Pri oblikovanju PCB plošča, s hitrim naraščanjem frekvence bo veliko motenj, ki se razlikujejo od motenj nizkofrekvenčnih PCB plošč. Poleg tega bodo s povečevanjem frekvence in protislovjem med miniaturizacijo in nizkimi stroški PCB plošče te motnje postajale vse bolj zapletene.

V dejanski raziskavi lahko sklepamo, da obstajajo predvsem štirje vidiki motenj, vključno s hrupom napajanja, motnjami v daljnovodu, sklopko in elektromagnetnimi motnjami (EMI). Z analizo različnih motenj visokofrekvenčnih PCB in združevanjem s prakso pri delu so predstavljene učinkovite rešitve.

Najprej hrup napajanja

V visokofrekvenčnem vezju hrup napajanja očitno vpliva na visokofrekvenčni signal. Therefore, the first requirement of the power supply is low noise. Čista tla so prav tako pomembna kot čista elektrika. Zakaj? Značilnosti moči so prikazane na sliki 1. Očitno ima napajalnik določeno impedanco in impedanca je porazdeljena po celotnem napajalniku, zato se hrupu doda napajanje.

Then we should minimize the impedance of the power supply, so it is best to have a dedicated power supply layer and grounding layer. Pri zasnovi hf vezja je veliko bolje oblikovati napajalnik kot plast kot v vodilu v večini primerov, tako da lahko zanka vedno sledi poti minimalne impedance.

In addition, the power board must provide a signal loop for all generated and received signals on the PCB. This minimizes the signal loop and thus reduces noise, which is often overlooked by low-frequency circuit designers.

Pri visokofrekvenčnem načrtovanju tiskanih vezij se pojavljajo motnje

Slika 1: Značilnosti moči

Pri oblikovanju tiskanih vezij obstaja več načinov za odpravo hrupa energije:

1. Note the through hole on the board: the through hole requires etched openings on the power supply layer to leave space for the through hole to pass through. If the opening of the power supply layer is too large, it is bound to affect the signal loop, the signal is forced to bypass, the loop area increases, and the noise increases. At the same time, if several signal lines are clustered near the opening and share the same loop, the common impedance will cause crosstalk. Glej sliko 2.

Pri visokofrekvenčnem načrtovanju tiskanih vezij se pojavljajo motnje

Slika 2: Skupna pot obvodne signalne zanke

2. The connection line needs enough ground: each signal needs to have its own proprietary signal loop, and the loop area of the signal and loop is as small as possible, that is to say, the signal and loop should be parallel.

3. Analogni in digitalni napajalnik za ločevanje: visokofrekvenčne naprave so na splošno zelo občutljive na digitalni šum, zato jih je treba ločiti, povezati skupaj na vhodu v napajalnik, če signal čez analogni in digitalni del besede, jih lahko v signal vstavimo čez zanko, da zmanjšamo površino zanke. Digitalno-analogni razpon, ki se uporablja za signalno zanko, je prikazan na sliki 3.

Pri visokofrekvenčnem načrtovanju tiskanih vezij se pojavljajo motnje

Figure 3: Digital – analog span for signal loop

4. Avoid overlapping of separate power supplies between layers: otherwise circuit noise can easily pass through parasitic capacitive coupling.

5. Isolate sensitive components: such as PLL.

6. Place the power cable: To reduce the signal loop, place the power cable on the edge of the signal line to reduce the noise, as shown in Figure 4.

Pri visokofrekvenčnem načrtovanju tiskanih vezij se pojavljajo motnje

Slika 4: Postavite napajalni kabel poleg signalne linije

Two, transmission line

V tiskanem vezju sta možna samo dva prenosna voda:

Največji problem tračne in mikrovalovne linije je odsev. Odsev bo povzročil številne težave. Na primer, signal obremenitve bo superpozicija izvirnega signala in odmevnega signala, kar bo povečalo težave pri analizi signala. Odsev povzroči povratno izgubo (povratna izguba), ki tako slabo vpliva na signal kot dodatne motnje hrupa:

1. Signal, ki se odbije nazaj v vir signala, bo povečal hrup sistema, zaradi česar bo sprejemnik težje ločil hrup od signala;

2. Any reflected signal will basically degrade the signal quality and change the shape of the input signal. Generally speaking, the solution is mainly impedance matching (for example, the impedance of the interconnection should very match the impedance of the system), but sometimes the calculation of impedance is more troublesome, you can refer to some transmission line impedance calculation software. The methods of eliminating transmission line interference in PCB design are as follows:

(a) Izogibajte se prekinitvi impedance prenosnih vodov. Točka diskontinuirane impedance je točka mutacije daljnovoda, na primer ravnega vogala, skozi luknjo itd., Se je treba čim bolj izogibati. Metode: Da bi se izognili ravnim vogalom črte, kolikor je mogoče, da gredo pod kotom 45 ° ali lokom, je lahko velik tudi kot; Uporabite čim manj skoznjih lukenj, ker je vsaka skoznja luknja impedančna diskontinuiteta, kot je prikazano na sl. 5; Signals from the outer layer avoid passing through the inner layer and vice versa.

