|
RF circuit layout principles
When designing RF layout, the following general principles must be met first:
(1) Isolate the high-power RF amplifier (HPA) and the low-noise amplifier (LNA) as much as possible. Simply put, keep the high-power RF transmitting circuit away from the low-power RF receiving circuit;
(2) Ensure that there is at least one whole ground in the high-power area of the PCB board, preferably without vias. Of course, the larger the copper foil area, the better;
(3) Circuit and power decoupling is also extremely important;
(4) RF output usually needs to be away from RF input;
(5) Sensitive analog signals should be as far away from high-speed digital signals and RF signals as possible.
Physical partitioning, electrical partitioning Design partitioning
Can be decomposed into physical partitioning and electrical partitioning. Physical partitioning mainly involves issues such as component layout, orientation and shielding; electrical partitioning can be further decomposed into partitions for power distribution, RF routing, sensitive circuits and signals, and grounding.
1. Let's discuss the issue of physical partitioning.
Component layout is the key to achieving an excellent RF design. The most effective technique is to first fix the components on the RF path and adjust their orientation to minimize the length of the RF path, keep the input away from the output, and separate the high-power circuit and the low-power circuit as far as possible.
The most effective circuit board stacking method is to arrange the main ground plane (main ground) on the second layer under the surface layer and run the RF line on the surface layer as much as possible. Minimizing the size of the vias on the RF path can not only reduce the path inductance, but also reduce the number of cold solder joints on the main ground and reduce the chance of RF energy leaking to other areas in the stacked board. In terms of physical space, linear circuits such as multi-stage amplifiers are usually sufficient to isolate multiple RF zones from each other, but duplexers, mixers, and intermediate frequency amplifiers/mixers always have multiple RF/IF signals interfering with each other, so this effect must be carefully minimized.
2. RF and IF traces should be crossed as much as possible, and a ground plane should be placed between them as much as possible.
The correct RF path is very important for the performance of the entire PCB board, which is why component layout usually takes up most of the time in mobile phone PCB board design. In the design of mobile phone PCB boards, the low noise amplifier circuit can usually be placed on one side of the PCB board, and the high power amplifier on the other side, and finally connected to the antenna on the RF end and the baseband processor end on the same side through a duplexer. Some skills are needed to ensure that the straight through hole does not transfer RF energy from one side of the board to the other side. The commonly used technique is to use blind holes on both sides. The adverse effects of the straight through hole can be minimized by arranging the straight through hole in an area on both sides of the PCB board that is not subject to RF interference.
Sometimes it is not possible to ensure sufficient isolation between multiple circuit blocks. In this 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 at an appropriate distance from the components, so it takes up valuable PCB board space. It is very important to ensure the integrity of the shield as much as possible. The digital signal lines entering the metal shield should be routed on the inner layer as much as possible, and it is best if the PCB layer below the routing 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, but as much ground as possible should be laid around the gap, and the ground on different layers can be connected together through multiple vias.
3. Proper and effective chip power decoupling is also very important.
Many RF chips with integrated linear circuits are very sensitive to power supply noise. Usually, each chip needs to use up to four capacitors and an isolation inductor to ensure that 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 power supply. The same principle also applies to decoupling the power supply at this inductor end.
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 placed in parallel because this will form an air-core transformer and induce interference signals. Therefore, the distance between them should be at least the height of one of the devices, or they should be arranged at right angles to minimize their mutual inductance.
4. The principle of electrical partitioning is generally the same as that of physical partitioning, but it also includes some other factors.
Some parts of the mobile phone use different operating voltages and control them with the help of software to extend the battery life. This means that the mobile phone needs to run multiple power supplies, which brings more problems to isolation.
The power supply is usually introduced from the connector and immediately decoupled to filter out any noise from outside the circuit board, and then distributed after passing through a set of switches or regulators. The DC current of most circuits on the mobile phone PCB is quite small, so the trace width is usually not a problem. However, a large current line as wide as possible must be run separately for the power supply of the high-power amplifier to minimize the transmission voltage drop. In order to avoid too much current loss, multiple vias are required to transfer current from one layer to another. In addition, if the power supply pin of the high-power amplifier is not adequately decoupled, high-power noise will radiate to the entire board and cause various problems.
