Do you know how to solve those difficult problems in RF circuits?

What is an RF circuit board? Do you know how to design it? RF circuit board design is generally referred to as “difficult and complicated diseases” because there are still many uncertain factors in theory. For fledgling engineers, they lack practical experience and have poor independent adaptability. This article is more suitable for engineers who have just entered the power supply industry. I hope it will help engineers deal with those “difficult and complicated diseases” calmly.

However, in actual design, the real practical skill is how to compromise these criteria and rules when they cannot be accurately implemented due to various design constraints. Of course, there are many important RF design topics worth discussing, including impedance and impedance matching, insulation layer materials and laminated boards, wavelengths and standing waves, etc. Careful planning under the premise of fully mastering various design principles is the guarantee of a one-time successful design.

Common problems in RF circuit design

1.Interference between digital circuit modules and analog circuit modules

If the analog circuit (RF) and the digital circuit work separately, they may work well separately. However, once the two are placed on the same circuit board and work together using the same power supply, the entire system is likely to be unstable. This is mainly because digital signals frequently swing between the ground and the positive power supply (>3 V), and the cycle is particularly short, often in the nanosecond level.

Due to the large amplitude and short switching time, 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. Obviously, if the digital signal and the RF signal cannot be separated well, the weak RF signal may be damaged, so that the working performance of the wireless device will deteriorate or even fail to work at all.

2.Noise interference of power supply

RF circuits are very sensitive to power supply noise, especially to glitch voltage and other high-frequency harmonics. The microcontroller will suddenly absorb most of the current in a short time in each internal clock cycle. This is because modern microcontrollers are manufactured using CMOS technology. Therefore, assuming that a microcontroller runs at an internal clock frequency of 1MHz, it will extract current from the power supply at this frequency. If proper power supply decoupling is not taken, voltage glitches on the power supply line will inevitably occur. If these voltage glitches reach the power pins of the RF part of the circuit, it may cause work failure in severe cases.

3.Unreasonable ground wire

If the ground wire of the RF circuit is not handled properly, some strange phenomena may occur. For digital circuit design, most digital circuit functions perform well even without a ground layer. In the RF band, even a very short ground wire will act like an inductor. Roughly calculated, the inductance per millimeter length is about l nH, and the inductance of a 10 toni PCB line at 433 MHz is about 27Ω. If a ground layer is not used, most ground wires will be longer and the circuit will not have the designed characteristics.

4.Antenna radiation interference to other analog circuit parts

In PCB circuit design, there are usually other analog circuits on the board. For example, many circuits have analog-to-digital conversion (ADC) or digital-to-analog converter (DAC). The high-frequency signal emitted by the antenna of the RF transmitter may reach the analog RF signal of the ADC. If the processing at the ADC input is not reasonable, the RF signal may self-excite in the ESD diode of the ADC input. This will cause ADC deviation.

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 are 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

It 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 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 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 mobile phone PCB board design, 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 where both sides of the PCB board are 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 that 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 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 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. Furthermore, if the power supply pins of the high-power amplifier are not adequately decoupled, high-power noise will radiate throughout the board and cause a variety of problems.

The grounding of high-power amplifiers is critical and often requires a metal shield

. In most cases, it is also critical to keep the RF output 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 the proper phase and amplitude, they may self-oscillate. In the best case, they will 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 routed back from the input to the output of the filter, this may seriously damage the filter’s passband characteristics. 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, grounding everywhere on the entire board must be very careful, otherwise a coupling path will be introduced. Sometimes you can choose to run 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 impedance changes at different frequencies very well, so that circuits do not interfere with each other. Buffers are very helpful in design. They can be placed right behind the circuit to be driven, making the high-power output traces very short.

Since the input signal level of the buffer is relatively low, they are not likely 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 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 set the VCO operating frequency and modulate voice or data onto the RF signal. All VCO design principles also 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 must be located very close 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.

Likewise, good grounding is essential, and the chip’s power supply must be well decoupled. If you have to run a long line at the input or output, it’s best to run it at the output, where the impedance is usually much lower and it’s 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.

Common analog ground and 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 as much ground copper 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 between them along the RF trace. 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.

Additionally, minimizing the distance between parallel RF traces can minimize inductive coupling.

Isolation is best when a solid, monolithic ground plane is placed directly below 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 ground planes on internal signal layers and power distribution layers, and adjust traces so that you can place ground connection vias to isolated ground planes on the surface layer. Avoid ground planes on Free ground is generated on each layer of PCB because it will pick up or inject noise like a small antenna. In most cases, if you can’t connect them to the main ground, then you’d better remove them.