Pri visokofrekvenčnem načrtovanju tiskanih vezij se pojavljajo motnje

Figure 5: Method for eliminating transmission line interference

(b) Do not use stake lines. Ker je vsak kupec vir hrupa. Če je pilotski vod kratek, ga lahko priključite na koncu daljnovoda; Če je kupec dolg, bo glavni vir daljnovoda vzel in ustvaril velik odsev, kar bo otežilo težavo. Priporočljivo je, da ga ne uporabljate.

Tretjič, sklopka

1. Common impedance coupling: it is a common coupling channel, that is, the interference source and the interfered device often share some conductors (such as loop power supply, bus, and common grounding), as shown in Figure 6.

Pri visokofrekvenčnem načrtovanju tiskanih vezij se pojavljajo motnje

Slika 6: Skupna impedančna sklopka

In this channel, the drop back of the Ic causes a common-mode voltage in the series current loop, affecting the receiver.

2. The field common-mode coupling will cause the radiation source to cause common-mode voltages in the loop formed by the interfered circuit and on the common reference surface.

If the magnetic field is dominant, the value of the common-mode voltage generated in the series ground circuit is Vcm=-(△B/△t)* area (where △B= change in magnetic induction intensity). If it is an electromagnetic field, when its electric field value is known, its induced voltage: Vcm=(L* H *F*E)/48, the formula is suitable for L(m)=150MHz, beyond this limit, the calculation of the maximum induced voltage can be simplified as: Vcm=2* H *E.

3. Differential mode field coupling: refers to the direct radiation by wire pair or circuit board on the lead and its loop induction received. If you get as close to the two wires as possible. Ta sklopka je močno zmanjšana, zato lahko obe žici zvijete skupaj, da zmanjšate motnje.

4. Inter-line coupling (crosstalk) can cause unwanted coupling between any line or parallel circuit, which will greatly damage the performance of the system. Its type can be divided into capacitive crosstalk and perceptual crosstalk.

The former is because the parasitic capacitance between the lines makes the noise on the noise source coupled to the noise receiving line through current injection. The latter can be thought of as the coupling of signals between the primary stages of an unwanted parasitic transformer. Velikost induktivnih preslušanj je odvisna od bližine dveh zank, velikosti območja zanke in impedance prizadetega bremena.

5. Priključitev napajalnega kabla: Napajalni ali enosmerni napajalni kabel motijo ​​elektromagnetne motnje

Transfer to other devices.

There are several ways to eliminate crosstalk in PCB design:

1. Obe vrsti navzkrižnih povezav se povečujeta s povečanjem impedance obremenitve, zato je treba signalne linije, občutljive na motnje, ki jih povzroča preslušanje, ustrezno prekiniti.

2. Povečajte razdaljo med signalnimi vodi za učinkovito zmanjšanje kapacitivnih preslušanj. Upravljanje tal, razmik med ožičenjem (kot so aktivni signalni vodi in ozemljitveni vodi za izolacijo, zlasti v stanju skoka med signalno linijo in ozemljitvijo do intervala) in zmanjšanje induktivnosti kabla.

3. Capacitive crosstalk can also be effectively reduced by inserting a ground wire between adjacent signal lines, which must be connected to the formation every quarter of a wavelength.

4. Za smiselno medsebojno povezovanje je treba območje zanke čim bolj zmanjšati, če je dovoljeno, pa je treba zanko odpraviti.

5. Avoid signal sharing loops.

6. Bodite pozorni na celovitost signala: oblikovalec mora v postopek varjenja implementirati konce, da reši celovitost signala. Oblikovalci, ki uporabljajo ta pristop, se lahko osredotočijo na dolžino mikropasov zaščitne bakrene folije, da bi dosegli dobro delovanje integritete signala. For systems with dense connectors in the communication structure, the designer can use a PCB as the terminal.

Four, electromagnetic interference

As the speed increases, EMI becomes more and more serious and presents in many aspects (such as electromagnetic interference at interconnects). High-speed devices are particularly sensitive to this and will receive high-speed spurious signals, while low-speed devices will ignore such spurious signals.

There are several ways to eliminate electromagnetic interference in PCB design:

1. Zmanjšaj zanke: Vsaka zanka je enakovredna anteni, zato moramo zmanjšati število zank, površino zank in antenski učinek zank. Make sure the signal has only one loop path at any two points, avoid artificial loops and use the power layer whenever possible.

2. Filtering: Filtering can be used to reduce EMI on both the power line and the signal line. There are three methods: decoupling capacitor, EMI filter and magnetic element. EMI filter is shown in Figure 7.

Pri visokofrekvenčnem načrtovanju tiskanih vezij se pojavljajo motnje

Slika 7: Vrste filtrov

3. The shielding. Zaradi dolžine vprašanja in številnih člankov, ki ščitijo razpravo, ni več posebnega uvoda.

4. Reduce the speed of high-frequency devices.

5. Povečajte dielektrično konstanto plošče PCB, kar lahko prepreči, da bi visokofrekvenčni deli, kot je daljnovod v bližini plošče, sevali navzven; Increase the thickness of PCB board, minimize the thickness of microstrip line, can prevent electromagnetic line spillover, can also prevent radiation.

At this point, we can conclude that in hf PCB design, we should follow the following principles:

1. Unification and stability of power supply and ground.

2. Carefully considered wiring and proper terminations can eliminate reflections.

3. Carefully considered wiring and proper terminations can reduce capacitive and inductive crosstalk.

4. Za izpolnjevanje zahtev EMC je potrebno zatiranje hrupa.