"In actual design, truly practical skills are how to compromise them when these criteria and rules cannot accurately implement them accurately due to various design constraints.
Of course, there are many important RF design issues to be discussed, including impedance and impedance matching, insulating materials and laminated plates and wavelengths and standings, careful planning under the premise of all kinds of design principles is a guarantee for a one-time successful design .
First, the common problem of RF circuit design
1. Interference between Digital Circuit Modules and Analog Circuit Modules
If the analog circuit (RF) and the digital circuit work separately, it may work well. However, once the two are placed on the same piece of circuit board, work with the same power supply, and the entire system is likely to be unstable.
This is mainly because digital signals are frequently swinging between and positive power (> 3 V), and the cycle is particularly short, often nanosecond. Due to a large amplitude and shorter switching time. The digital signals contain high frequency components that are large and independent of the switching frequency. In the analog portion, the signal transmitted from the wireless tuning circuit to the wireless device receiving portion is generally less than lμV.
Therefore, the difference between the digital signal and the radio frequency signal will reach 120 dB. Obviously. If the digital signal cannot be greatly separated from the RF signal. The weak radio frequency signal may be destroyed, so that wireless devices will deteriorate, and even don't work at all.
2, noise interference from power supply
RF circuits are quite sensitive to power supply noise, especially for glitch voltages and other high-frequency harmonics. The microcontroller suddenly sucks most of the current in a short time in each internal clock cycle, which is because modern microcontrollers are manufactured in CMOS process.
Therefore, it is assumed that a microcontroller operates in an internal clock frequency of LMHz, which will extract current from the power source with this frequency.
If a suitable power supply is not taken, it will cause voltage burrs on the power line. If these voltage burrs arrive at the power pins of the circuit RF part, it may cause the work to fail when it is severe.
3, unreasonable ground
If the groundwork of the RF circuit is improper, some strange phenomena may occur. For digital circuit design, most digital circuit functions are good even if there is no ground layer. In the RF band, even if a short ground line is like in the inductor.
Roughly calculated, the inductance amount of 10 Toni PCB lines is about 27 Ω at a temperature of about L NH, 433 MHz. If the ground layer is not used, most of the grounds will be longer, and the circuit will not be designed.
4, the antenna of radiation interference to other analog circuit parts
In the PCB circuit design, there are usually other analog circuits on the board.
For example, there are many circuits, a digital conversion (ADC) or several / analog converter (DAC). The high-frequency signal emitted by the radio transmitter antenna may arrive at the ADC simulation 淙 淙 蛭 缏 缏 范 范 缣 缣 煜 谎 谎 谎If the processing of the ADC input is unreasonable, the RF signal may be self-excited in the ESD diode input by the ADC. Thereby, the ADC deviation is caused.
Second, five major experience summary
1, radio frequency circuit layout principle
When designing the RF layout, you must give priority to the following general principles:
(1) Separate high-power RF amplifier (HPA) and low noise amplifier (LNA) as much as possible, simply, is to keep high power RF transmit circuits away from low power RF receiving circuits;
(2) Make sure there is at least one of the high-power regions on the PCB board, preferably no overall, of course, the larger, the better the copper foil;
(3) The circuit and power supply are also extremely important;
(4) RF output usually needs to be kept away from RF;
(5) Sensitive analog signals should be as far as possible away from high-speed digital signals and RF letters;
2, physical partition, electrical partition design partition
It can be broken down into physical partitions and electrical partitions. Physical partitions mainly involve problems such as component layout, orientation, and shields; electrical partitions can continue to decompose into power distribution, RF traces, sensitive circuits, and signals, and grounding.
1) We discuss physical partition issues:
Component layout is a key to achieving an excellent RF design. The most effective technology is to first secure components located on the RF path, and adjust its orientation to minimize the length of the RF path, so as far as possible High power circuits and low power circuits are separated.
The most effective circuit board stacking method is to arrange the main ground (primary) in the second layer under the surface and take the RF line on the surface. Reduce the vias size size on the RF path to minimize the path inductance, but also reduce the solder spots on the main ground, and reduce RF energy leaks to other regions within the laminate.
On physical space, linear circuits such as multi-stage amplifiers are typically sufficient to isolate multiple RF regions, but duplexers, mixers, and intermediate frequency amplifiers / mixers always have multiple RF / IF. The signals are interfere with each other, so this effect should be carefully minimized.
