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Riassunto dell'esperienza del layout e del cablaggio della scheda PCB RF del telefono cellulare
Blog PCB
Riassunto dell'esperienza del layout e del cablaggio della scheda PCB RF del telefono cellulare

Riassunto dell'esperienza del layout e del cablaggio della scheda PCB RF del telefono cellulare

2022-01-19
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Author:pcb

Radio frequency (RF) printed circuit boards design is often described as a "black art" due to its theoretical uncertainty, but this view is only partially true, e ci sono molte linee guida per la progettazione del circuito RF che può e non dovrebbe essere seguita regola trascurata. However, quando si tratta di progettazione effettiva, the real trick is how to compromise these guidelines and laws when they cannot be implemented accurately due to various design constraints. Naturalmente, there are many important RF design topics worth discussing, compresa la corrispondenza di impedenza e impedenza, insulating layer materials and laminates, lunghezza d'onda e onde standing, so these have a great impact on the EMC and EMI of mobile phones. The conditions that must be met when designing an RF layout are summarized:

Scheda PCB

1. The high power RF amplifier (HPA) and the low noise amplifier (LNA) should be separated
To isolate the high power RF amplifier (HPA) and the low noise amplifier (LNA) as much as possible, simply put the high power RF transmit circuit away from the low power RF receive circuit. Mobile phones have many functions and many components, ma il Scheda PCB space is small, and considering the limitations of the wiring design process, tutti questi richiedono competenze di progettazione relativamente elevate. At this time, può essere necessario progettare da quattro a sei strati Scheda PCB, and let them work alternately instead of working at the same time. High power circuits may also sometimes include RF buffers and voltage controlled oscillators (VCOs). Make sure that there is at least one whole ground in the high-power area on the PCB without vias. Naturalmente, the more copper, meglio è. Sensitive analog signals should be kept as far away as possible from high-speed digital and RF signals.

2. Physical partition, electrical partition
Design partitions can be broken down into physical and electrical partitions. Physical partitions mainly involve issues such as component placement, orientation, and shielding; electrical partitions can continue to be broken down into partitions for power distribution, Tracce RF, sensitive circuits and signals, and grounding.

2.1 Discutiamo il partizionamento fisico.
Component placement is the key to implementing an RF design. An effective technique is to prima fix the components located on the RF path and adjust their orientation to minimize the length of the RF path, keep the input away from the output, e separare i componenti per quanto possibile. power circuits and low power circuits. An effective board stacking method is to arrange the main ground plane (main ground) on the second layer below the surface layer, and run the RF lines on the surface layer as much as possible. Reducing the via size on the RF path not only reduces the path inductance, but also reduces ghost solder joints on the main ground and reduces the chance of RF energy leaking to other areas within the stack. In physical space, linear circuits like multistage amplifiers are usually sufficient to isolate multiple RF zones from each other, ma duplex, mixers, and IF amplifiers/mixers always have multiple RF/IFs The signals interfere with each other, so care must be taken to minimize this effect.

2.2 The RF and IF traccias should be crossed as much as possible, e un terreno dovrebbe essere distanziato tra di loro il più possibile.
The correct RF path is very important to the performance of the entire PCB, che è il motivo per cui il posizionamento dei componenti di solito occupa la maggior parte del tempo nella progettazione del PCB del telefono cellulare. In the design of the mobile phone Scheda PCB, il circuito dell'amplificatore a basso rumore può essere solitamente posizionato su un lato del Scheda PCB, and the high power amplifier can be placed on the other side, e sono infine collegati all'estremità RF e all'elaborazione della banda base sullo stesso lato attraverso un duplexer. on the antenna of the device. Some tricks are required to ensure that straight-through vias do not transfer RF energy from one side of the board to the other, and a common technique is to use blind vias on both sides. The detrimental effects of straight-through vias can be minimized by arranging the straight-through vias in areas where both sides of the PCB are free from RF interference. Sometimes it is not possible to ensure sufficient isolation between multiple circuit blocks, in which case it is necessary to consider using a metal shield to shield the RF energy in the RF area. The metal shield must be soldered to the ground and must be kept from the components. una distanza adeguata, thus taking up valuable Scheda PCB space. It is very important to ensure the integrity of the shielding cover as much as possible. Le linee di segnale digitali che entrano nella copertura di schermatura metallica dovrebbero andare allo strato interno il più possibile, and the Scheda PCB below the wiring layer is the ground layer. The RF signal line can go out from the small gap at the bottom of the metal shield and the wiring layer at the ground gap, ma più terreno possibile dovrebbe essere distribuito intorno al divario, and the ground on different layers can be connected together through multiple vias .

