With the increase of integrated circuit output switching speed and PCB ploča density, Signal Integrity has become one of the issues that must be concerned in high-speed digital PCB design. The parameters of components and PCB board, the layout of components on PCB board, the wiring of high-speed Signal line and other factors, Can cause problems with signal integrity.

For PCB layouts, signal integrity requires a board layout that does not affect signal timing or voltage, while for circuit wiring, signal integrity requires termination elements, layout strategies, and wiring information. High signal speed on a PCB, incorrect placement of end components, or incorrect wiring of high-speed signals can cause signal integrity problems, which may cause the system to output incorrect data, the circuit to work improperly or not work at all. How to take signal integrity into full consideration and take effective control measures in PCB design has become a hot topic in PCB design industry.

Signal integrity Problem Good signal integrity means that the signal can respond with the correct timing and voltage level values when required. Suprotno tome, kada signal ne reagira ispravno, postoji problem s integritetom signala. Signal integrity problems can lead to or directly lead to signal distortion, timing errors, incorrect data, address and control lines, and system misoperation, or even system crash. U procesu dizajna PCB-a, ljudi su nakupili mnogo pravila dizajna PCB-a. In PCB design, the signal integrity of PCB can be better achieved by carefully referring to these design rules.

When designing PCB, we should first understand the design information of the whole circuit board, which mainly includes:

1. The number of devices, device size, device package, chip rate, whether PCB is divided into low speed, medium speed and high speed area, which is the interface input and output area;

2. The overall layout requirements, device layout location, whether there is a high power device, chip device heat dissipation special requirements;

3. Vrsta signalne linije, brzina i smjer prijenosa, zahtjevi kontrole impedancije signalne linije, smjer brzine sabirnice i situacija u vožnji, ključni signali i mjere zaštite;

4. Vrsta napajanja, vrsta uzemljenja, zahtjevi tolerancije buke za napajanje i uzemljenje, postavljanje i segmentacija napajanja i uzemljenja;

5. Vrste i brzine taktnih linija, izvor i smjer linija sata, zahtjevi za kašnjenje takta, zahtjevi za najdužu liniju.

PCB slojeviti dizajn

Nakon razumijevanja osnovnih podataka o pločici, potrebno je odvagnuti zahtjeve dizajna cijene tiskane pločice i integriteta signala te odabrati razuman broj slojeva ožičenja. At present, the circuit board has gradually developed from single layer, double layer and four layer to more multi-layer circuit board. Multi-layer PCB design can improve the reference surface of signal routing and provide backflow path for signal, which is the main measure to achieve good signal integrity. When designing PCB layering, follow the following rules:

1. The reference plane shall preferably be the ground plane. Both power supply and ground plane can be used as reference plane, and both have certain shielding function. Međutim, zaštitni učinak ravnine napajanja je mnogo niži od uzemljenja zbog njezine veće karakteristične impedancije i veće potencijalne razlike između ravnine napajanja i referentne razine uzemljenja.

2. Digital circuit and analog circuit are layered. Where design costs permit, it is best to arrange digital and analog circuits on separate layers. If must want to arrange in same wiring layer, can use ditch, add earthing line, the method such as dividing line to remedy. Analog and digital power and ground must be separated, never mixed.

3. Usmjeravanje ključnog signala susjednih slojeva ne prelazi područje segmentacije. Signals will form a large signal loop across the region and generate strong radiation. If the signal cable must cross the area when the ground cable is divided, a single point can be connected between the ground to form a connection bridge between the two ground points, and then the cable can be routed through the connection bridge.

4. Ispod komponente mora postojati relativno potpuna ravnina tla. Za višeslojnu ploču potrebno je održati cjelovitost uzemljenja što je više moguće. Obično nije dopušteno prolaziti signalnim vodovima u ravnini tla.

5, visoke frekvencije, velike brzine, sat i druge ključne signalne linije trebaju imati susjednu uzemljenu ravninu. In this way, the distance between signal line and ground line is only the distance between PCB layers, so the actual current always flows in the ground line directly below the signal line, forming the smallest signal loop area and reducing radiation.