The grounding of the high-power amplifier is quite critical and often requires a metal shielding case to be designed for it. In most cases, it is also critical to ensure that the RF output is far away from the RF input. This also applies to amplifiers, buffers and filters. In the worst case, if the outputs of amplifiers and buffers are fed back to their inputs with appropriate phase and amplitude, they may produce self-oscillation. In the best case, they will be able to operate stably under any temperature and voltage conditions. In fact
, they may become unstable and add noise and intermodulation signals to the RF signal. If the RF signal line has to be looped back from the input of the filter to the output, this may seriously damage the bandpass characteristics of the filter. In order to achieve good isolation between the input and output, first a circle of ground must be laid around the filter, and secondly a ground must be laid in the lower area of the filter and connected to the main ground around the filter. It is also a good idea to keep the signal lines that need to pass through the filter as far away from the filter pins as possible.
In addition, the grounding of various places on the entire board must be very careful, otherwise a coupling channel will be introduced. Sometimes you have the choice of running single-ended or balanced RF signal lines, and the same principles about crosstalk and EMC/EMI apply here. Balanced RF signal lines can reduce noise and crosstalk if they are routed correctly, but their impedance is usually higher, and it may be difficult to maintain a reasonable line width to obtain an impedance match between the signal source, the line, and the load. Buffers can be used to improve isolation because they can split the same signal into two parts and use them to drive different circuits. In particular, the local oscillator may need a buffer to drive multiple mixers.
When the mixer reaches the common mode isolation state at the RF frequency, it will not work properly. Buffers can isolate the impedance changes at different frequencies very well, so that the circuits will not interfere with each other. Buffers are very helpful in design. They can be placed right after the circuit that needs to be driven, so that the high-power output traces are very short. Because the input signal level of the buffer is relatively low, they are not easy to interfere with other circuits on the board. Voltage-controlled oscillators (VCOs) can convert changing voltages into changing frequencies. This feature is used for high-speed channel switching, but they also convert traces of noise on the control voltage into small frequency changes, which adds noise to the RF signal.
5. To ensure that no noise is added, 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 a wide bandwidth of noise by filtering; second, the VCO control line is usually part of a feedback loop that controls the frequency, and it may introduce noise in many places, so the VCO control line must be handled very carefully. Make sure that the ground of the lower layer of the RF trace is solid, and all components are firmly connected to the main ground and isolated from other traces that may bring noise. In addition
, make sure that the power supply of the VCO has been fully 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, the VCO is often placed at the end of the RF area, and sometimes it also requires a metal shielding cover. 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 set the VCO operating frequency and modulate voice or data onto the RF signal. All VCO design principles apply to resonant circuits. Resonant circuits are usually very sensitive to noise because they contain a relatively large number of components, have a wide distribution area on the board, and usually operate 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 that these inductors and capacitors be located very close to each other and connected back to a control loop that is very sensitive to noise. This is not easy to achieve.
Automatic gain control (AGC) amplifiers are also prone to problems. AGC amplifiers are present in both transmit and receive circuits. AGC amplifiers are usually effective in filtering out noise, but because mobile phones have the ability to handle rapid changes in transmit and receive signal strength, the AGC circuits require a fairly wide bandwidth, which makes it easy for AGC amplifiers on certain critical circuits to introduce noise. Designing AGC circuits must follow good analog circuit design techniques, which are related to very short op amp input pins and very short feedback paths, both of which must be far away from RF, IF or high-speed digital signal traces.
Similarly, good grounding is essential, and the chip power supply must be well decoupled. If you have to run a long line at the input or output, it is best to run it at the output, where the impedance is usually 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 designs. The common analog ground and the ground used to shield and separate signal lines are usually equally important, so careful planning, thoughtful component layout and thorough layout * estimation are very important in the early stages of design. RF lines should also be kept away from analog lines and some critical digital signals. All RF traces, pads and components should be filled with ground 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 lay a layer of ground connected to the main ground along the RF trace between them. If this is not possible, make sure they are cross-crossed, which can minimize capacitive coupling, and try to lay more ground around each RF trace and connect them to the main ground.
In addition, minimizing the distance between parallel RF traces can minimize inductive coupling. Isolation works best when a solid, monolithic ground plane is placed directly under the surface layer on the first layer, although other approaches can work with careful design. On each layer of the PCB, place as many ground planes as possible and connect them to the main ground plane. Place traces as close together as possible to increase the number of land planes on internal signal layers and power distribution layers, and adjust traces appropriately so that you can place ground connection vias to isolated land planes on the surface layer. Avoid creating free ground planes on each PCB layer because they can pick up or inject noise like a small antenna. In most cases, if you can't connect them to the main ground, then you're better off removing them.
|
提升卡
变色卡
千斤顶