2.3 Proper and effective chip power decoupling is also very important.
Many RF chips with integrated linear lines are very sensitive to noise from the power supply, typically requiring up to four capacitors and an isolation inductor per chip to ensure all power supply noise is filtered out. An integrated circuit or amplifier often has an open-drain output, so a pull-up inductor is required to provide a high-impedance RF load and a low-impedance DC supply. The same principle applies to decoupling the supply on this inductor side. Some chips require multiple power supplies to work, so you may need two or three sets of capacitors and inductors to decouple them separately, inductors are rarely close together in parallel, in quanto questo creerebbe un trasformatore di nucleo dell'aria e inducerebbe interferenze tra di loro il segnale, so the distance between them should be at least the height of one of the devices, or they should be arranged at right angles to reduce their mutual inductance.

2.4 I principi del partizionamento elettrico sono generalmente gli stessi del partizionamento fisico, but there are some other factors involved.
Alcune parti del telefono funzionano a tensioni diverse e sono controllate dal software per prolungare la durata della batteria. This means the phone needs to run on multiple power sources, che crea più problemi con l'isolamento. Power is usually brought in at the connector and is immediately decoupled to filter out any noise from outside the board before being distributed through a set of switches or voltage regulators. La maggior parte dei circuiti sui PCB del telefono cellulare hanno correnti DC abbastanza piccole, so trace width is usually not an issue, tuttavia, a separate high-current trace as wide as possible must be run for the power supply of the high-power amplifier to minimize the transmission voltage drop. Per evitare troppe perdite di corrente, multiple vias are needed to pass current from one layer to another. Inoltre, if the high power amplifier is not sufficiently decoupled at its power supply pins, Il rumore ad alta potenza si irradia su tutta la scheda e causerà vari problemi. Grounding of high power amplifiers is critical and often requires a metal shield. Nella maggior parte dei casi, it is also critical to ensure that the RF output is kept away from the RF input. Ciò vale anche per gli amplificatori, buffers and filters. Nel peggiore dei casi, amplifiers and buffers have the potential to self-oscillate if their outputs are fed back to their inputs with the proper phase and amplitude. In any case, they will work stably under any temperature and voltage conditions. In fact, they can become unstable and add noise and intermodulation signals to the RF signal. If the RF signal lines have to be looped back from the input of the filter to the output, questo può danneggiare gravemente le caratteristiche di passaggio del nastro del filtro. In order to obtain good isolation between input and output, first, a ground must be placed around the filter, and secondly, a ground should be placed in the lower area of the filter and connected to the main ground around the filter. È anche una buona idea mantenere le linee di segnale che devono passare attraverso il filtro il più lontano possibile dai pin del filtro. Anche, Fai attenzione con la messa a terra ovunque sulla tavola, or you will introduce a coupling channel. A volte è possibile scegliere linee di segnale RF monoterminale o bilanciate, and the same principles regarding cross-interference and EMC/EMI apply here. Balanced RF signal lines can reduce noise and cross-interference if they are routed correctly, but their impedance is usually high, and a reasonable line width should be maintained to obtain an impedance that matches the source, trace, and load. Il cablaggio effettivo può Ci saranno alcune difficoltà. A buffer can be used to improve isolation because it can split the same signal into two parts and use it to drive different circuits, especially if the LO may need a buffer to drive multiple mixers. When the mixer reaches common mode isolation at RF frequencies, non funzionerà correttamente. Buffers are good at isolating impedance changes at different frequencies so that circuits do not interfere with each other. I buffer sono un grande aiuto nella progettazione, they can be placed right after the circuit that needs to be driven, in modo che le tracce di uscita ad alta potenza siano molto brevi, because the input signal level of the buffer is relatively low, so they are not easy to be affected by other circuits on the board. circuito che provoca interferenze. Voltage Controlled Oscillators (VCOs) convert changing voltages to changing frequencies, una funzione utilizzata per la commutazione di canale ad alta velocità, but they also convert small amounts of noise on the control voltage into small frequency changes, che danno segnali RF aggiungono rumore.

