The objective of this article is to give 5 RF circuit design tutorials to circuit designers who would like to get acquainted with RF circuit designs. A step-by-step tutorial approach is adopted. It is the hope of the author that by the end of this tutorial session, the user will know how to create a schematic, and perform simulations regarding the RF circuit.
RF circuit layout principles
In the design of the RF layout, priority must be given to meet the following general principles.
(1) as far as possible, the high-power RF amplifier (HPA) and low-noise amplifier (LNA) are isolated, simply put, so that the high-power RF transmitter circuit is away from the low-power RF receiver circuit.
(2) ensure that there is at least one whole ground in the high power area on the PCB board, preferably without over holes on it, and 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 the RF input.
(5) sensitive analog signals should be as far away as possible from high-speed digital signals and RF letters.
Physical Partitioning, Electrical Partitioning Design Partitioning
Can be broken down into physical partitioning and electrical partitioning. 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 alignment, sensitive circuits and signals, and grounding.
We discuss physical partitioning issues
The component layout is key to achieving a good RF design, and 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 so that the inputs are away from the outputs, and to separate high and low power circuits as far as possible.
The most efficient way to stack the board is to arrange the main ground plane (main ground) on the second layer under the surface layer and route the RF lines as far as possible on the surface layer. Minimizing the size of the vias in the RF path not only reduces path inductance but also reduces false solder points on the main ground and can reduce the chance of RF energy leakage to other areas within the laminate.
In physical space, linear circuits like multi-stage amplifiers are usually sufficient to isolate multiple RF areas from each other, but duplexers, mixers, and IF amplifiers/mixers always have multiple RF/IF signals interfering with each other, so care must be taken to minimize this effect.
RF and IF alignment should be crossed as far as possible, and as far as possible in between them separated by a piece of ground.
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 the design of cell phone PCBs.
On the cell phone PCB board design, you can usually put the low-noise amplifier circuit on one side of the PCB board, while the high-power amplifier on the other side, and eventually connect them to the RF side and the antenna on the baseband processor side through the duplexer on the same side.
Proper and effective chip power decoupling is also very important.
Many RF chips with integrated linear lines 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, and the same principle applies to decoupling the power supply at the end of this inductor.
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, as this would form a null-core transformer and generate interference signals by mutual inductance, so the distance between them should be at least equal to the height of one of the devices, or arranged at right angles to minimize their mutual inductance.
The principles of electrical partitioning are largely the same as physical partitioning but also include some other factors.
Some parts of the phone use different operating voltages and control them with the help of software to extend battery operating life. This means that the phone needs to run multiple power supplies, and this poses additional problems for isolation.
Power is usually brought in from a connector and immediately decoupled to filter out any noise from outside the board, then distributed after a set of switches or regulators. The DC current of most circuits on a cell phone PCB is quite small, so the alignment width is usually not an issue, however, a separate high current line 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 required to pass current from one layer to another. In addition, if the power supply pins of the high-power amplifier cannot be sufficiently decoupled at its end, then high-power noise will radiate throughout the board and cause a variety of problems.
The grounding of high-power amplifiers is quite critical and often requires a metal shield to be designed for them. 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 are likely to generate self-excited oscillations if their outputs are fed back to their inputs with the proper phase and amplitude. In the best case, they will be able to operate stably at any temperature and voltage conditions.
In reality, they may become unstable and add noise and intermodulation signals to the RF signal. If the RF signal line has to be wound from the input of the filter back to the output, this can seriously damage the bandpass characteristics of the filter.
In order to get good isolation of the input and output, the first ground must be laid around the filter, secondly, the ground should be laid in the lower 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 from the filter pins as possible.
To ensure that no noise is added must be considered from the following aspects
First, the desired bandwidth of the control line may range from DC until 2MHz, and it is almost impossible to remove such a wide band of noise by filtering; second, the VCO control line is usually part of a feedback loop that controls the frequency, and it has the potential to introduce noise in many places, so the VCO control line must be handled very carefully.
Be sure that the ground under the RF alignment is solid, and that all components are firmly connected to the main ground and isolated from other alignments that may introduce noise.
In addition, to ensure that the VCO power supply has been fully decoupled because the RF output of the VCO is often a relatively high level, the VCO output signal can easily interfere with other circuits, so the VCO must pay special attention to the VCO. In fact, the VCO is often placed at the end of the RF area, and sometimes it needs a metal shield.
Resonant circuits (one for the transmitter, the other for the receiver) and VCO-related, but also has their own characteristics. Simply put, a resonant circuit is a parallel resonant circuit with capacitive diodes that help set the VCO operating frequency and modulate voice or data to the RF signal. All VCO design principles apply to resonant circuits as well. Resonant circuits are usually very sensitive to noise because they contain a significant number of components, have a wide distribution area on the board, and usually operate at a very high RF frequency.
The 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 a control loop that is sensitive to noise. This is not easy to achieve.