2) RF and IF lines should be cross-crossing as much as possible, and they are as close as possible to them:
The correct RF path is very important for the performance of the entire PCB board, which is why component layout usually accounts for most of the time in mobile phone PCB board design.
On the mobile phone PCB board design, the low noise amplifier circuit can usually be placed on a side of the PCB board, while the high power amplifier is placed on the other side, and eventually connects them to the RF end and baseband processing on the same surface through the duplexer. The antenna of the device.
There are some techniques to ensure that the direct passing hole will not pass the RF energy from the plate to the other, and the commonly used technology is to use blind holes on both sides.
The adverse effects of direct passing holes can be minimized by arranging straight through holes on both sides of the PCB board.
Sometimes it is less likely to ensure sufficient isolation between multiple circuit blocks, in which case the metal shield must be considered in the RF area, the metal shield must be welded to the ground, must be held with the components A proper distance, therefore needs to take up the valuable PCB board space.
To ensure that the full and very important shield, the digital signal line entering the metal shield should take the inner layer as much as possible, and the lower layer of the lower layer of the line layer is the formation.
The RF signal line can go out from the wiring layer at the bottom of the metal shield and the wiring layer at the bottom of the ground, but the gap should be as much as possible around the gap, and the ground can be connected together on different layers. .
3) Appropriate and effective chip power decoupling is also very important:
Many RF chips integrated with linear lines are very sensitive to the noise of the power supply. Usually, each chip requires up to four capacitors and an isolation inductance to ensure filtering all power supply noise.
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, and the same principle is also suitable for decoupling the power supply of this inductive end.
Some chips require multiple power to work, so you may need two to three sets of capacitances and inductors to decouple them, and the inductance is very small, because this will form an air core transformer and interference Signals, the distance between them is at least equivalent to the height of one of the devices, or is arranged in a right angle to minimize it.
4) The principle of electrical partition is substantially the same as the physical partition, but it also contains other factors:
Some parts of your phone use different operating voltages and control it by software to extend battery life. This means that mobile phones need to run multiple power supplies, and this has brought more problems with isolation.
The power source is typically introduced from the connector and immediately performs decoupling to filter out any noise from the external line plate, and then distribute it after a set of switches or regulators.
The direct current of most circuits on the mobile phone PCB board is quite small, so the width of the trace is not a problem, however, a high-power amplifier must take a high power supply line to minimize transmission pressure to the lowest .
In order to avoid too much current loss, multiple vias are needed to pass current from a layer to another. In addition, if it is not possible to adequate decoupling on the high power amplifier's power leader, high power noise will radiate to the entire board and bring a variety of problems.
The high-power amplifier is grounded is quite key and often needs to design a metal shield. In most cases, the same critical is to ensure that the RF output is remote from the RF input. This also applies to amplifiers, buffers, and filters.
In worst cases, if the output of the amplifier and the buffer feeds back to their input at an appropriate phase and amplitude, then they may generate self-excitation oscillations. In the best case, they will be able to operate stably at any temperature and voltage conditions.
In fact, they may become unstable and add noise and intermodulated signals to the RF signal. If the RF signal line has to wrap the output end from the input of the filter, this may seriously damage the band passage of the filter.
In order to obtain good isolation in the input and output, it is first necessary to arrange a circle around the filter, and the next layer region of the second filter is also placed and connected to the main land around the filter. It is also a good way to keep the signal lines that need to pass through the filter.
In addition, the ground on the entire board is very careful, otherwise a coupling channel will be introduced. Sometimes you can choose to take a single-ended or balance RF signal line, and the principles of cross-interference and EMC / EMI are equally applicable.
Balancing RF signal lines If the wire is correct, you can reduce noise and cross-interference, but their impedance is usually relatively high, and to maintain a reasonable line width to obtain a matching signal source, trace and load impedance, actual wiring There will be some difficulties.
The buffer can be used to improve the isolation effect because it can divide the same signal into two parts and is used to drive different circuits, especially the local oscillator may require a buffer to drive multiple mixers.
When the mixer reaches the common mode isolation state at the RF frequency, it will not work. The buffer can isolate the impedance change at different frequencies, so that the circuit does not interfere with each other.
The buffer is very helpful to the design, which can keep back in the back of the drive circuit, so that the high power output trace is very short, because the input signal level of the buffer is relatively low, so they are not easy to panel other Circuit causes interference.