Pay attention to several aspects when designing PCB boards

1.Handling of power and ground wires

Even if the wiring in the entire PCB board is completed well, the interference caused by the lack of consideration of power and ground wires will reduce the performance of the product and sometimes even affect the success rate of the product. Therefore, the wiring of power and ground wires should be taken seriously, and the noise interference generated by power and ground wires should be reduced to a minimum to ensure the quality of the product. For every engineer engaged in electronic product design, they understand the cause of noise between ground wires and power wires. Now they only describe the reduction method of noise suppression:

(1) It is well known that decoupling capacitors are added between power and ground wires.

(2) Try to widen the width of the power supply and ground wires. It is best that the ground wire is wider than the power supply wire. The relationship between them is: ground wire > power supply 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 supply wire is 1.2~2.5 mm. For the PCB of digital circuits, a wide ground wire can be used to form a loop, that is, to form a ground network for use (the ground of analog circuits cannot be used in this way)

(3) Use a large area of ​​copper layer as a ground wire, and connect all unused areas on the printed circuit board to the ground as a ground wire. Or make a multi-layer board, with the power supply and ground wire occupying one layer each.

2.Common ground processing of digital circuits and analog circuits

Nowadays, many PCBs are no longer single-function circuits (digital or analog circuits), but are composed of a mixture of digital circuits and analog circuits. Therefore, when wiring, it is necessary to consider the mutual interference between them, especially the noise interference on the ground wire. The frequency of digital circuits is high, and the sensitivity of analog circuits is strong. For signal lines, high-frequency signal lines should be as far away from sensitive analog circuit devices as possible. For ground lines, the entire PCB has only one node to the outside world, so the problem of digital and analog common ground must be handled inside the PCB. In fact, the digital ground and analog ground are separated inside the board. They are not connected to each other, but only at the interface where the PCB is connected to the outside world (such as plugs, etc.). There is a short circuit 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.Signal lines are laid on the power (ground) layer

When wiring a multi-layer printed circuit board, since there are not many lines left in the signal line layer, adding more layers will cause waste and increase the workload of production, and the cost will increase accordingly. To solve this contradiction, you can consider wiring on the power (ground) layer. First, you should consider using the power layer, and then the ground layer. Because it is best to preserve the integrity of the ground layer.

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 connecting legs needs to be comprehensively considered. In terms of electrical performance, it is better for the pads of the component legs to be fully connected to the copper surface, but there are some bad hidden dangers for the welding and assembly of components, such as: ① Welding requires a high-power heater. ② It is easy to cause cold solder joints. Therefore, taking into account both electrical performance and process requirements, a cross-shaped pad is made, which is called heat shield, commonly known as thermal pad. In this way, the possibility of cold solder joints caused by excessive heat dissipation in the cross section during welding can be greatly reduced. The treatment of the connecting (ground) layer legs of multilayer boards is the same.

5.The role of network system in wiring

In many CAD systems, wiring is determined by the network system. The grid is too dense, and although the passages have increased, the step is too small, and the amount of data in the drawing field is too large, which will inevitably have higher requirements for the storage space of the equipment, and also have a great impact on the computing speed of computer-related electronic products. Some paths are invalid, such as those occupied by the pads of the component legs or by the mounting holes and fixed holes. Too sparse grids and too few paths have a great impact on the routing rate. Therefore, a reasonable grid system is required to support the routing. The distance between the two legs of a standard component is 0.1 inches (2.54 mm), so the basis of the grid system is generally set to 0.1 inches (2.54 mm) or an integer multiple of less than 0.1 inches, such as: 0.05 inches, 0.025 inches, 0.02 inches, etc.

