges.

One thing to note, PCB layout no strict rules, no special rules to cover all PCB routing. Most PCB layout is limited to the size of the board and copper layers.

Some techniques can be applied to a wiring circuit, but not for another. This will mainly depend on the experience of cabling engineers.

However, there are some general rules, the following sections discuss them. These rules will be treated as a general guideline. Any person should remember that poor PCB layout EMC can cause more problems rather than eliminate these problems, in many cases, even with the filters and components can not solve these problems. In the end, had to rewire the entire board. Thus, at the beginning to develop good habits PCB layout is the most economical way.

1. The basic characteristics of PCB

Constitutes a PCB is laminated using a series of vertical stack traces and prepreg. In the multi-layer PCB, the designer in order to facilitate debugging, will signal the outermost line of cloth. Wiring on the PCB there is resistance, capacitance and inductance characteristics.

• Impedance: wiring impedance is determined by weight of copper and cross-sectional area of ​​the decision. For example, one ounce of copper there 0.49mΩ / Single Impedance-bit area.

• Capacitance: wiring capacitance is determined by an insulator (EoEr), the current reach of the (A) and trace spacing (h) decision. Expressed by the equation

C = EoErA / h, Eo is the permittivity of free space (8.854pF / m), Er is the relative permittivity of the PCB substrate (FR4 laminate was 4.7 in)

• Inductance: wiring inductance wiring evenly distributed, about 1nH / m. For one ounce copper, under the 0.25mm (10mil) FR4 case thick compacted located above the ground plane 0.5mm (20mil) wide, 20mm (800mil) to produce a long line of 9.8mΩ impedance, 20nH between the inductor and the ground 1.66pF coupling capacitance.

The above values ​​are compared with the parasitic components, these are negligible, but the sum of all the wiring can be parasitics can be exceeded.

Therefore, the designer must take this into account.

Here are some general guidelines for PCB layout:
• Increase the distance between traces to reduce capacitive coupling crosstalk;
• parallel to the power line and ground cloth to make the PCB capacitors to achieve the best;
• The sensitive high-frequency noise wire cloth in the power cord away from high places;
• widening the power and ground to reduce the power and ground impedance.

2. Segmentation

Segmentation refers to the physical separation of different types to reduce the coupling between the lines, especially through the power and ground.

3 . Partial decoupling between the power supply and the IC

Partial decoupling can reduce the spread of noise power along the trunk . Connected to the bulk power between the input port and the PCB plays a role in the low frequency bypass capacitor ripple filter , and as a potential reservoir for emergency power requirements . In addition, between each IC power and ground should have decoupling capacitors , these decoupling capacitors should be as close to the pin. This will help filter out the switching noise of the IC.

4 . RF current datum

Whether the reference path multilayer PCB ground layer or a single layer PCB ground , the current from the load back to the electricity always sources. The lower the impedance of the return path , the better the electromagnetic compatibility of the PCB . RF current flows due to the power supply between the load and length of the return path will be generated in the mutual coupling between them . Therefore, the return path should be as short loop area should be as small as possible .

5. Wiring separation

Separation is the role of the wiring within the same layer PCB noise coupling and crosstalk between adjacent lines is minimized. 3W specification indicates that all signals ( clock , video, audio , reset , etc. ) have to be like that shown in Figure 20 , be isolated between online and offline , edge to edge . In order to further reduce the magnetic coupling , the reference ground cloth placed near the key signal to isolate the other signal line coupling noise generated .

6. Protection and shunt line

The clock circuit, local decoupling capacitors to reduce noise transmission power along Route has a very important role. However, the same clock line requires protection against interference from other sources of electromagnetic interference , otherwise disturbed clock signal will cause problems elsewhere in the circuit.

7 . Grounding Technology

Grounding techniques used in both multilayer PCB, is also used in a single layer PCB. The goal is to minimize grounding techniques grounding impedance , thereby reducing the potential returns from the circuit between the power supply to ground loops .

a) single-layer PCB ground wire

In the single ( single ) PCB , the width of the grounding line should be as wide , and should be at least 1.5mm (60mil). Since the star can not be achieved on a single-layer wiring PCB, thus changing the jumper and ground should be kept to a minimum width , otherwise it will cause the line impedance and inductance changes.

b) The ground layer PCB

In the double ( double-sided ) PCB , for digital circuits priority to grill / dot wiring , this wiring can reduce ground impedance , ground loops and signal loops. As in the single-layer PCB , the ground and power lines should be at least the width of 1.5mm.