The voltage controlled oscillator (VCO) can convert the changed voltage into a changed frequency, which is used for high-speed channel switching, but they also convert trace noise on the control voltage into a small frequency change, and this gives it The RF signal adds noise.
5) To ensure that no noise must be considered from the following aspects:
First, the desired bandwidth of the control line may be from DC until 2 MHz, and the noise of the wide frequency band is almost impossible by filtering, secondly, the VCO control line is usually part of a feedback loop of a control frequency, which is there. There may be noise in places.
Therefore, you must have a very careful handle of the VCO control cable. To ensure that the lower layer of the RF traverse is solid, and all components are firmly connected to the main place, and they are separated from other traces that may cause noise.
In addition, to ensure that the power supply of VCO has been sufficiently decoupled. Since the RF output of VCO is often a relatively high level, the VCO output signal is easily interfered with other circuits, so it must be special attention to the VCO.
In fact, VCO is often placed at the end of the RF area, sometimes it also requires a metal shield. Resonance circuit (a transmitter, another for the receiver) is related to VCO, but also has its own characteristics.
Briefly, the resonant circuit is a parallel resonant circuit with capacitive diode, which helps to set the VCO operating frequency and modulate voice or data to the RF signal. All VCO design principles are equally applicable to resonant circuits. Since the resonant circuit contains a quite amount of components, the distribution area on the board is wide and the resonant circuit is usually very sensitive to noise at a very high RF frequency.
Signals are typically arranged on the adjacent feet of the chip, but these signal pins need to work with relatively large inductances and capacitors, which in turn requires the position of these inductance and capacitance to be close, and A control loop that is sensitive to noise. It is not easy to do this.
Automatic Gain Control (AGC) amplifier is also a place that is prone to problems, whether it is an emission or receiving circuit, there is an AGC amplifier. The AGC amplifier can usually filter out noise, but because the mobile phone has the ability to process the rapid changes in the intensity of the transmitting and reception signal.
Therefore, the AGC circuit is required to have a relatively wide bandwidth, which makes it easy to introduce noise on some critical circuits. Designing the AGC line must comply with good analog circuit design techniques, which is related to a short op amp input pin and a short feedback path, both of which must be away from RF, IF or high speed digital signal traces.
Similarly, good grounding is also indispensable, and the power supply of the chip must be well decoupled. If you have to walk a long line on the input or output, it is preferable that the impedance of the output is much lower in the output, and it is not easy to induce noise.
The higher the signal level, the easier the noise is easily introduced into other circuits. In all PCB designs, the digital circuit is kept away from the analog circuit as much as possible, which also applies to RF PCB design.
The public simulation is usually equally important to shield and spaced apart signal lines. Therefore, in the early stage, careful plans, considering the surrounding component layout and thorough layout * are very important, and should also make RF The line away from the analog lines and some very critical digital signals, all RF traces, pads and components should be filled with as much as possible as much as possible, and are connected to the main land.
If the RF trace must pass through the signal line, then as far as possible, it is connected to the main ground along the RF trace. If it is impossible, be sure to ensure that they are cross-crossing, which can minimize capacitive coupling, while as much as possible around each RF trace, and connect them to the main ground.
In addition, the distance between the parallel RF trace is minimized to minimize the sensitive coupling. When a solid whole ground floor is placed directly under the surface layer, the isolation effect is best, although careful when designing other practices is also used.
At each layer of the PCB board, we should be as much as possible and connect them to the main ground. Use the traces to increase the number of blocks of the internal signal layer and the power distribution layer and appropriately adjust the trace so that you can connect the via the acoustic block on the surface layer.
Free ground should be avoided on the PCB layers because they pick or inject noise like a small antenna. In most cases, if you can't connect them to the mainland, then you should bestThey remove.
3. When designing the mobile phone PCB board, pay attention to several aspects
1) Power supply, groundline processing:
Both the entire PCB board is completed well, but due to the disturbance caused by the power supply, the ground wire considers, it will reduce the performance of the product, and sometimes even affect the success rate of the product.
Therefore, the wiring of electricity and ground wire should be taken seriously, the noise interference generated by the electricity and ground is minimized to ensure the quality of the product.
