RF circuit layout 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 layout design also has many guidelines that can be followed and rules that should not be ignored.
In the actual RF circuit layout design, the really practical skill is how to trade off these guidelines and rules when they cannot be implemented accurately due to various design constraints.
There are many important RF design topics worth discussing, including impedance and impedance matching, insulating layer materials and laminates, wavelength and standing waves, etc. Careful planning under the premise of a comprehensive grasp of various design principles is a guarantee for a successful one-time design.
First. Common problems in RF circuit layout design
Interference between digital circuit modules and analog circuit modules
If the analog circuit (radio frequency) and the digital circuit work separately, they may each work well. However, once the two are placed on the same circuit board and work together with the same power supply, the entire system is likely to be unstable.
This is mainly because the digital signal frequently swings between the ground and the positive power supply (>3 V), and the period is extremely short, often in the order of nanoseconds. The larger amplitude and shorter switching time make these digital signals contain a large number of high-frequency components that are independent of the switching frequency.
In the analog part, the signal transmitted from the wireless tuning loop to the receiving part of the wireless device is generally less than 1μV. Therefore, the difference between the digital signal and the RF signal can reach 120 dB.
If you can’t make the digital signal and the radio frequency signal well separated. Weak radio frequency signals may be destroyed, and as a result, the performance of the wireless device will deteriorate, or even not work at all.
Noise interference from the power supply
RF circuits are quite sensitive to power noise, especially to glitch voltage and other high-frequency harmonics. Microcontrollers will suddenly draw most of the current in a short period of time within each internal clock cycle. This is due to the fact that modern microcontrollers are manufactured using CMOS technology.
Assuming a microcontroller is running at an internal clock frequency of 1MHz, it will draw current from the power supply at this frequency. If proper power decoupling is not taken, it will inevitably cause voltage glitches on the power line. If these voltage glitches reach the power supply pins of the RF part of the circuit, they may cause work failure in severe cases.
Unreasonable RF circuit layout ground wire
If the ground wire of the RF circuit layout is not handled properly, some strange phenomena may occur. For digital circuit design, most RF circuit layout functions perform well even if there is no ground plane.
In the RF frequency band, even a short ground wire acts as an inductor. In the rough calculation, the inductance per mm length is about 1 NH, and the inductance of a 10 mm PCB circuit at 433 MHz is about 27Ω. If the ground wire layer is not used, most ground wires will be longer and the circuit will not have the characteristics of the design.
The antenna's radiation interference to other analog circuit parts
In PCB circuit design, there are usually other analog RF circuits on the board. For example, there is AD/DA on many circuits. The high-frequency signal from the antenna of the radio frequency transmitter may reach the analog input of the ADC. Because any circuit line may send or receive RF signals like an antenna.
If the processing of the ADC input is unreasonable, the RF signal may self-excite in the ESD diode of the ADC input, causing ADC deviation.
Second. The principle of RF circuit layout
When designing the RF circuit layout, the following general principles must be met first:
(1) Separate the high-power RF amplifier (HPA) and the low-noise amplifier (LNA) as much as possible. Simply put, keep the high-power RF transmitter circuit away from the low-power RF receiver circuit;
(2) Ensure that there is at least a whole piece of 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 supply decoupling is also extremely important;
(4) The RF output usually needs to be far away from the RF input;
(5) Sensitive analog signals should be as far away from high-speed digital signals and RF signals as possible;
Third. Physical partition, electrical partition design partition in RF circuit layout
Decompose physical and electrical partitions
Physical partitioning mainly involves issues such as component layout, orientation, and shielding;
Electrical partitions can continue to be decomposed into partitions for power distribution, RF wiring, sensitive circuits and signals, and grounding.
Physical partition problem in RF circuit layout
The component layout is the key to achieving a good RF circuit layout design. The most effective technique is to fix the components on the RF path and adjust their orientation to minimize the length of the RF path and keep the RF input away from the RF output. Keep away from high-power circuits and low-noise circuits as much as possible.
The most effective circuit board stacking method is to arrange the main ground plane on the second layer below the surface layer and route the RF lines 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 virtual solder joints on the main ground and reduce the chance of RF energy leaking to other areas in the laminate.
In physical space, linear circuits like 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/IFs. The signals interfere with each other, care must be taken to minimize this effect.
The RF and IF traces in RF circuit layout
The RF and IF traces in the RF circuit layout should be crossed as much as possible, and the space between them should be a piece of ground as far as possible
The correct RF path is very important to the performance of the entire PCB board, which is why the component layout usually accounts for most of the time in the mobile phone PCB board design.
