In actual design, the real practical skill is how to compromise these guidelines and laws when they cannot be implemented accurately 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 laminates, wavelengths and standing waves, etc. Careful planning based on a comprehensive grasp of various design principles is the guarantee of a successful design at the first time .

Frequently asked questions about RF circuit design

1. Interference between digital circuit modules and analog circuit modules
If analog circuits (radio frequency) and digital circuits work independently, they may each work well. However, once the two are put 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 ground and positive supplies (>3 V), and the periods are extremely short, often on the order of nanoseconds.

Due to the larger amplitude and shorter switching time. As a result, 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 1 μV. Therefore the difference between the digital signal and the RF signal can reach 120 dB.

Obviously. If the digital signal cannot be well separated from the RF signal. Weak RF signals can be damaged, causing wireless device performance to deteriorate or even fail to work at all.

2. Noise interference from power supply
RF circuits are quite sensitive to power supply noise, especially glitch voltages and other high-frequency harmonics.

Microcontrollers will suddenly draw most of the current for a short period of time during each internal clock cycle. This is because modern microcontrollers are manufactured using CMOS processes.

therefore. Suppose a microcontroller is running at an internal clock frequency of lMHz, it will draw current from the power supply at this frequency. If proper power supply decoupling is not taken. It will definitely cause voltage glitches on the power line.

If these voltage glitches reach the power pins of the RF part of the circuit, they 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 designs, most digital circuit functions perform well even without a ground plane.

In the RF band, even a short ground wire can act like an inductor. Roughly calculated, the inductance per millimeter of length is about l nH, and the inductive reactance of a 10 toni PCB line at 433 MHz is about 27Ω.

If a ground layer is not used, most ground wires will be long and the circuit will not have the designed characteristics.

4. Radiation interference from the antenna 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 converters (ADCs) or digital-to-analog converters (DACs).

The high-frequency signal emitted by the antenna of the RF transmitter may reach the analog signal of the ADC.

If the ADC input is not handled properly, the RF signal may self-excite within the ESD diode of the ADC input. This causes ADC deviation.

1. RF circuit layout principles

When designing the RF layout, the following general principles must be prioritized:

(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 the high-power area on the PCB board has at least a whole piece of ground, preferably without via holes. Of course, the larger the copper foil area, the better;

(3) Circuit and power supply decoupling is also extremely important;

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

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

2. Physical partition, electrical partition design partition

Can be broken down into physical partitions and electrical partitions. 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. Let’s discuss the issue of physical partitioning

Component layout is the key to achieving a good RF design. The most effective technique is to first fix the components located in the RF path and adjust their orientation to minimize the length of the RF path, keeping the input as far away from the output as possible. Ground separates high power circuits from low power circuits.

The most effective way to stack circuit boards is to arrange the main ground plane (main ground) on the second layer below the surface layer, and run the RF lines on the surface layer as much as possible.

Minimizing the size of vias in the RF path not only reduces path inductance, but also reduces false solder joints on the main ground and reduces the opportunity for RF energy to leak into other areas within the stackup.

In physical space, linear circuits like multistage amplifiers are usually sufficient to isolate multiple RF zones from each other, but diplexers, mixers, and IF amplifiers/mixers always have multiple RF/IF Signals interfere with each other, so care must be taken to minimize this effect.

2. RF and IF traces should be crisscrossed as much as possible, and a piece of ground should be placed between them as much as possible.

The correct RF path is very important to 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, it is usually possible to place the low-noise amplifier circuit on one side of the PCB board, and the high-power amplifier on the other side, and finally connect them to the RF end and baseband processing on the same side through a duplexer on the antenna on the transmitter side.

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, typically requiring up to four capacitors and an isolation inductor per chip.

Make sure to filter out all power supply noise. An integrated circuit or amplifier often has an open-drain output and therefore requires a pull-up inductor to provide a high-impedance RF load and a low-impedance DC supply. The same principle applies to decoupling the supply at this inductor end.

Some chips require multiple power supplies to operate, so you may need two or three sets of capacitors and inductors to decouple them individually. Inductors are rarely placed close together in parallel because this will form an air-core transformer and induce interference with each other. signals, so they should be at least as far apart as the height of one of the devices, or arranged at right angles to minimize their mutual inductance.

4. The principles of electrical zoning are generally the same as physical zoning, but also include some other factors.

Certain parts of the phone operate at different voltages and are controlled with the help of software to extend battery life. This means the phone needs to run on multiple power sources, which creates more problems with isolation.

Power is typically brought in at the connector and immediately decoupled to filter out any noise coming from outside the board, before being distributed through a set of switches or regulators.

The DC current of most circuits on mobile phone PCBs is quite small, so trace width is usually not an issue. However, a separate high-current trace as wide as possible must be run for the power supply of the high-power amplifier to minimize the transmission voltage drop. .