2.5 To ensure that no noise is added, the following aspects must be considered:
First, the desired bandwidth of the control line may range from DC to 2MHz, and filtering to remove noise in such a wide band is almost impossible; second, la linea di controllo VCO è solitamente parte di un ciclo di feedback che controlla la frequenza, which is in many Noise can be introduced everywhere, quindi le linee di controllo VCO devono essere gestite con grande cura. Make sure that the ground below the RF traces is solid and that all components are firmly connected to the main ground and isolated from other traces that may introduce noise. Inoltre, make sure that the power supply of the VCO is adequately decoupled, poiché l'uscita RF del VCO tende ad essere un livello relativamente alto, the VCO output signal can easily interfere with other circuits, quindi particolare attenzione deve essere prestata al VCO. In fact, il VCO è spesso posizionato alla fine dell'area RF, and sometimes it requires a metal shield. The resonant circuit (one for the transmitter and the other for the receiver) is related to the VCO, but also has its own characteristics. In poche parole, a resonant circuit is a parallel resonant circuit with a capacitive diode that helps set the VCO operating frequency and modulate speech or data onto an RF signal. Tutti i principi di progettazione VCO si applicano ugualmente ai circuiti risonanti. Resonant circuits are often very sensitive to noise due to their considerable number of components, ampia distribuzione sul bordo, and typically operating at a very high RF frequency. Signals are usually arranged on adjacent pins of the chip, but these signal pins need to work with relatively large inductors and capacitors, which in turn requires these inductors and capacitors to be located close together and connected back to on a noise-sensitive control loop. It is not easy to do this. The automatic gain control (AGC) amplifier is also a problem-prone place, e ci sarà un amplificatore AGC sia nei circuiti di trasmissione che di ricezione. AGC amplifiers are usually effective at filtering out noise, ma grazie alla capacità dei telefoni cellulari di gestire rapidi cambiamenti nella potenza del segnale trasmesso e ricevuto, AGC circuits are required to have a fairly wide bandwidth, che rende facile introdurre amplificatori AGC su alcuni circuiti critici. Good analog circuit design techniques must be followed when designing AGC lines, e questo ha a che fare con pin di ingresso ampli op molto brevi e percorsi di feedback molto brevi, both of which must be kept away from RF, IF, or high-speed digital signal traces.
Also, good grounding is essential, e l'alimentazione del chip deve essere ben disaccoppiata. If you have to run a long wire at the input or output, it's at the output, which usually has a much lower impedance and is less prone to inductive noise. Di solito più alto è il livello del segnale, the easier it is to introduce noise into other circuits. In all PCB design, it is a general principle to keep digital circuits away from analog circuits as much as possible, and it also applies to RF PCB design. The common analog ground is often as important as the ground used to shield and separate the signal lines, così attenta pianificazione, thoughtful component placement, and thorough placement* estimation are all important in the early stages of design. Likewise, RF should be Lines should be kept away from analog lines and some very critical digital signals. Tutte le tracce RF, pads and components should be filled with ground copper as much as possible, e collegato al terreno principale il più possibile. If the RF traces must pass through the signal lines, cercare di instradare uno strato di terra collegato al terreno principale lungo le tracce RF tra di loro. If not possible, Assicurarsi che siano incrociati per ridurre al minimo l'accoppiamento capacitivo, and as much ground as possible around each RF trace, e collegarli al terreno principale. Also, Ridurre la distanza tra tracce RF parallele può ridurre l'accoppiamento induttivo. A solid monolithic ground plane placed directly under the surface layer, l'effetto di isolamento, although a little careful design other practices also work. Su ogni strato del Scheda PCB, lay as many grounds as possible and connect them to the main ground. Posizionare le tracce il più vicino possibile per aumentare il numero di pad sui livelli interni del segnale e della distribuzione di energia, and adjust the traces so that you can route ground connection vias to isolated pads on the surface. I terreni liberi su vari strati del PCB dovrebbero essere evitati in quanto possono raccogliere o iniettare rumore come una piccola antenna. In most cases, se non riesci a collegarli al terreno principale, then you remove them.