For each engineering personnel engaged in electronic product, it is clear that the noise between the ground and the power cord is now only suppressed to suppress noise to the expression:
(1), well known is to add a decoupling capacitor between the power supply and the ground.
(2), try to broaden the power supply, the ground width, preferably the ground wire is wide than the power line, their relationship is: ground line> power cord> signal line, usually the signal line width is: 0.2 ~ 0.3mm, most The fine width can be 0.05 ~ 0.07mm, and the power cord is 1.2 to 2.5 mm. The PCB of the digital circuit can be used to form a loop with a wide ground wire, which constitutes a ground network to use (the way the analog circuit cannot be used)
(3), use a large-scale copper layer to be used, and the place that is not used on the printed board is connected to the ground as a ground. Or make a multi-layer board, power supply, and ground lines each.
2) Treatment of digital circuits and analog circuits
There are now many PCBs no longer a single functional circuit (digital or analog circuit), but is mixed by the digital circuit and analog circuit.
Therefore, in the wiring, it is necessary to consider mutual interference problems between them, in particular, noise interference on the ground. 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 is as far from the sensitive analog circuitry, and the entire PCB is only one node.
Therefore, it is necessary to perform the number of processing in the PCB, the problem of the template, and in the interior of the board, and the simulation is actually unconnected between them, just at the interface of the PCB and the external connection (such as plug, etc.) .
Digitally is a little short, please note that there is only one connection point. There is also a discharge of the PCB, which is determined by the system design.
3) Signal line cloth in the electricity (floor) layer
When the multi-layer printed board wiring, since there is not much in the line of the signal line layer, there is still no more, and the number of additions will cause waste, it will increase the amount of work, and the cost increases accordingly.
To solve this contradiction, you can consider wiring on the electricity (floor) layer. First, the power layer should be considered, followed by the formation. Because it is best to retain the integrity of the formation.
4) Treatment of connecting legs in large-area conductors
In large-area grounding (electricity), the legs of the common components are connected to them, and the treatment of the connection leg needs to be synthesized. In terms of electrical performance, the welding of the element legs is fully connected to the copper surface, but There are some adverse hidden dangers such as welding assembly such as: 1 welding requires high-power heater; 2 easily cause a dot soldering point.
Therefore, it is taking into account the electrical performance and process, it is used as a crush pad, which is called thermal isolation (Heat Shield), which is the possibility of producing a solder point due to excessive heat dissipation due to excessive heat during welding. Sex a big decrease. The processing of the multi-layer plate is the same.
5) The role of network system in wiring
In many CAD systems, the wiring is determined by the network system. The grid is over secret, although the path is increased, but the stepper is too small, the data amount is too large, which must have a higher requirements for the storage space of the equipment, and the computers of computer electronic products have the computing speed. Extremely affected.
Some pathways are invalid, such as occupying or mounted holes by the pads of the elements, etc., which are occupied by the apertures. The grid is too low, the pathway is too much impact on the radical rate. So there is a clear and reasonable mesh system to support the wiring.
The distance between the two legs between the standard components is 0.1 inches (2.54mm), so the foundation of the grid system is generally set to 0.1 inches (2.54 mm) or less than 0.1 inches, such as: 0.05 inches, 0.025 inches, 0.02 Inches.
4, high frequency PCB design techniques and methods
(1) The corner of the transmission line should be 45 ° angle to reduce the loss.
(2) To adopt insulation constant values, high-performance insulated circuit boards that are strictly controlled by hierarchy. This method is conducive to effective management of the electromagnetic field between the insulating material and the adjacent wiring.
(3) Improve the PCB design specification for high precision etching. It is to be considered that the specified line width total error is +/- 0.0007 inches, and the decut and cross section are managed and the wiring side wall plating conditions are specified. Total the wiring (wire) geometry and the surface of the coating, which is important to solve the problem of skin effects associated with microwave frequencies and to implement these specifications.
(4) The outstanding lead existence is a tap inductor, avoiding components with leads. In high frequency environments, it is best to use surface mount assemblies.
(5) For signal via, avoid using a hypertension (PTH) process on the sensitive board because the process causes a lead inductor at the via.
(6) To provide a rich grounding layer. To connect these ground layers to the effect of 3 dimensional electromagnetic fields from 3D electromagnetic fields.
(7) To select a non-electrolytic nickel plating or immersion process, do not use the HASL method for electroplating.