In mobile phone PCB board design, usually, the low-noise amplifier RF circuit layout can be placed on one side of the PCB board, and the high-power amplifier is placed on the other side, and finally, they are connected to one end of the RF antenna on the same side by a duplexer. And the other end of the baseband processor.
This requires some tricks to ensure that RF energy does not pass through the vias from one side of the board to the other. A common technique is to use blind holes on both sides. The adverse effects of vias can be minimized by arranging blind vias in areas where both sides of the PCB board are not subject to RF interference.
Proper and effective chip power decoupling in RF circuit layout
Proper and effective chip power decoupling is also very important.
Many RF chips with integrated linear circuits are very sensitive to power noise. Usually, each chip needs to use up to four capacitors and an isolation inductor to ensure that all power noise is filtered out.
An integrated circuit or amplifier often has an open-drain output, and a pull-up inductor is required to provide a high-impedance RF load and a low-impedance DC power supply. The same principle applies to decoupling the power supply at this inductor side.
Some chips require multiple power supplies to work, you may need two or three sets of capacitors and inductors to decouple them separately. If there is not enough space around the chip, the decoupling effect may not be good.
In particular, it is necessary to pay special attention to:
Inductances are rarely close together in parallel because this will form an air-core transformer and induce interference signals with each other, so the distance between them must be at least the height of one of the devices, or arranged at right angles to minimize their mutual inductance.
The principle of electrical zoning in RF circuit layout
The principle of electrical zoning in RF circuit layout design is roughly the same as that of physical zoning, but it also contains some other factors
Some parts of the mobile phone use different working voltages and are controlled by software to extend the battery life. This means that mobile phones need to run multiple power sources, and this brings more problems to isolation.
The power is usually introduced from the connector and is immediately decoupled to filter out any noise from the outside of 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 board is quite small, so the trace width is usually not a problem. However, a large current line as wide as possible must be routed 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 needed to transfer current from one layer to another.
In addition, if it cannot be sufficiently decoupled at the power supply pin of the high-power amplifier, high-power noise will radiate to the entire board and cause various problems.
The grounding of high-power amplifiers is critical, and it is often necessary to design a metal shield 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 output of the amplifier and buffer is fed back to their input with appropriate phase and amplitude, then they may have self-oscillation.
In the best case, they will be able to work 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 from the input end of the filter back to the output end, this may seriously damage the bandpass characteristics of the filter.
In order to get good isolation between the input and the output, a ground must be laid around the filter first, and ground must be laid in the lower layer area of the filter and connected to the main ground surrounding the filter.
It is also a good way to keep the signal lines that need to pass through the filter as far away as possible from the filter pins.
Metal shield in RF circuit layout design
Sometimes, it is impossible to keep enough separation between multiple circuit blocks. In this case, it is necessary to consider the use of a metal shield to shield the RF energy in the RF area, but metal shields also have side effects, such as manufacturing cost and assembly costs are both high.
It is difficult to ensure high precision when manufacturing metal shields with irregular shapes. Rectangular or square metal shields restrict the RF circuit layout of components; metal shields are not conducive to component replacement and fault displacement; metal shields must be Solder on the ground plane and must maintain a proper distance from the components, so it takes up valuable PCB board space.
It is very important to ensure the integrity of the metal shield as much as possible. Therefore, the digital signal line entering the metal shield should go as far as possible to the inner layer, and it is best to set the next layer of the signal circuit layer as the ground layer.
The RF signal line can be routed out from the small gap at the bottom of the metal shield and the wiring layer at the ground gap, but the gap should be surrounded by a large ground area as much as possible. The ground on different signal layers can use multiple vias to link up.
Despite the above shortcomings, metal shields are still very effective and are often the only solution to isolate critical circuits.
No noise increases in RF circuit layout design
To ensure that noise is not increased in RF circuit layout design, the following aspects must be considered.
Firstly, the expected bandwidth of the control line may range from DC to 2MHz, and it is almost impossible to remove such a wide band of noise through filtering;
Secondly, the VCO 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.
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.
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 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 a transmitting or receiving circuit that 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 the 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 for shielding and separating signal lines are usually equally important. Therefore, in the early stages of design, careful planning, well-considered component layout, and thorough layout estimation are all very important. RF Keeps 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.
Fourth. 5 aspects should be paid attention to when designing the PCB board RF circuit layout
Treatment of power supply and ground wire in RF circuit layout design
Every engineer engaged in the RF circuit layout 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 in the RF circuit layout.
(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 the ground wire > power wire > signal wire, usually the signal wire width is 0.2～0.3mm, the thinnest width can reach 0.05～0.07mm, and the power cord is 1.2～2.5 mm.