3. In the design of mobile phone Scheda PCB, should pay attention to several aspects
3.1 Handling of power supply and ground wire
Even if the wiring in the entire Scheda PCB è ben completato, the interference caused by the lack of thoughtful consideration of the power supply and the ground wire will reduce the performance of the product, e talvolta anche influenzare il tasso di successo del prodotto. Therefore, il cablaggio dei cavi di alimentazione e di terra dovrebbe essere preso sul serio, and the noise interference generated by the power and ground wires should be minimized to ensure the quality of the products. Per ogni ingegnere impegnato nella progettazione di prodotti elettronici, the reason for the noise between the ground wire and the power line is understood, and now only the reduced noise suppression is expressed:
(1) It is well known that a decoupling capacitor is added between the power supply and the ground wire.
(2) Try to widen the width of the power supply and ground wire as much as possible. Il cavo di massa è più largo del cavo di alimentazione elettrica. 0.05ï½ 0.07mm, the power cord is 1.2ï½ 2.5mm. Per il Scheda PCB of the digital circuit, un ampio filo di terra può essere utilizzato per formare un anello, that is, a ground net can be used (the ground of the analog circuit cannot be used in this way)
(3) Use a large-area copper layer as a ground wire, and connect the unused places on the printed board to the ground as a ground wire. O fare una scheda multistrato, power supply, Il filo di terra occupa ciascuno uno strato.

3.2 Common ground processing of digital circuits and analog circuits
Now there are many Scheda PCBs that are no longer a single function circuit (digital or analog circuit), but are composed of a mixture of digital and analog circuits. Therefore, è necessario considerare l'interferenza reciproca tra loro durante il cablaggio, especially the noise interference on the ground wire. The frequency of the digital circuit is high, and the sensitivity of the analog circuit is strong. For the signal line, the high-frequency signal line should be kept away from the sensitive analog circuit devices as much as possible. For the ground line, the whole Scheda PCB ha un solo nodo al mondo esterno. Therefore, Il problema del terreno comune digitale e analogico deve essere affrontato all'interno del Scheda PCB, while the digital ground and the analog ground are actually separated inside the board, and they are not connected to each other, only at the interface between the Scheda PCB and the outside world (such as plugs). Wait). The digital ground is a little shorted to the analog ground, note that there is only one connection point. There are also different grounds on the Scheda PCB, which are determined by the system design.

3.3 Signal lines are routed on the electrical (ground) layer
In the wiring of multi-layer printed boards, poiché non ci sono molte linee rimaste nel livello della linea del segnale, adding more layers will cause waste and increase the production workload, and the cost will increase accordingly. To solve this contradiction, we can consider wiring on the electrical (ground) layer. The power plane should be considered first, seguito dal piano di terra. Because the integrity of the formation is preserved.