To avoid too much current loss, multiple vias are needed to pass current from one layer to another. Additionally, if the high-power amplifier is not adequately decoupled at its supply pins, the high-power noise will radiate throughout the board and cause all sorts of problems.

The grounding of high-power amplifiers is critical and often requires the design of a metal shield. In most cases, it is also critical to ensure that the RF output is kept away from the RF input.

This also applies to amplifiers, buffers and filters. In the worst case, amplifiers and buffers have the potential to self-oscillate if their outputs are fed back to their inputs with the proper phase and amplitude.

In the best case, they will work reliably under any temperature and voltage conditions.

In fact, they can become unstable and add noise and intermodulation signals to the RF signal. If the RF signal line has to be routed from the input back to the output of the filter, this can seriously compromise the filter's bandpass characteristics.

In order to achieve good isolation between input and output, first a circle of ground must be laid around the filter, and secondly a piece of ground must be laid in the lower area of the filter and connected to the main ground surrounding 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.

5. To ensure that noise is not increased, the following aspects must be considered:

First, the desired 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; second, the VCO control line is usually part of a feedback loop that controls the frequency, and it is used in many applications. Noise can be introduced anywhere, so the VCO control lines must be handled with great care.

Make sure that the ground underneath the RF traces is solid, and that 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 at 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 has its own characteristics. Simply put, a resonant circuit is a parallel resonant circuit with a capacitive diode that helps set the VCO operating frequency and modulate voice or data onto the RF signal.

All VCO design principles apply equally to resonant circuits. Resonant circuits are often very sensitive to noise because they contain a significant number of components, are spread over a wide board area, and typically 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 together and connected back to on a control loop that is very sensitive to noise. It is not easy to do this.

The automatic gain control (AGC) amplifier is also a problem-prone place. Whether it is a transmitting or receiving circuit, there will be an AGC amplifier.

AGC amplifiers can usually effectively filter out noise. However, due to the ability of mobile phones to handle rapid changes in transmitting and receiving signal strengths, the AGC circuit is required to have a fairly wide bandwidth, which makes it easy to introduce AGC amplifiers on certain critical circuits. noise.

Designing the AGC circuit must comply with good analog circuit design techniques, and this very short op amp input pin is related to a very short feedback path, both of which must be kept 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 wire at the input or output, it's best to run it at the output, which usually has much lower impedance and is less likely 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 as far away from analog circuits as possible, and it also applies to RF PCB design.

Common analog grounds and grounds used to shield and separate signal lines are often equally important, so careful planning, thoughtful component placement, and thorough layout evaluation are all important in the early stages of design. The same should apply to RF The lines should 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 trace, try to lay a layer of ground connected to the main ground along the RF trace between them. If this is not possible, make sure they are criss-crossed. This will minimize capacitive coupling. Also try to put as much ground around each RF trace as possible and connect them to the main ground.

3. Several aspects should be paid attention to when designing PCB boards.

1. Handling of power supply and ground wires
Every engineer who is engaged in the design of electronic products understands the causes of noise between the ground wire and the power wire. Now we only describe the reduced noise suppression method:

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

(2) Try to widen the width of the power and ground wires. It is best to make the ground wire 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 digital circuit PCBs, wide ground wires can be used to form a loop, that is, to form a ground network (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 it can be made into a multi-layer board, with power supply and ground wires 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 and analog circuits. Therefore, it is necessary to consider the mutual interference between them when wiring, especially the noise interference on the ground line.

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 dealt with inside the PCB. However, the digital ground and analog ground are actually separated inside the board. They are not connected to each other, but are only at the interface where the PCB connects to the outside world (such as plugs, etc.).

The digital ground is a little shorted to the analog ground, please note that there is only one connection point. There are also different ground on the PCB, which is determined by the system design.

3. The signal lines are laid on the electrical (ground) layer
When wiring multi-layer printed boards, there are not many unfinished lines left on the signal line layer. Adding more layers will cause waste and increase the workload of production, and the cost will also increase accordingly. To resolve this contradiction, you can consider wiring on the electrical (ground) layer.

The power layer should be considered first, followed by the ground layer. 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 commonly used components are connected to it. The handling of the 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 for There are some hidden dangers in the welding assembly of components.

Therefore, taking into account the electrical performance and process requirements, a cross-shaped solder pad is made, which is called heat shield, commonly known as thermal pad (Thermal). In this way, the possibility of virtual solder joints due to excessive cross-section heat dissipation during welding can be eliminated. Sex is greatly reduced. The treatment of the power (ground) layer legs of multi-layer boards is the same.

5. The role of network system in wiring
In many CAD systems, routing is determined based on the network system.

If the grid is too dense, although the number of channels is increased, the steps are too small and the amount of data in the image field is too large. This will inevitably put higher requirements on the storage space of the device. At the same time,It also has a great impact on the computing speed of electronic products such as computers.