Automatic gain control (AGC) amplifiers are also a problem-prone area, whether the transmit or receive circuitry will have AGC amplifiers. AGC amplifiers are usually effective at filtering out the noise, but because the phone has the ability to handle rapid changes in transmitting and receiving signal strength, it requires a fairly wide bandwidth for AGC circuits, and this makes AGC amplifiers on certain key circuits very easily introduce noise.
Good analog circuit design techniques must be followed to design AGC lines, and this has to do with very short op-amp input pins and very short feedback paths, both of which must be away from RF, IF, or high-speed digital signal alignments.
Likewise, good grounding is essential, and the chip’s power supply must be well decoupled. If you have to take a long line at the input or output, then it is best to be at the output, which is usually a much lower impedance and less likely to induce noise.
Usually the higher the signal level, the easier it is to introduce noise into other circuits. In all PCB designs, as far as possible, the digital circuit away from the analog circuit is a general principle, it also applies to RF PCB design.
Public analog ground and shielding and isolating the signal lines are usually equally important, so in the early stages of design, careful planning, well-thought-out component layout, and thorough layout * estimate are very important, the same should make RF lines away from analog lines and some very critical digital signals, all RF lines, pads and components should be filled as much as possible around the ground copper, and as far as possible with the main ground connected.
If the RF alignment must cross the signal lines, then try to lay a layer of ground between them along the RF alignment and the main ground connected. If this is not possible, make sure they are crossed, which minimizes the capacitive coupling, while laying as much ground as possible around each RF alignment and connecting them to the main ground.
PCB board design should pay attention to several aspects
Power, ground processing
For each of the engineers engaged in the design of electronic products understands the ground and power lines between the noise generated by the reasons, now only to reduce the type of noise suppression to express.
- The public
- Try to widen the power supply, ground line width, preferably wider than the power line, their relationship is ground line > power line > signal line, usually the signal line width: 0.2 ~ 0.3mm, the most by the thin width of up to 0.05 ~ 0.07mm, power line for 1.2 ~ 2.5 mm. on the digital circuit PCB available wide ground wire to form a circuit, that constitutes A ground network to use (analog circuit ground can not be used in this way)
- With a large area of copper layer for the ground, the printed circuit board is not used on the place are connected to the ground as the ground. Or make a multilayer board, power, ground each occupies a layer.
Digital circuits and analog circuits for common ground processing
Nowadays, many PCBs are no longer single-function circuits (digital or analog) but are composed of a mixture of digital and analog circuits.
Therefore, it is necessary to consider the problem of mutual interference between them when wiring, especially noise interference on the ground.
Digital circuits are high frequency, and analog circuits are sensitive, for signal lines, high-frequency signal lines as far away as possible from sensitive analog circuit devices, for ground, the entire PCB to the outside world only a junction, so the PCB must be processed inside the digital and analog common ground, and the board is actually separated from the digital and analog ground they are not connected to each other, only in the PCB and the external connection the interface (such as plugs, etc.).
Digital ground and analog ground have a short connection, please note that there is only one connection point. There is also no common ground on the PCB, which is determined by the system design.
Signal lines are laid on the electrical (ground) layer
In the multilayer printed circuit board wiring, due to the signal line layer is not finished cloth line left has not much, and then add more layers will cause waste will also add a certain amount of work to the production, the cost also increased accordingly, in order to solve this contradiction, you can consider wiring on the electrical (ground) layer. The first consideration should be to use the power layer, followed by the ground layer. This is because it is best to retain the integrity of the ground layer.
Handling of connecting legs in large-area conductors
In a large area of ground (electrical), commonly used components of the leg and its connection, the processing of the connection leg needs to be comprehensive consideration, in terms of electrical performance, the pad of the component leg and copper surface full connection is good, but the welding assembly of the components there are some bad potential problems.
So taking into account the electrical performance and process needs, made of cross flower pads, called thermal isolation (heat shield) commonly known as thermal pads, so that the possibility of false solder points due to excessive heat dissipation in the cross-section when welding is greatly reduced. Multi-layer board of the electrical (ground) layer leg of the same treatment.
The role of the network system in cabling
In many CAD systems, wiring is based on the network system decision. The network is too dense, the pathway is increased, but the step is too small, and the amount of data in the figure field is too large, which inevitably has higher requirements for the storage space of the equipment, and also has a great impact on the computing speed of computer-type electronic products.
And some of the pathways are invalid, such as the pad occupied by the component leg or by the installation hole, fixed their holes occupied by the. The grid is too sparse, with too little access to the cloth through the rate of great impact.
So there should be a reasonable grid system to support the wiring process. Standard components between the two legs of the distance of 0.1 inches (2.54mm), so the basis of the grid system is generally set at 0.1 inches (2.54 mm) or an integer multiple of fewer than 0.1 inches, such as 0.05 inches, 0.025 inches, 0.02 inches, etc.
High-frequency PCB design tips and tricks
Transmission line corners to use a 45 ° angle to reduce the return loss.
To use the insulation constant value by the level strictly controlled high-performance insulated circuit board. This method facilitates the effective management of electromagnetic fields between the insulating material and the adjacent wiring.