3.4 Handling of connecting legs in large area conductors
In a large area of grounding (electricity), the legs of commonly used components are connected to it, and the handling of the connecting legs needs to be comprehensively considered. Ci sono alcuni pericoli nascosti nella saldatura e nell'assemblaggio dei componenti, such as: ‘  Welding requires high-power heaters. '¡È facile causare giunti di saldatura virtuali. Therefore, taking into account the electrical performance and process needs, a cross-shaped pad is made, which is called heat shield, commonly known as thermal pad. Il sesso è notevolmente ridotto. The electrical (ground) leg of a multilayer board is treated the same way.

3.5 The role of network system in wiring
In many CAD systems, Il cablaggio è determinato dal sistema di rete. If the grid is too dense, although the number of channels is increased, the step is too small, e la quantità di dati nel campo immagine è troppo grande, which must have higher requirements on the storage space of the equipment, and also affect the computing speed of computer electronic products. great influence. And some vias are invalid, such as those occupied by pads of component legs or occupied by mounting holes and fixed holes. Griglie troppo scarse e canali troppo pochi hanno un grande impatto sul tasso di distribuzione. Therefore, ci deve essere un sistema di rete con densità ragionevole per sostenere il cablaggio. The distance between the legs of standard components is 0.1 inches (2.54mm), so the basis of the grid system is generally set to 0.1 inches (2.54 mm) or less than an integral multiple of 0.1 inches, such as: 0.05 inches, 0.025 inches, 0.02 inches etc.

4. Alta frequenza Scheda PCB design skills and methods
4.1 Use a 45° angle for the corners of the transmission line to reduce return loss

4.2 High-performance insulating circuit boards whose insulation constant values are strictly controlled by level shall be adopted. This approach facilitates efficient management of electromagnetic fields between insulating materials and adjacent wiring.

4.3 It is necessary to improve the Scheda PCB design specifications for high-precision etching. Considera di specificare un errore totale di ++/- 0.0007 inches in line width, managing undercuts and cross-sections of wiring shapes, and specifying wiring sidewall plating conditions. Overall management of wiring (conductor) geometry and coating surface is important to address skin effect issues associated with microwave frequencies and to achieve these specifications.

4.4 There is a tap inductance on the protruding leads, so avoid using leaded components. For high frequency environments, use surface mount components.

4.5 For signal vias, avoid using the via processing (pth) process on sensitive boards, in quanto questo processo si tradurrà in induttanza di piombo alla via.

4.6 Fornire un piano di terra ricco. Molded vias are used to connect these ground planes to prevent the effects of 3D electromagnetic fields on the board.

4.7 To choose electroless nickel plating or immersion gold plating process, non utilizzare il metodo HASL per la galvanizzazione. This plated surface provides better skin effect for high frequency currents (Figure 2). Inoltre, this highly solderable coating requires fewer leads, helping to reduce environmental pollution.

4.8 Maschera di saldatura impedisce il flusso della pasta di saldatura. However, covering the entire board surface with solder mask material will result in large variations in the electromagnetic energy in the microstrip design due to thickness uncertainty and unknown insulating properties. Solder dam is generally used as a solder mask. il campo elettromagnetico. In this case, we manage the transition between microstrip and coax. In a coaxial cable, the ground planes are interwoven in a ring and evenly spaced. In microstrip, il piano di terra è al di sotto della linea attiva. This introduces certain edge effects that need to be understood, previsto e preso in considerazione al momento della progettazione. Of course, this mismatch also results in return loss, which must be reduced to avoid noise and signal interference.

5. Electromagnetic Compatibility Design
Electromagnetic compatibility refers to the ability of electronic equipment to work harmoniously and effectively in various electromagnetic environments. Lo scopo della progettazione di compatibilità elettromagnetica è quello di consentire alle apparecchiature elettroniche di sopprimere varie interferenze esterne, so that electronic equipment can work normally in a specific electromagnetic environment, and at the same time reduce the electromagnetic interference of electronic equipment itself to other electronic equipment.