(8) The solder resist layer prevents the flow of solder paste. However, due to the unknown of thickness uncertainty and insulation performance, the surface of the entire plate covering the solder resist will result in a large change in electromagnetic energy in the microstrip design. Generally, the solder Dam is used to make the electromagnetic field of the solder resist layer.
In this case, we manage the conversion between the microber to the coaxial cable. In the coaxial cable, the ground layer is annular, and the interval is uniform. In the microstrip, the ground plane is under the source line.
This introduces certain edge effects, need to be understood, predict and consider when designing. Of course, this mismatch can also cause loss, and must minimize this mismatch to avoid noise and signal interference.
5, electromagnetic compatibility design
Electromagnetic compatibility means that the electronic device can still coordinate and effectively work in various electromagnetic environments.
The purpose of electromagnetic compatibility design is to enable electronic devices to inhibit various external interferences, enabling electronic devices to work in a particular electromagnetic environment while reducing electromagnetic interference of other electronic devices.
1) Select a reasonable wire width:
Since the impact interference generated by transient currents on printed lines is mainly caused by inductive components of the printed wire, the inductance of the printed wire should be minimized. The inductance of the printed wire is proportional to its length, which is inversely proportional to its width, and there is a shorter wire to suppress interference.
The signal line of the clock lead, the row driver or the bus driver often carries a large transient current, and the printed wire should be as short as possible. For discrete components circuitry, the printed wire width is about 1.5mm, which is fully satisfied; for the integrated circuit, the printed wire width can be selected between 0.2 to 1.0 mm.
2) Adopt the correct wiring strategy:
Wire inductance can be reduced by equality traces, but the mutual inductance and distribution capacitance between the wires increases. If the layout allows, it is best to use the well-shaped mesh wiring structure, and the specific method is a lateral wiring of the printed board, and the other side is longitudinally wiring, Then, in the intersection, a metallized hole is connected.
3) In order to suppress crosstalk between printed board wires, the equal line of long distance should be avoided when designing the wiring:
Take the distance between the line and the line as much as possible, the signal line and the ground wire and the power cord do not cross. At some of some, a grounding line is disposed between the interference very sensitive signal lines, which can effectively suppress crosstalk.
4) In order to avoid the electromagnetic radiation generated by the high-frequency signal by the printed wire, the following points should be noted when the circuit board wiring is printed:
(1) minimize the discontinuities of the printed wire, such as the wire width is not mutated, and the corner of the wire should be greater than 90 degrees to prohibit the annular trace.
(2) The clock signal lead is most likely to generate electromagnetic radiation interference. When the trace should be close to the ground circuit, the drive should be next to the connector.
(3) The bus driver should be close to the bus to be driven. For leads that leave the printed circuit board, the driver should be tightly smashed.
(4) The wiring of the data bus should be folded between each two signal lines. It is best to take a loop in the least important address leads, because the latter often carries high frequency current.
(5) When the printed board is arranged at high speed, medium speed, and low speed logic circuits, the devices should be arranged in the manner of Figure 1.
5) Inhibition of reflex interference
In order to suppress the reflection interference from the printed line terminal, except for special needs, the length of the printed line should be shortened and the slow circuit is used. When necessary, the final match can be added, that is, the matching resistance of the same resistance value is added to the ground and the power supply end on the end of the transmission line.
Depending on the experience, terminal matching measures should be employed when the printed line is longer than 10 cm or more. The resistance of the matching resistor should be determined based on the output drive current and the maximum value of the absorption current.
6) Differential signal line wiring strategy during board design
The differential signals that are very close to each other will be closely coupled to each other, and the coupling between this mutual decreases EMI emission, and is usually (of course, there are some exceptions) The differential signal is also a high-speed signal, so high-speed design rules are usually applicable. This is especially true when the wiring of the differential signal, in particular the signal line for designing the transmission line.
This means that we must design the wiring of the signal line very carefully to ensure that the characteristic impedance of the signal line is continuous and maintained along the signal line.
During the layout of the differential line pair, we want the two PCB lines in the differential line to be completely consistent.
This means that in practical applications, we should do our best to ensure that the PCB line in the differential line pair has exactly the same impedance and the length of the wiring is also identical. The differential PCB line is usually always pair of wiring, and the direction of the distance between them remains in any position as a constant in any position. Normally, the layout wiring of differential line pairs is always as close as possible.
Source: EDA360 "
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