Summary of RF PCB Layout and Louting

Radiofrequency (RF) circuit board design is often described as a kind of "black art" because there are still many theoretical uncertainties, but this view is only partially correct. RF circuit board design also has many guidelines that can be followed and should not be Ignored by law.

In the actual design of RF, the really practical skill is how to compromise these guidelines and rules when they cannot be implemented accurately due to various design constraints. Of course, there are many important RF design topics worth discussing, including impedance and impedance matching, insulating layer materials and laminates, wavelength, and standing waves, so these have a great impact on the EMC and EMI of mobile phones. The conditions that must be met when designing the RF layout are summarized:

Separate the high-power RF amplifier (HPA) from the low-noise amplifier (LNA) as much as possible.

 Simply put, it is to keep the high-power RF transmitting circuit away from the low-power RF receiving circuit. The mobile phone has many functions and many components, but the PCB space is small. At the same time, considering that the wiring design process has the highest limit, all of these have relatively high requirements for design skills. At this time, it may be necessary to design a four- to six-layer PCB and let them work alternately instead of working at the same time. High-power circuits sometimes include RF buffers and voltage-controlled oscillators (VCO). Make sure that there is at least a whole piece of ground in the high-power area of ​​the PCB, preferably without vias. Of course, the more copper, the better. Sensitive analog signals should be as far away as possible from high-speed digital signals and RF signals.

 
The design partition can be decomposed into a physical partition and electrical partition.

Physical partitioning mainly involves issues such as component layout, orientation, and shielding; electrical partitioning can continue to be decomposed into partitions for power distribution, RF routing, sensitive circuits and signals, and grounding.

To ensure that noise is not increased, the following aspects must be considered

First, the expected bandwidth of the control line may range from DC to 2MHz, and it is almost impossible to remove such wide-band noise through filtering; secondly, VCO The control line is usually part of a feedback loop that controls the frequency. It may introduce noise in many places, so the VCO control line must be handled very carefully. Make sure that the ground below the RF trace is solid, and all components are firmly connected to the main ground and isolated from other traces that may cause noise. In addition, it is necessary to ensure that the power supply of the VCO has been sufficiently decoupled. Since the RF output of the VCO is often a relatively high level, the VCO output signal can easily interfere with other circuits, so special attention must be paid to the VCO. In fact, VCO is often placed at the end of the RF area, and sometimes it needs a metal shield. The resonant circuit (one for the transmitter and the other for the receiver) is related to the VCO, but it also has its own characteristics. Simply put, the resonant circuit is a parallel resonant circuit with a capacitive diode, which helps to set the VCO operating frequency and modulate the voice or data to the RF signal. All VCO design principles also apply to resonant circuits. Because the resonant circuit contains a considerable number of components, has a wide distribution area on the board, and usually runs at a very high RF frequency, the resonant circuit is usually very sensitive to noise. The signals are usually arranged on the 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 very close and connected back On a control loop that is sensitive to noise. It is not easy to do this.

Automatic gain control (AGC) amplifier is also a problem-prone place, whether it is transmitting or receiving circuit will have an AGC amplifier. AGC amplifiers can usually effectively filter out noise, but because mobile phones have the ability to deal with the rapid changes in the strength of transmitted and received signals, the AGC circuit is required to have a fairly wide bandwidth, which makes it easy to introduce AGC amplifiers on some key circuits noise. Designing AGC circuits must comply with good analog circuit design techniques, which are related to the short op-amp input pins and short feedback paths, both of which must be far away from RF, IF, or high-speed digital signal traces. Similarly, good grounding is also essential, and the chip's power supply must be well decoupled. If it is necessary to run a long wire at the input or output end, it is best to go at the output end, usually, the impedance of the output end is much lower, and it is not easy to induce noise. Generally, the higher the signal level, the easier it is to introduce noise into other circuits.

In all PCB designs, 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. Common analog ground and ground used to shield and separate signal lines are usually equally important. Therefore, in the early stages of design, careful planning, well-thought-out component layout, and thorough layout * evaluation are all very important. The same should be used for RF Keep the line away from analog lines and some very critical digital signals. All RF traces, pads, and components should be filled with grounded copper as much as possible and connected to the main ground as much as possible. If the RF trace must pass through the signal line, try to route a layer of ground connected to the main ground along the RF trace between them. If it is not possible, make sure that they are crossed. This minimizes capacitive coupling. At the same time, place as much ground as possible around each RF trace and connect them to the main ground. In addition, minimizing the distance between parallel RF traces can minimize inductive coupling.