To improve the PCB design specifications regarding high-precision etching. Consider specifying a total line width error of +/-0.0007 inch, managing the undercut and cross-section of the wiring shape, and specifying the wiring sidewall plating conditions. Overall management of wiring (wire) geometry and coating surfaces is important to address skin effect issues associated with microwave frequencies and to achieve these specifications.
Prominent leads have tapped inductance, and components with leads should be avoided. For high-frequency environments, surface mount components are preferred.
For signal vias, avoid using the vias processing (pth) process on sensitive boards, as the process can lead to lead inductance at the vias.
To provide a rich grounding layer. To use molded holes to connect these ground layers to prevent the influence of 3-dimensional electromagnetic fields on the board.
Choose a non-electrolytic nickel or immersion gold plating process, and do not use the HASL method for plating.
The solder resists layer prevents the flow of solder paste. However, due to thickness uncertainty and unknown insulation properties, covering the entire board surface with solder resist material will result in large variations in electromagnetic energy in microstrip designs. Solder dams (solder dams) are generally used for the electromagnetic field of the solder resist layer.
In this case, we manage the transition between microstrip and coaxial cable. In coaxial cables, the ground layers are interwoven in a circular pattern and are evenly spaced. In microstrip, the ground layer is below the active line.
This introduces certain edge effects that need to be understood, predicted, and taken into account during design. Of course, this mismatch also leads to return loss, which must be minimized to avoid noise and signal interference.
Electromagnetic compatibility design
Electromagnetic compatibility refers to the ability of electronic equipment in a variety of electromagnetic environments can still work in a coordinated and effective manner.
The purpose of electromagnetic compatibility design is to enable electronic equipment to suppress all kinds of external interference so that electronic equipment in a specific electromagnetic environment can work properly, but also to reduce the electromagnetic interference of electronic equipment itself to other electronic equipment.
Select a reasonable wire width
As the transient current in the printed lines generated by the impact interference is mainly caused by the inductive components of the printed wire, so 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, thus a short and precise wire is beneficial for suppressing interference.
Clock lead, line driver, or bus driver signal lines often contain large transient currents, printed wire to be as short as possible. For discrete component circuits, the printed wire width of about 1.5 mm can fully meet the requirements; for integrated circuits, the printed wire width can be selected between 0.2 and 1.0 mm.
Use the correct wiring strategy
The use of equal alignment can reduce the wire inductance, but the mutual inductance and distributed capacitance between the wires increase, if the layout allows, it is best to use a tic-tac-toe mesh wiring structure, the specific practice is to print the board on one side of the horizontal wiring, the other side of the vertical wiring, and then connected at the cross-hole with a metalized hole.
Effective crosstalk suppression
In order to suppress the crosstalk between the printed board wires, the long distance of equal alignment should be avoided as far as possible when designing the wiring, and the distance between the lines should be pulled apart as far as possible, and the signal lines should not cross with the ground and power lines as far as possible. In some very sensitive to interference between the signal lines setting a grounded printed line, can effectively suppress crosstalk.
In order to avoid electromagnetic radiation generated when high-frequency signals pass through the printed wire, the following points should also be noted when wiring the printed circuit board.
(1) minimize the discontinuity of the printed wire, for example, the width of the wire should not change abruptly, the corner of the wire should be greater than 90 degrees to prohibit loop alignment, etc.
(2) the clock signal leads are most likely to generate electromagnetic radiation interference, the alignment should be close to the ground loop, and the driver should be next to the connector.
(3) bus driver should be next to the bus he wants to drive. For those leads leaving the printed circuit board, the driver should be next to the connector.
(4) The wiring of the data bus should be sandwiched between every two signal lines with the signal ground. It is best to place the ground loop immediately next to the least important address lead because the latter often contains 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 accordance.
Suppression of reflection interference
In order to suppress the reflection interference that appears at the terminal of the printed lines, in addition to special needs, the length of the printed lines should be shortened and slow-speed circuits should be used as much as possible. If necessary, terminal matching can be added, that is, at the end of the transmission line to the ground and power supply terminal each with a matching resistor of the same resistance.
According to experience, the general speed of the TTL circuit, the printed line is longer than 10cm should be used when the terminal matching measures. The resistance value of the matching resistor should be decided according to the maximum value of the output driving current and absorption current of the integrated circuit.
Circuit board design process using differential signal line wiring strategy
Wiring very close to the differential signal pairs will be closely coupled with each other, and 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 applied to differential signal wiring, especially when designing the signal lines of transmission lines is so.
This means that we must be very careful to design the wiring of the signal lines to ensure that the characteristic impedance of the signal lines is continuous and constant along with all parts of the signal lines.
During the layout and wiring of a differential pair, we want the two PCB lines in the differential pair to be identical. This means that in practice every effort should be made to ensure that the PCBs in the differential pair have exactly the same impedance and that the wiring lengths are identical.
Differential PCB lines are usually always wired in pairs, and the distance between them remains constant at any position along the direction of the pair. Typically, the layout of differential pairs is always routed as close as possible.
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