How to design the signal of integrity PCB

PCB layout design

Ključ dizajna integriteta signala tiskane ploče je raspored i ožičenje, što je izravno povezano s performansama PCB-a. Prior to layout, the PCB size must be determined to meet the function at the lowest possible cost. If the PCB is too large and distributed, the transmission line may be very long, resulting in increased impedance, reduced noise resistance, and increased cost. If the components are placed together, heat dissipation is poor, and coupling crosstalk may occur in adjacent wiring. Stoga se raspored mora temeljiti na funkcionalnim jedinicama kruga, uzimajući u obzir elektromagnetsku kompatibilnost, rasipanje topline i čimbenike sučelja.

When laying out a PCB with mixed digital and analog signals, do not mix digital and analog signals. If analog and digital signals must be mixed, be sure to line vertically to reduce the effect of cross-coupling. The digital circuit, analog circuit, and noise-generating circuit on the circuit board should be separated, and the sensitive circuit should be routed first, and the coupling path between the circuits should be eliminated. Posebno uzmite u obzir linije za sat, resetiranje i prekide, nemojte ih uspoređivati ​​s visokostrujnim prekidačima, inače ih je lako oštetiti signalima elektromagnetskog spajanja, što može uzrokovati neočekivano resetiranje ili prekid. The overall layout should follow the following principles:

1. Functional partition layout, analog circuit and digital circuit on PCB should have different spatial layout.

2. U skladu s procesom signalnog kruga organizirati funkcionalne jedinice kruga, tako da protok signala održava isti smjer.

3. Take the core components of each functional circuit unit as the center, and other components are arranged around it.

4. Shorten the connection between high-frequency components as much as possible and try to reduce their distribution parameters.

5. Easily disturbed components should not be too close to each other, input and output components should be far away.

How to design the signal of integrity PCB

Dizajn ožičenja PCB -a

All signal lines should be classified before PCB wiring. First of all, clock line, sensitive signal line, and then high-speed signal line, in order to ensure that this kind of signal through the hole is enough, distribution parameters of good characteristics, and then general unimportant signal line.

Incompatible signal lines should be far away from each other and do not parallel wiring, such as digital and analog, high speed and low speed, high current and small current, high voltage and low voltage. Signal cables on different layers should be routed vertically to each other to reduce crosstalk. Raspored signalnih vodova najbolje je rasporediti prema smjeru protoka signala. Linija izlaznog signala u krugu ne smije se vratiti unatrag u područje ulaznog signala. High-speed signal lines should be kept as short as possible to avoid interfering with other signal lines. Na dvostrukoj ploči, ako je potrebno, može se dodati izolacijska žica za uzemljenje s obje strane signalne linije velike brzine. Sve brze linije sata na višeslojnoj ploči trebaju biti zaštićene u skladu s duljinom linija sata.

Opća načela za ožičenje su:

1. As far as possible to choose low density wiring design, and signal wiring as far as possible thickness consistent, conducive to impedance matching. For rf circuit, the unreasonable design of signal line direction, width and line spacing may cause cross interference between signal transmission lines.

2. As far as possible to avoid adjacent input and output wires and long-distance parallel wiring. Za smanjenje preslušavanja paralelnih signalnih vodova, razmak između signalnih vodova može se povećati ili se između signalnih vodova mogu umetnuti izolacijski pojasevi.

3. Širina linije na PCB-u mora biti ujednačena i neće doći do mutacije širine linije. Savijanje ožičenja PCB -a ne smije koristiti kut od 90 stupnjeva, treba koristiti luk ili kut od 135 stupnjeva, koliko je god moguće za održavanje kontinuiteta impedancije linije.

4. Minimize the area of the current loop. The external radiation intensity of current-carrying circuit is proportional to the current passing through, the loop area and the square of signal frequency. Reducing the current loop area can reduce the ELECTROMAGNETIC interference of PCB.

5. As far as possible to reduce the length of the wire, increase the width of the wire, is conducive to reducing the impedance of the wire.

6. For switch control signals, the number of SIGNAL PCB wiring that changes the state at the same time should be reduced as far as possible.