U dizajnu od PCB ploča, s brzim povećanjem frekvencije, bit će mnogo smetnji koje se razlikuju od onih na niskofrekventnim PCB pločama. Štoviše, s povećanjem frekvencije i kontradikcijom između minijaturizacije i niske cijene PCB ploče, te smetnje postat će sve kompliciranije.

U stvarnom istraživanju možemo zaključiti da postoje uglavnom četiri aspekta smetnji, uključujući šum napajanja, smetnje u dalekovodu, spajanje i elektromagnetske smetnje (EMI). Analizom različitih problema smetnji visokofrekventnih PCB-a i kombiniranjem s praksom u radu, dolaze se do učinkovitih rješenja.

Prvo, buka napajanja

U visokofrekventnom krugu šum napajanja ima očit utjecaj na visokofrekventni signal. Therefore, the first requirement of the power supply is low noise. Clean floors are just as important as clean electricity. Why? The power characteristics are shown in Figure 1. Očito, napajanje ima određenu impedanciju, a impedancija je raspoređena po cijelom napajanju, pa će se napajanju dodati i buka.

Then we should minimize the impedance of the power supply, so it is best to have a dedicated power supply layer and grounding layer. Kod projektiranja visokofrekvencijskog kruga, mnogo je bolje napajanje projektirati kao sloj nego kao sabirnicu u većini slučajeva, tako da petlja uvijek može slijediti put minimalne impedancije.

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.

U visokofrekventnom dizajnu PCB-a javljaju se smetnje

Slika 1: Karakteristike snage

Postoji nekoliko načina za uklanjanje šuma napajanja u dizajnu PCB -a:

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. Pogledajte sliku 2.

U visokofrekventnom dizajnu PCB-a javljaju se smetnje

Slika 2: Zajednički put petlje zaobilaznog signala

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. Analogno i digitalno napajanje za odvajanje: visokofrekventni uređaji općenito su vrlo osjetljivi na digitalnu buku, pa ih treba odvojiti i povezati zajedno na ulazu u izvor napajanja, ako signal preko analognog i digitalnog dijela riječi, mogu se postaviti u signal preko petlje kako bi se smanjilo područje petlje. Digitalno-analogni raspon koji se koristi za signalnu petlju prikazan je na slici 3.

U visokofrekventnom dizajnu PCB-a javljaju se smetnje

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.

U visokofrekventnom dizajnu PCB-a javljaju se smetnje

Slika 4: Kabel za napajanje postavite pored signalnog voda

Two, transmission line

U PCB -u postoje samo dva moguća prijenosna voda:

Najveći problem vrpce i mikrovalne linije je refleksija. Razmišljanje će uzrokovati mnoge probleme. Na primjer, signal opterećenja bit će superpozicija izvornog signala i eho signala, što će povećati poteškoće u analizi signala. Refleksija uzrokuje povratni gubitak (povratni gubitak), što utječe na signal jednako kao i aditivna smetnja:

1. Signal koji se reflektira natrag do izvora signala povećat će šum sustava, što će prijemniku otežati razlikovanje šuma 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) Izbjegavajte prekid impedancije dalekovoda. Točka diskontinuirane impedancije je točka mutacije dalekovoda, poput ravnog kuta, rupe itd., Koju treba izbjegavati koliko je god moguće. Metode: Kako biste izbjegli ravne uglove crte, što je više moguće do kuta ili luka od 45 °, veliki kut također može biti; Upotrijebite što je moguće manje prolaznih otvora, jer je svaki prolazni otvor diskontinuitet impedanse, kao što je prikazano na Sl. 5; Signals from the outer layer avoid passing through the inner layer and vice versa.

U visokofrekventnom dizajnu PCB-a javljaju se smetnje

Figure 5: Method for eliminating transmission line interference

(b) Do not use stake lines. Budući da je svaka linija hrpa izvor buke. Ako je vod za hrpu kratak, može se spojiti na kraju dalekovoda; Ako je vod gomile dugačak, uzeti će glavni dalekovod kao izvor i proizvesti veliku refleksiju, što će zakomplicirati problem. Preporuča se ne koristiti.

Treće, spojnica

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.

U visokofrekventnom dizajnu PCB-a javljaju se smetnje

Slika 6: Spojnica zajedničke impedancije

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. Ova spojka je uvelike smanjena pa se dvije žice mogu uviti zajedno kako bi se smanjile smetnje.

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. Veličina induktivnog preslušavanja ovisi o blizini dvije petlje, veličini područja petlje i impedanciji utjecaja opterećenja.

5. Spoj kabela za napajanje: Kablovi za napajanje izmjeničnog ili istosmjernog napona ometani su elektromagnetskim smetnjama

Transfer to other devices.

There are several ways to eliminate crosstalk in PCB design:

1. Both types of crosstalk increase with the increase of load impedance, so the signal lines sensitive to interference caused by crosstalk should be properly terminated.

2. Povećajte udaljenost između signalnih vodova za učinkovito smanjenje kapacitivnih preslušavanja. Upravljanje uzemljenjem, razmakom između ožičenja (kao što su aktivni signalni vodovi i vodovi uzemljenja za izolaciju, osobito u stanju skoka između signalnog voda i mase prema intervalu) i smanjenje induktivnosti olova.

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. Radi razumnog preslušavanja, područje petlje treba minimizirati, a ako je dopušteno, petlju treba ukloniti.

5. Avoid signal sharing loops.

6. Obratite pozornost na integritet signala: projektant bi trebao implementirati krajeve u postupak zavarivanja kako bi riješio integritet signala. Dizajneri koji koriste ovaj pristup mogu se usredotočiti na duljinu mikrotrake zaštitne bakrene folije kako bi postigli dobre performanse integriteta 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. Smanjite petlje: Svaka petlja je ekvivalentna anteni, pa moramo minimizirati broj petlji, površinu petlji i antenski učinak petlji. 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.

U visokofrekventnom dizajnu PCB-a javljaju se smetnje

Slika 7: Vrste filtera

3. The shielding. Kao rezultat duljine izdanja i puno diskusijskih članaka, više nije bilo posebnog uvoda.

4. Reduce the speed of high-frequency devices.

5. Povećajte dielektričnu konstantu PCB ploče, što može spriječiti da visokofrekventni dijelovi, poput dalekovoda u blizini ploče, zrače prema van; 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. Suzbijanje buke potrebno je kako bi se zadovoljili zahtjevi EMC -a.