5.1 Choose a reasonable wire width
Since the impulse interference produced by the transient current on the printed wires is mainly caused by the inductive components of the printed wires, the inductance of the printed wires should be minimized. The inductance of the printed wire is proportional to its length and inversely proportional to its width, I fili così corti e precisi sono utili per sopprimere le interferenze. Clock traces, signal lines of row drivers or bus drivers often carry large transient currents and traces should be kept as short as possible. For discrete component circuits, quando la larghezza del filo stampato è di circa 1.5mm, può soddisfare pienamente i requisiti; per circuiti integrati, the width of the printed wire can be selected between 0.2 e 1.0 mm.

5.2 Adopt the correct wiring strategy
The use of equal wiring can reduce the wire inductance, ma l'induttanza reciproca e la capacità distribuita tra i fili aumentano. If the layout allows, use the grid-shaped mesh wiring structure. The specific method is to route horizontally on one side of the printed board and vertically on the other side. The cross holes are connected by metallized holes.

5.3 In order to suppress the crosstalk between the conductors of the printed board, the long-distance and equal wiring should be avoided as far as possible when designing the wiring, and the distance between the lines should be kept as wide as possible, and the signal line, the ground line and the power line should not cross as much as possible. . Setting a grounded trace between some signal lines that are very sensitive to interference can effectively suppress crosstalk.

5.4 In order to avoid electromagnetic radiation generated when high-frequency signals pass through the printed wires, the following points should also be paid attention to when wiring the printed circuit board:
(1) Minimize the discontinuity of the printed conductors, for example, la larghezza dei conduttori non deve essere bruscamente cambiata, the corners of the conductors should be greater than 90 degrees, and ring-shaped routing is prohibited.
(2) The lead of the clock signal is prone to electromagnetic radiation interference. The wiring should be close to the ground loop, e il driver dovrebbe essere vicino al connettore.
(3) The bus driver should be close to the bus it wants to drive. For those leads that leave the printed circuit board, the driver should be right next to the connector.
(4) The wiring of the data bus should sandwich a signal ground wire between every two signal wires. The ground return is placed right next to the unimportant address leads, since the latter often carry high frequency currents.
(5) When arranging high-speed, circuiti logici a media velocità e bassa velocità sulla scheda stampata.

5.5 Suppress reflection interference
In order to suppress the reflection interference that appears at the end of the printed line, except for special needs, la lunghezza della linea stampata dovrebbe essere accorciata il più possibile e dovrebbero essere utilizzati circuiti lenti. If necessary, può essere aggiunta la corrispondenza del terminale, that is, una resistenza corrispondente con lo stesso valore di resistenza viene aggiunta alla fine della linea di trasmissione al suolo e all'estremità dell'alimentazione elettrica. According to experience, per il circuito TTL generalmente più veloce, the terminal matching measures should be adopted when the printed lines are longer than 10cm. Il valore di resistenza della resistenza corrispondente dovrebbe essere determinato in base alla corrente di azionamento in uscita e al valore della corrente di sink del circuito integrato.

6. Use the differential signal line routing strategy in the circuit board design process
Differential signal pairs that are routed very close to each other are also tightly coupled to each other. This mutual coupling reduces EMI emissions. Usually (with some exceptions) differential signals are also high-speed signals, so high-speed design rules usually apply. Ciò vale soprattutto per il cablaggio dei segnali differenziali, especially when designing signal lines for transmission lines. Ciò significa che dobbiamo progettare il routing delle linee di segnale con molta attenzione per garantire che l'impedenza caratteristica della linea di segnale sia continua e costante su tutta la linea di segnale. During the layout and routing process of the differential pair, speriamo che i due Scheda PCB lines in the differential pair are exactly the same. Ciò significa che, in practice, Occorre fare ogni sforzo per garantire che le tracce PCB nella coppia differenziale abbiano esattamente la stessa impedenza e che le tracce siano della stessa lunghezza. Differential Scheda PCB Le tracce sono solitamente sempre instradate in coppia, and the distance between them remains constant anywhere along the direction of the pair.