Microwave Tech - Summary of 5 major experiences in RF circuit design​

In electronics theory, when current flows through a conductor, a magnetic field is formed around the conductor; when an alternating current passes through the conductor, an alternating electromagnetic field is formed around the conductor, which is called electromagnetic wave.

When the electromagnetic wave frequency is lower than 100khz, the electromagnetic wave will be absorbed by the surface and cannot form effective transmission, but when the electromagnetic wave frequency is higher than 100khz, the electromagnetic wave can propagate in the air and be reflected by the ionosphere at the outer edge of the atmosphere to form long-distance transmission capability. Therefore, the alternating current that changes less than 1000 times per second is called low-frequency current, and the alternating current that changes more than 1000 times is called high-frequency current, and radio frequency is such a high-frequency current. Radio Frequency is referred to as RF.

The radio frequency circuit is composed of passive components, active devices and passive networks. The frequency characteristics of components used in radio frequency circuits are different from those in low frequency circuits. In addition to the different frequency characteristics of components and low-frequency circuits, the characteristics of radio frequency circuits in the field of electronic technology are also different from low-frequency circuits. Under high frequency conditions, stray capacitance and stray inductance have a great influence on the circuit. In low-frequency circuits, these spurious parameters have little effect on the performance of the circuit. As the frequency increases, the influence of the spurious parameters becomes greater. In the high-frequency heads of early VHF-band television receivers and the front-end circuits of communication receivers, the influence of stray capacitance is so great that it is no longer necessary to add additional capacitors.

In addition, the circuit has a skin effect under radio frequency conditions. Unlike direct current, current flows in the entire conductor under direct current conditions, while current flows on the surface of the conductor under high-frequency conditions. As a result, the high frequency AC resistance is greater than the DC resistance.

Another problem in high-frequency circuits is the electromagnetic radiation effect. As the frequency increases, when the wavelength is comparable to the circuit size of 12, the circuit becomes a radiator. At this time, various coupling effects will occur between the circuits and between the circuits and the external environment, leading to many interference problems.

RF circuit board design, like electromagnetic interference (EMI), has always been the most difficult part for engineers to control. Although there are still many uncertainties in the design of RF circuit boards, there are still certain rules to follow in the design of RF circuit boards. The following will discuss various issues related to the partition design of the RF circuit board.

Five experience summary

1. Principles of RF circuit layout

When designing the RF 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) Decoupling of chip and power supply is also extremely important;

(4) RF output usually needs to be far away from RF input;

(5) Sensitive analog signals should be as far away from high-speed digital signals and RF signals as possible;

Two, physical partition, electrical partition design 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 wiring, sensitive circuits and signals, and grounding.

1. Physical partition problem

The layout of the components shows the quality of the RF 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, keep the input away from the output, and as far as possible Ground separation of high-power circuits and low-power circuits.

The most effective circuit board stacking method is to arrange the main ground plane (main ground) 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.

2. RF wiring principles

The RF and IF traces should be crossed as much as possible, and spaced between them as much as possible. The correct RF path is very important to the performance of the entire PCB board, which is why the component layout is usually on the mobile phone PCB board. The reason that accounts for most of the time in the design. In the mobile phone PCB board design, usually the low-noise amplifier circuit 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 the RF end and baseband processing on the same side through a duplexer. On the antenna at the end of the device. Some tricks are needed to ensure that the straight through holes do not transfer RF energy from one side of the board to the other. A common technique is to use blind holes on both sides. The adverse effects of the straight-through holes can be minimized by arranging the straight-through holes in areas that are free from RF interference on both sides of the PCB board.

Sometimes it is impossible to ensure sufficient isolation 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. The metal shield must be soldered to the ground and must be kept with the components. A proper distance, so it needs to take up valuable space on the PCB board. It is very important to ensure the integrity of the shielding cover as much as possible. The digital signal lines entering the metal shielding cover should go to the inner layer as much as possible, and it is best that the PCB layer below the wiring layer is the ground layer. RF signal lines 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 around the gap, the ground on different layers can be connected together through multiple vias .

3. Decoupling of chip and power supply

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, so 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, so you may need two or three sets of capacitors and inductors to decouple them separately. The inductors are rarely close together in parallel, because this will form an air-core transformer and induce interference with each other. Signals, so the distance between them must be at least equal to the height of one of the devices, or arranged at right angles to minimize their mutual inductance.

4. Principles of electrical zoning

The principle of electrical zoning 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 fully 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 a good isolation between the input and the output, a ground must be laid around the filter first, and then a 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.