For the PCB of the digital RF circuit layout, 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 a 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 RF circuit layout and analog RF circuit layout
Nowadays, many PCBs are no longer single-function circuits (digital or analog circuits) but are composed of a mixture of digital and analog circuits.
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 RF circuit layout is high, and the sensitivity of the analog RF circuit layout 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 entire PCB has only one node to the outside world, so it must be a problem of digital and analog common ground is handled inside the PCB.
While the digital ground and analog ground are actually separated inside the board and they are not connected to each other, but only at the interface (such as a plug, 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 RF circuit layout
In the multi-layer printed board wiring, 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 need to consider wiring on the electrical (ground) layer.
The power layer should be considered at the first and the ground layer at the second. 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 commonly used components are connected to it, and 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 hidden dangers in the welding and assembly of components.
Taking into account the electrical performance and process needs, the cross-pattern pad is made, called a heat shield, commonly known as a thermal pad (Thermal), the possibility of virtual solder joints due to excessive cross-section heat during soldering can be generated decrease very much. The processing of the electrical connection (ground) leg of the multilayer board is the same.
The role of the network system in cabling in RF circuit layout design
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 equipment, and also the computing speed of the computer-type electronic products has a 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.
There must be a dense 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 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.
Fifth. High-frequency RF circuit layout PCB design skills and methods
- The corner of the transmission line should be 45° to reduce the return loss;
- Use high-performance insulated circuit boards whose insulation constant values are strictly controlled by level. This method is conducive to the effective management of the electromagnetic field between the insulating material and the adjacent wiring.
- To improve the RF circuit layout PCB design specifications related to high-precision etching. It is necessary to consider that the total error of the specified line width is +/-0.0007 inches, the undercut and cross-section of the wiring shape should be managed, and the plating conditions of the wiring sidewall should be specified.
The overall management of wiring geometry and coating surface is very important to solve the skin effect problem related to microwave frequency and realize these specifications.
- The protruding leads have tap inductance, so avoid using components with leads. In high-frequency environments, it is best to use surface mount components.
- For signal vias, avoid using a visa processing (PTH) process on sensitive boards because this process will cause lead inductance at the vias.
- Provide abundant ground planes. Use molded holes to connect these ground planes to prevent the 3D electromagnetic field from affecting the circuit board.
- To choose electroless nickel plating or immersion gold plating process, do not use the HASL method for electroplating.
- The solder mask can prevent the flow of solder paste. However, due to the uncertainty of the thickness and the unknown of the insulation performance, the entire surface of the board is covered with solder mask material, which will cause a large change in the electromagnetic energy in the microstrip design.
Generally, a solder dam is used as the electromagnetic field of the solder mask.
In this case, we manage the conversion from microstrip to coaxial cable. In the coaxial cable, the ground layer is interwoven ring-shaped, and evenly spaced. In a microstrip, the ground plane is below the active line.
This introduces certain edge effects, which need to be understood, predicted, and considered during RF circuit layout design. Of course, this mismatch will also cause return loss, and this mismatch must be minimized to avoid noise and signal interference.
Sixth. EMC design in RF circuit layout design
Electromagnetic compatibility refers to the ability of electronic equipment to work in a coordinated and effective manner in various electromagnetic environments.
The purpose of electromagnetic compatibility design in RF circuit layout design is to enable electronic equipment to suppress all kinds of external interference so that the electronic equipment can work normally in a specific electromagnetic environment, and at the same time to reduce the electromagnetic interference of the electronic equipment itself to other electronic equipment.
Choose a reasonable wire width in RF circuit layout design
Since the impact interference generated by the transient current on the printed lines is mainly caused by the inductance 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, so short and precise wires are beneficial to suppress interference.
The signal lines of clock lead, row drivers, or bus drivers often carry large transient currents, and the printed wires should be as short as possible.
For discrete component circuits, when the printed wire width is about 1.5mm, it can fully meet the requirements; for integrated circuits, the printed wire width can be selected between 0.2mm and 1.0mm.
Adopt the correct wiring strategy in RF circuit layout design
The use of equal routing can reduce the wire inductance, but the mutual inductance and distributed capacitance between the wires increase. If the layout permits, it is best to use a grid-shaped wiring structure. The specific method is to wire one side of the printed board horizontally and the other side of the printed board. Then connect with metalized holes at the cross holes.
Effectively suppress crosstalk in RF circuit layout design
In order to suppress the crosstalk between the conductors of the printed circuit board, when designing the wiring in RF circuit layout design, you should try to avoid long-distance equal wiring, extend the distance between the wires as much as possible, and try not to cross the signal wires with the ground wires and the power wires. Setting a grounded printed line between some signal lines that are very sensitive to interference can effectively suppress crosstalk.