When designing the mobile phone PCB board, great attention should be paid to the following aspects

Treatment of power supply and ground wire

Even if the wiring in the entire PCB board is completed very well, the interference caused by the improper consideration of the power supply and the ground wire will reduce the performance of the product, and sometimes even affect the success rate of the product. Therefore, the wiring of the electric and ground wires must be taken seriously, and the noise interference generated by the electric and ground wires should be minimized to ensure the quality of the product. Every engineer engaged in the design of electronic products understands the cause of the noise between the ground wire and the power wire, and now only the reduced noise suppression is described:

• (1) It is well-known to add decoupling capacitors between the power supply and ground.
• (2) Widen the width of the power and ground wires as much as possible, preferably the ground wire is wider than the power wire, their relationship is: ground wire>power wire>signal wire, usually the signal wire width is: 0.2～0.3mm, the most The thin width can reach 0.05～0.07mm, and the power cord is 1.2～2.5mm. For the PCB of the digital circuit, a wide ground wire can be used to form a loop, that is, to form a ground net to use (the ground of the analog circuit cannot be used in this way)
• (3) Use a large area of ​​the copper layer as ground wire, and connect the unused places on the printed circuit board as the ground wire. Or it can be made into a multilayer board, and the power supply and ground wires occupy one layer each.

Common ground processing of digital circuit and analog circuit

Many PCBs are no longer single-function circuits (digital or analog circuits) but are composed of a mixture of digital and analog circuits. Therefore, it is necessary to consider the mutual interference between them when wiring, 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 as far away as possible from the sensitive analog circuit device. For the ground line, the whole PCB has only one node to the outside world, so The problem of digital and analog common ground must be dealt with inside the PCB, and the digital ground and analog ground inside the board are actually separated and they are not connected to each other, but at the interface (such as plugs, etc.) connecting the PCB to the outside world. There is a short connection between the digital ground and the analog ground. Please note that there is only one connection point. There are also non-common grounds on the PCB, which is determined by the system design.
 
The signal line is laid on the electric (ground) layer

In the multi-layer printed board wiring, because there are not many wires left in the signal line layer that have not been laid out, adding more layers will cause waste and increase the production workload, and the cost will increase accordingly. To solve this contradiction, you can consider wiring on the electrical (ground) layer. The power layer should be considered first, and the ground layer second. Because it is best to preserve the integrity of the formation.

Treatment of connecting legs in large area conductors

In large-area grounding (electricity), the legs of common components are connected to it. The treatment of the connecting legs needs to be considered comprehensively. In terms of electrical performance, it is better to connect the pads of the component legs to the copper surface. There are some undesirable hidden dangers in the welding assembly of components, such as ① Welding requires a high-power heater. ②It is easy to cause virtual solder joints. Therefore, both electrical performance and process requirements are made into cross-patterned pads, called heat shields, commonly known as thermal pads (Thermal), so that the possibility of virtual solder joints due to excessive cross-section heat during soldering can be generated decrease very much. The processing of the power (ground) leg of the multilayer board is the same.

The role of the network system in cabling

In many CAD systems, wiring is determined by the network system. The grid is too dense and the path has increased, but the step is too small, and the amount of data in the field is too large. This will inevitably have higher requirements for the storage space of the device, and also the computing speed of the computer-based electronic products. Great influence. Some paths are invalid, such as those occupied by the pads of the component legs or by mounting holes and fixed holes. Too sparse grids and too few channels have a great impact on the distribution rate. Therefore, there must be a well-spaced and reasonable grid system to support the wiring. The distance between the legs of standard components is 0.1 inches (2.54mm), so the basis of the grid system is generally set at 0.1 inches (2.54mm) or less than an integral multiple of 0.1 inches, such as 0.05 inches, 0.025 inches, 0.02 Inches, etc.

More experience summary knowledge about RF PCB layout and louting, please read the next article: How do You Make High-Frequency PCB of RF?