In addition, the grounding of various places on the whole board must be very careful, otherwise a coupling channel will be introduced. Sometimes you can choose to take single-ended or balanced RF signal lines. The principles of cross-interference and EMC/EMI are also applicable 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 must be maintained to obtain a matching signal source, trace and load impedance. The actual wiring may be 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 used to drive different circuits, especially 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. The buffer can well isolate the impedance changes at different frequencies, so that the circuits will not interfere with each other. Buffers are very helpful to the design. They can follow 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 on the board. The circuit is causing interference. Voltage-controlled oscillators (VCOs) can convert varying voltages into varying frequencies. This feature is used for high-speed channel switching, but they also convert trace noise on the control voltage into tiny frequency changes, which gives The RF signal adds noise.

5. Solve the noise problem

First, 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. Noise may be introduced everywhere, 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) amplifiers are also prone to problems. Both the transmitting and receiving circuits will have AGC amplifiers. 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 evaluation are all very important, and RF circuits should also be used Keep 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 a solid ground plane is placed directly on the first layer below the surface, the isolation effect is best, although other methods of designing with care will also work. On each layer of the PCB board, place as many grounds as possible and connect them to the main ground. Place the traces as close together as possible to increase the number of plots of the internal signal layer and power distribution layer, and adjust the traces appropriately so that you can arrange the ground connection vias to the isolated plots on the surface. Free grounds should be avoided on the various layers of the PCB because they can pick up or inject noise like a small antenna. In most cases, if you can't connect them to the main land, then you'd better remove them.

3, the PCB board design should pay attention to several aspects

1. 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 thinnest The width can reach 0.05～0.07mm, and the power cord is 1.2～2.5 mm. 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 copper layer as a ground wire, and connect the unused places on the printed circuit board to the ground as a ground wire. Or it can be made into a multi-layer board, and the power supply and ground wire 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 entire PCB has only one node to the outside world, so it must The problem of digital and analog common ground is dealt with 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 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.

3. 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.

4. 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 and assembly of components, such as: 1. Welding requires high-power heaters. 2. It 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 virtual solder joints may be generated due to excessive cross-section heat during soldering. Sex is greatly reduced. The processing of the power (ground) leg of the multilayer board is the same.

5. 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 to 0.1 inches (2.54 mm) or an integral multiple of less than 0.1 inches, such as: 0.05 inches, 0.025 inches, 0.02 Inches etc.

4, high-frequency PCB design skills and methods

1. The corner of the transmission line should be 45° to reduce the return loss

2. Use high-performance insulated circuit boards whose insulation constant values are strictly controlled by level. This method is conducive to effective management of the electromagnetic field between the insulating material and the adjacent wiring.

3. To improve the 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 side wall should be specified. The overall management of wiring (wire) geometry and coating surface is very important to solve the skin effect problem related to microwave frequency and realize these specifications.

4. The protruding leads have tap inductance, so avoid using components with leads. In high frequency environments, it is best to use surface mount components.

5. For signal vias, avoid using a via processing (pth) process on sensitive boards, because this process will cause lead inductance at the vias.

6. Provide abundant ground planes. Use molded holes to connect these ground planes to prevent the 3D electromagnetic field from affecting the circuit board.

7. To choose electroless nickel plating or immersion gold plating process, do not use HASL method for electroplating.

8. 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 microstrip, the ground plane is below the active line. This introduces certain edge effects, which need to be understood, predicted and considered during design. Of course, this mismatch will also cause return loss, and this mismatch must be minimized to avoid noise and signal interference.

5, electromagnetic compatibility 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 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.

1. Choose a reasonable wire width

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 leads, 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.

2. Adopt the correct wiring strategy

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 metallized holes at the cross holes.

3. Effectively suppress crosstalk

In order to suppress the crosstalk between the conductors of the printed circuit board, when designing the wiring, 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.

4. Wiring points to avoid electromagnetic radiation

In order to avoid electromagnetic radiation generated when high-frequency signals pass through the printed wires, the following points should also be noted when wiring the printed circuit board:

(1) Minimize the discontinuity of printed wires. For example, the width of the wires should not change suddenly, 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 to be driven. 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. Suppress reflection interference

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, that is, 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.

6. Adopt differential signal line routing strategy in the circuit board design process

Differential signal pairs with very close wiring will also be tightly coupled to each other. This mutual coupling will reduce EMI emissions. Usually (of course there are some exceptions) differential signals are also high-speed signals, so high-speed design rules usually apply. This is especially true for the routing of differential signals, especially when designing signal lines for transmission lines. 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 layout and routing process of the differential pair, we hope that the two PCB lines in the differential pair are exactly the same. This means that 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 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 is always as close as possible.