Avoid electromagnetic radiation generated in RF circuit layout design
In order to avoid electromagnetic radiation generated when high-frequency signals pass through printed wires, the following points should also be noted when wiring the printed circuit board:
(1) Minimize the discontinuity of the printed wires, for example, do not change the width of the wires, and the corners of the wires should be greater than 90 degrees to prohibit circular routing.
(2) The clock signal lead is most likely to produce electromagnetic radiation interference. When routing the wire, it should be close to the ground loop, and the driver should be close to the connector.
(3) The bus driver should be close to the bus he wants to drive. For those leads that leave the printed circuit board, the driver should be next to the connector.
(4) The wiring of the data bus should clamp a signal ground wire between every two signal wires. It is best to place the ground loop next to the least important address lead because the latter often carries high-frequency currents.
(5) When arranging high-speed, medium-speed, and low-speed logic circuits on the printed board, the devices should be arranged in the manner shown below in the figure.
Suppress reflection interference in RF circuit layout design
In order to suppress the reflection interference that appears at the terminal of the printed line, in addition to special needs, the length of the printed line should be shortened as much as possible and a slow circuit should be used.
Terminal matching can be added when necessary, a matching resistor of the same resistance is added to the end of the transmission line to the ground and the power terminal.
According to experience, for general faster TTL circuits, terminal matching measures should be adopted when the printed lines are longer than 10cm. The resistance value of the matching resistor should be determined according to the maximum value of the output drive current and the absorption current of the integrated circuit.
Adopt differential signal line routing strategy in the RF circuit layout design process
Differential signal pairs with very close wiring will also be tightly coupled to each other. This mutual coupling will reduce EMI emissions. Generally, differential signals are also high-speed signals. High-speed design rules usually apply to the wiring of differential signals, especially this is especially true when designing the signal line of the transmission line.
This means that we must carefully design the wiring of the signal line to ensure that the characteristic impedance of the signal line is continuous and constant along the signal line.
In the RF circuit layout and routing process of the differential pair, we hope that the two PCB lines in the differential pair are exactly the same.
In practical applications, the greatest effort should be made to ensure that the PCB lines in the differential pair have exactly the same impedance and that the length of the wiring is exactly the same.
Differential PCB lines are usually routed in pairs, and the distance between them is kept constant at any position along the line pair direction. Under normal circumstances, the placement and routing of differential pairs are always as close as possible.
Seventh. The points of attention when matching the RF circuit layout
- The wiring in the matching RF circuit layout (including the wiring from the RF port of the chip, but not including the wiring of the DC power supply before the choke coil) should be as short as possible, and the line width should not change suddenly, and the line width should be changed. The line width change is processed with an approximate effect such as teardrop to obtain the effect of continuous line width change.
- The width of the trace in the matching RF circuit layout is preferably the same as the width of the device pad
- The topology in the matching RF circuit layout should try to avoid a U-shape to prevent the influence of parasitic capacitance
- In theory, the wiring in the matching RF circuit layout should avoid a 45-degree angle as much as possible, and circular arc wiring is preferred.
- The distance between the grounding pads of the grounding capacitor in the matching RF circuit layout is as large as possible to reduce the influence of parasitic capacitance.
- There should be enough shielding ground near the matching RF circuit layout, and more vias should be punched on the shielding ground to ensure the three-dimensional shielding effect and provide a low impedance loop. The distance between two adjacent vias is recommended to be less than one-twentieth of the wavelength but not too dense to form a slotting effect.
- If the space layout permits, keep the shielding ground and the digital signal line as far away as possible, refer to the 3W principle
- Except for the copper foil of the ground network under the high-frequency signal loop, no other signal lines can be used to ensure the continuity of impedance
- The silkscreen cannot be printed on the high-frequency signal line, which will cause impedance changes; the best method is not to add the silkscreen
- If necessary, add 50-ohm impedance control to the filter network after the matching balun, but it will cause the cost to increase by 2 to 5 times.
- If the antenna does not appear directly at the end of the matching RF circuit layout, the part of the connection line from the end of the matching circuit to the antenna also needs 50-ohm impedance control.
- If the matching device does not appear directly near the RF port of the device, theoretically this part of the connection line from the RF port of the device to the matching device needs impedance control, but its impedance is determined by the internal parameters of the chip.
- Impedance control calculations use the impedance routing function of Altium Designer or use the PCB Toolkit tool to calculate.
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