6. High-frequency PCB design skills and methods

1. The corners of the transmission line should be 45° to reduce return loss
2. A high-performance insulating circuit board with a strictly controlled insulation constant value according to the level should be used. This method is conducive to the effective management of the electromagnetic field between the insulating material and the adjacent wiring.
3. The PCB design specifications for high-precision etching should be improved. Consider the total tolerance of the specified line width of +/-0.0007 inches, manage the undercut and cross-section of the wiring shape, and specify the plating conditions of the wiring sidewall. Overall management of the wiring (wire) geometry and coating surface is very important to solve the skin effect problems related to microwave frequencies and achieve these specifications.
4. There is a tap inductance for protruding leads, so avoid using components with leads. In high-frequency environments, it is best to use surface-mount components.
5. For signal vias, avoid using the through-hole 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 influence of 3D electromagnetic fields on the circuit board.
7. Choose non-electrolytic nickel plating or immersion gold plating processes, and do not use HASL for electroplating.
8. The solder mask prevents the flow of solder paste. However, due to the uncertainty of thickness and unknown insulation performance, covering the entire board surface with solder mask will lead to large changes in electromagnetic energy in microstrip designs. Solder dams are generally used to create the electromagnetic field of the solder mask layer.

In this case, we manage the transition between microstrip and coaxial cable. In coaxial cable, the ground layer is annularly interwoven and evenly spaced. In microstrip, the ground layer is below the active line. This introduces certain edge effects that need to be understood, predicted and considered during design. Of course, this mismatch will also cause return loss, which must be minimized to avoid noise and signal interference.

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 various external interferences, enable electronic equipment to work normally in a specific electromagnetic environment, and at the same time reduce the electromagnetic interference of electronic equipment itself to other electronic equipment.

1. Choose a reasonable wire width

Since the impact interference generated by transient current on the printed line is mainly caused by the inductance component of the printed wire, the inductance of the printed wire should be minimized. The inductance of the printed wire is proportional to its length and inversely proportional to its width, so short and fine wires are beneficial for suppressing 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, the printed wire width of about 1.5mm can fully meet the requirements; for integrated circuits, the printed wire width can be selected between 0.2 and 1.0mm.

2. Use the correct wiring strategy

Using equal routing can reduce wire inductance, but the mutual inductance and distributed capacitance between wires increase. If the layout allows, it is best to use a tic-tac-toe mesh wiring structure. The specific method is to wire horizontally on one side of the printed board and vertically on the other side, and then connect them with metallized holes at the cross holes.

3. Effectively suppress crosstalk

In order to suppress crosstalk between printed circuit board conductors, long-distance equal routing should be avoided as much as possible when designing wiring, and the distance between wires should be kept as far as possible. Signal wires should not cross ground wires and power wires as much as possible. Setting a grounded printed wire between some signal wires that are very sensitive to interference can effectively suppress crosstalk.

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

(1) Minimize the discontinuity of printed wires, such as the width of the wire should not change suddenly, the corner of the wire should be greater than 90 degrees, and loop routing is prohibited.

(2) Clock signal leads are most likely to generate electromagnetic radiation interference. When routing, they 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 it wants to drive. For those leads that leave the printed circuit board, the driver should be close to the connector.

(4) The wiring of the data bus should sandwich a signal ground wire between every two signal lines. It is best to place the ground loop right next to the least important address lead, because the latter often carries high-frequency current.

(5) When arranging high-speed, medium-speed and low-speed logic circuits on the printed circuit board, the devices should be arranged as shown in Figure 1.

5. Suppressing reflection interference

In order to suppress the reflection interference that appears at the terminal of the printed line, the length of the printed line should be shortened as much as possible and a slow circuit should be used, except for special needs. If necessary, terminal matching can be added, that is, a matching resistor of the same resistance value is connected to the ground and power supply ends at the end of the transmission line. According to experience, for TTL circuits with generally faster speeds, terminal matching measures should be adopted when the printed line is longer than 10cm. The resistance value of the matching resistor should be determined based on the maximum output drive current and absorption current of the integrated circuit.

6. Use differential signal line routing strategy during PCB design

Differential signal pairs that are routed very close to each other will also be tightly coupled to each other. This mutual coupling will reduce EMI emission. Usually (of course there are some exceptions) differential signals are also high-speed signals, so high-speed design rules are usually applicable to differential signal routing, especially when designing signal lines of transmission lines. This means that we must be very careful in designing the routing of signal lines to ensure that the characteristic impedance of the signal lines is continuous and constant everywhere along the signal lines.

During the layout and routing of differential line pairs, we hope that the two PCB lines in the differential line pairs are exactly the same. This means that in practical applications, we should make every effort to ensure that the PCB lines in the differential line pairs have exactly the same impedance and the routing lengths are exactly the same. Differential PCB lines are usually always routed in pairs, and the distance between them is kept constant at any position along the direction of the line pair. Under normal circumstances, the layout and routing of differential line pairs are always as close as possible. The above is the analysis of RF circuit boards, I hope it can help you.