Another one is the ground floor layout aside, signal and power lines put on the other side. This arrangement will further reduce ground loops and impedance , decoupling capacitors can be placed at a distance between the IC power supply line and the ground plane as close as possible .

c) protection ring

A guard ring can be noisy environment ( e.g., RF current ) isolated exocyclic grounding techniques , because no current flows through the protective ring in a normal operation.

d) PCB capacitors

On the plywood , separated by a thin insulating power plane and ground to produce a PCB capacitors. In the single- board power source and ground laying parallel capacitance will cause this effect . An advantage of the capacitor PCB is that it has very high. The frequency response and evenly distributed over the entire surface or the entire low series inductance. It is equivalent to a uniform distribution. Throughout board decoupling capacitors . No single discrete element has this feature.

e) high-speed and low-speed circuit circuit

Laying high-speed circuit should bring it closer to the ground plane , and low power circuits should be so close to the surface .

f) copper fill places

In some analog circuit board is not used by a large area is to cover the ground , thereby providing increased shielding and decoupling capability. But if this piece of copper area is vacant ( ie not connected ) , then it may appear as an antenna and cause electromagnetic compatibility problems.

g) in multi-layer PCB ground plane and power planes

In a multilayer PCB , it is recommended to power and ground planes placed as close as possible in adjacent layers so that the entire board to produce a large PCB capacitors. The key signals should be the fastest ground near side , the signal is non-critical deployment as close to the supply side .

h) Power Requirements

When the circuit requires more than one power supply , the use of the ground will be separated from each power supply . However, in a single layer PCB multi- point grounding is impossible. One solution is to elicit from a power line and ground power supply with other power line and ground separated . This also helps to avoid noise coupling between the power supply .

8. Layout techniques

The following sections discuss some of the rules on PCB layout .

a) through the hole

Vias typically used in multi-layer printed circuit version . When the high-speed signal vias to produce an inductor and 0.3 4nH Capacitance of 0.8pF to the path .

Accordingly, when the high-speed signal path laying vias should be kept to an absolute minimum . For high-speed parallel lines ( e.g., address and data lines ) , if the change is inevitable layers , vias should ensure that the same number of each signal line .

b) the angle of the path 45

And had a similar hole , turning at right angles to the path should be avoided because it can produce at the edge of the internal electric field is concentrated .

The coupling to the adjacent field to produce acoustic noise paths , and therefore , when all orthogonal rotational paths should be used 45 degrees.

c) stub

Stub of reflection , but also a potential increase may be divided wavelength antenna to the circuit. Although the stub length may not be any integer in the quarter system known signal wavelength , but the radiation may resonate with the stub line. Therefore , to avoid using stub on the transmission high frequency and sensitive signals .

d) arranged in star-shaped signal

While the star -shaped arrangement applies to the ground from multiple PCB printed circuit board connections, but it has to produce multiple Stub signal path lines. Therefore, it should be avoided with a star -shaped arrangement on the speed and sensitivity of the signal .

e) radiation signal arrangement

Radiation signal arrangement usually have the shortest path , and minimal delay from source to sink , but it can also produce multiple reflection and radiation interference , so it should be avoided in the fast and sensitive high signal -type arrangement with radiation .

f) the same path Width

Width of the signal from the drive to the load path should be constant. Change the path width on the path impedance ( resistors, inductors, and capacitors ) to produce change , which can cause reflections and line impedance imbalance. So it is best to keep the width of the path unchanged .

g) holes and vias intensive

After intensive power and ground planes vias will produce localized differences in impedance vias close to the place . This area has not only become active signal "hot spots" , but at this point is the supply side impedance , the same as the RF current inefficient .

h) segmentation pore

Dense with holes and vias same segmentation porosity ( ie long hole or wide channels ) produce inconsistent regional power planes within the scope and status of the surface , and the same as shield to reduce their effectiveness , but also locally incremental power planes and status of surface impedance.

i) the ground metal mold

All of the metal mold should be connected to ground , otherwise, the large metal area can act as a radiation antenna

j) minimize loop area

Keeping the signal path and its return lines close together will help to minimize ground loop , thus avoiding potential antenna ring . For high-speed single-ended signal , sometimes if the signal does not go along the path of low resistance of the status of surface and ground loops may also be along the signal path.

A good ground can protect normal operations and personal safety equipment or systems that can eliminate electromagnetic interference and lightning . So grounding design is very important, but it is also more difficult task. Many different types of ground , logically , the signal , the shield to protect the land. Grounded approach can be divided into single-point ground , multi-point ground , mixed ground and suspended land. Ideal ground should be zero potential, no potential difference between the ground. But in fact, any " " or both ground resistance . When a current is passed , it will generate pressure drop, the potential of the ground line is not zero , it will exist between the two ground -ground voltage . When multi-point grounding circuit , and a signal contact , it will constitute a ground loop interference voltage. Therefore , very careful grounding techniques , such as signal ground and power ground to separate , complex multi-point grounding circuit and public land.

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