Some paths are invalid, such as those occupied by the pads of component legs or occupied by mounting holes and positioning holes. Too sparse mesh and too few channels will have a great impact on the routing rate.

Therefore, there must be a grid system with reasonable density to support wiring. The distance between the legs of a standard component 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 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 corners of the transmission line should be at a 45° angle to reduce return loss.

2. Use high-performance insulating circuit boards whose insulation constant values are strictly controlled according to levels. This approach facilitates efficient management of electromagnetic fields between the insulating material and adjacent wiring.

3. It is necessary to improve PCB design specifications for high-precision etching. Consider specifying a total line width tolerance of +/-0.0007 inches, managing the undercut and cross-section of the wiring shape, and specifying wiring sidewall plating conditions.

Overall management of wiring (conductor) geometry and coating surfaces is important to address skin effect issues associated with microwave frequencies and to achieve these specifications.

4. There is a tap inductance in the 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 via processing (pth) process on sensitive boards, because this process will cause lead inductance at the vias.

6. Provide a rich ground layer. Molded holes should be used to connect these ground planes to prevent the influence of 3D electromagnetic fields on the circuit board.

7. Choose electroless nickel plating or immersion gold plating process instead of HASL method for electroplating.

8. The solder mask layer can prevent the flow of solder paste. However, due to thickness uncertainty and unknown insulation properties, covering the entire board surface with solder mask would

Leads to large changes in electromagnetic energy in microstrip designs. Generally, a solder dam is used as the electromagnetic constant of the solder mask layer.

In this case, we manage the conversion between microstrips and coaxial cables. In coaxial cable, the ground planes are interwoven in a ring and evenly spaced.

In microstrip, the ground plane is below the active lines. This introduces certain edge effects that need to be understood, anticipated, and accounted for during design. Of course, this mismatch also results in return loss, which must be minimized to avoid noise and signal interference.

5. Electromagnetic compatibility design

Electromagnetic compatibility refers to the ability of electronic equipment to still work harmoniously and effectively 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 specific electromagnetic environments, and at the same time reduce the electromagnetic interference of the electronic equipment itself to other electronic equipment.

1. Choose a reasonable wire width

Since the impact interference caused by the transient current on the printed lines is mainly caused by the inductance component of the printed wires, the inductance of the printed wires should be minimized.

The inductance of a printed wire is directly proportional to its length and inversely proportional to its width, so short and precise wires are beneficial to suppressing interference. Clock leads, row driver or bus driver signal lines often carry large transient currents, and printed conductors should be kept as short as possible.

For discrete component circuits, the printed wire width can fully meet the requirements when it is about 1.5mm; for integrated circuits, the printed wire width can be selected between 0.2 and 1.0mm.

2. Adopt the correct wiring strategy

Using equal wiring can reduce the wire inductance, but the mutual inductance and distributed capacitance between the wires will 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. Then connect them with metallized holes at the intersection holes.

3. Effectively suppress crosstalk

In order to suppress crosstalk between printed circuit board wires, long-distance equal wiring should be avoided when designing wiring, the distance between wires should be as wide as possible, and signal wires should not cross with ground wires and power wires as much as possible.

Setting a grounded printed line between some signal lines 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 also be noted when wiring printed circuit boards:

(1) Minimize the discontinuity of printed conductors. For example, the width of the conductors should not change suddenly, the corners of the conductors should be greater than 90 degrees, and circular routing is prohibited.

(2) The clock signal leads are most likely to cause electromagnetic radiation interference. They should be routed 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 exit the PCB, the driver should be located immediately next to the connector.

(4) The data bus wiring should include a signal ground wire between every two signal wires. It is best to place the ground return immediately next to the least important address leads, which often carry high-frequency currents.

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

5. Suppress reflection interference

In order to suppress the reflection interference that appears at the terminals of printed lines, except for special needs, the length of printed lines should be shortened as much as possible and slow circuits should be used.

If necessary, terminal matching can be added, that is, a matching resistor with the same resistance value can be added to the end of the transmission line to the ground and power supply ends.

According to experience, for generally fast TTL circuits, terminal matching measures should be used when the printed lines are longer than 10cm. The resistance 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 strategies in the circuit board design process

Pairs of differential signals routed very close to each other 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 generally apply. This is especially true for differential signal wiring, especially when designing signal lines for transmission lines.

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 constant along the signal line.

During the layout and routing process of the differential wire pair, we hope that the two PCB lines in the differential wire pair are completely consistent.

This means that in practical applications, every effort should be made to ensure that the PCB traces in the differential pair have exactly the same impedance and that the trace lengths are exactly the same.

Differential PCB lines are usually always routed in pairs, and the distance between them remains constant at any position along the direction of the pair. Normally, differential pairs are always routed as close together as possible.