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Successful high-frequency circuit board design requires careful attention to every step and detail throughout the design process, which means that thorough planning must be done at the beginning of the design process. This means that thorough planning must be carried out at the beginning of the design process, and the progress of each design step must be thoroughly and continuously evaluated.
In recent years, due to Bluetooth devices. The demand and growth of Bluetooth devices, wireless LAN (WLAN) devices, and mobile phones has led to an increased focus on high-frequency circuit design techniques. From the past to the present, high-frequency circuit board design, like electromagnetic interference (EMI) issues, has always been one of the most difficult, if not a nightmare, for engineers to master. It takes careful planning and attention to detail to get it right the first time.
High-frequency circuit board design is often described as a form of ‘black art’ because of the many theoretical uncertainties associated with it. However, this is a biased view and there are still many rules that can be followed in HF circuit board design. However, the real practical skill in design is to compromise these rules when they cannot be implemented due to various constraints. Important high frequency design topics include: impedance and impedance matching. Insulation data and laminates. Wavelength and harmonics, etc. We will now focus on various issues related to HF circuit board partition design.
1.Types of microvias for HF circuit board design
High-frequency circuit boards of different nature of the circuit must be separated, but not to produce electromagnetic interference in the best case of connection, which requires the use of microvias (microvia). Usually the diameter of microvia is 0.05mm to 0.20mm. These vias are generally classified into three categories, i.e. blind via, buried via and buried via. These vias are generally classified into three types, namely blind via, bury via and through via. Blind vias are located on the top and bottom surfaces of the printed wiring boards and have a certain depth for the connection of the surface wiring to the inner wiring underneath, and the depth of the vias usually does not exceed a certain ratio (aperture diameter). Buried holes are connection holes located in the inner layer of the printed circuit board, which do not extend to the surface of the board. Both of these types of holes are located in the inner layer of the board and are made using a through-hole forming process before lamination, and may overlap several inner layers during the through-hole formation process. The third type is called through-hole. This type of hole goes through the entire circuit board and can be used to realise internal interconnections or as adhesive positioning holes for components.
2.Adoption of Partitioning Techniques
When designing a high-frequency circuit board, it is important to separate the high-power high-frequency amplifiers (HPAs) from the low-noise amplifiers (LNAs) as much as possible. Simply put, HPA is to keep the high power HF transmitter circuit away from the low power receiver circuit. This can be easily done if there is a lot of space on the PCB. But usually when there are many components, the PCB space becomes very small and it is difficult to achieve this with commemorative items. It is possible to put them on both sides of the PCB or to have them work alternately rather than simultaneously. High-power circuits can sometimes include high-frequency buffers and voltage-controlled oscillators (VCOs).
Design partitioning can be divided into physical partitioning and electrical partitioning. Physical partitioning is mainly concerned with component layout, orientation and masking. Electrical partitioning can be further divided into power distribution. High-frequency routing. Sensitive circuits and signals. Grounding, etc.
High Frequency Circuit Board
3.Physical Partitioning for High Frequency PCB Designs
Component layout is the key to achieving a superior HF design. The most effective technique is to first fix the components on the HF path and adjust their orientation to minimise the length of the HF path. The most effective technique is to first fix the components on the HF path and adjust their orientation so that the length of the HF path is minimised and the HF inputs are far away from the HF outputs and as far away as possible from the high power and low power circuits.
The most efficient way to stack the boards is to place the main ground on the second layer below the surface layer and route the HF traces on the surface layer as far as possible. Minimising the size of the vias on the HF path not only reduces path inductance, but also reduces the number of virtual solder joints on the main ground and reduces the chance of HF energy leakage to other areas of the laminate.
In physical space, linear circuits such as multistage amplifiers are usually sufficient to isolate multiple high-frequency areas from each other, but diplexers, mixers, and IF amplifiers are always a good idea. Mixers and IF amplifiers will always have multiple HF/IF signals interfering with each other, and care must be taken to minimise this effect. HF and IF routes should be criss-crossed wherever possible and separated by a ground plane wherever possible. Correct HF routing is very important for the performance of the whole PCB, which is why component layout usually takes up most of the time in mobile phone PCB design.
On a mobile phone PCB, it is usually possible to place the low noise amplifier circuit on one side of the PCB and the high power amplifier on the other, and ultimately connect them to the HF antenna at one end and the baseband processor at the other end by means of a duplexer on the same side. This requires some finesse to ensure that the high-frequency energy is not transferred from one side of the board to the other through vias, and a common technique is to use blind vias on both sides. The adverse effects of vias can be minimised by placing blind vias on both sides of the PCB in areas that are not subject to high-frequency interference.
4.Metal Masks for High Frequency PCBs
Sometimes, it is not possible to maintain sufficient separation between multiple circuit blocks, in which case it is necessary to consider using metal masks to shield the high frequency energy in the high frequency region, but metal masks also have side effects, such as: manufacturing costs and assembly costs are very high.
Metal masks with irregular shapes are difficult to manufacture with high precision, and rectangular or square metal masks impose some restrictions on component layout; metal masks are not favourable for component replacement and fail-over; and since metal masks have to be soldered to a ground plane and must be kept at a suitable distance from components, they take up valuable PCB board space.
It is important to keep the metal mask as intact as possible, so digital signal lines entering the metal mask should be routed as far as possible through the inner layer, and it is a good idea to make the layer below the signal line layer the grounding layer. High frequency signal lines can be routed through the small notch at the bottom of the metal mask and through the wiring layer at the grounding notch, but the notch should be surrounded by as large a grounding area as possible, and the grounding on the different signal layers can be connected together through multiple vias.
Despite these drawbacks, metal masks are very effective and are often the only solution for isolating critical circuits.
5.Power Decoupling Circuits for High Frequency Boards
In addition, proper and effective on-chip power decoupling circuits are also very important. Many high-frequency chips that incorporate linear circuits are very sensitive to power noise, and often up to four capacitors and an isolation inductor per chip are required to filter out the full power noise.
An integrated circuit or amplifier often has an open collector output, which requires a pull-up inductor to provide a high-impedance high-frequency load and a low-impedance DC supply, and the same principle applies to decoupling the power side of this inductor. Some wafers require more than one power supply to operate, so two or three sets of capacitors and inductors may be required to decouple them separately, which may not be effective if there is not enough space around the wafer.
Particular attention should be paid to the fact that inductors are rarely placed parallel to each other, as this creates a hollow-core transformer with interfering signals, and the distance between them should be at least the height of one of them, or arranged at right angles to minimise their mutual inductance.
6.Power partitioning for high-frequency circuit board design
Power partitioning is in principle the same as physical partitioning, but there are some other factors involved. Some parts of modern mobile phones use different operating voltages and are controlled by software to extend battery life. This means that mobile phones need to run multiple power sources, which creates more isolation problems. Power is usually brought in by a connection line, immediately decoupled to filter out any noise from outside the circuit board, then passed through a set of switches or regulators, after which the power is distributed.
In mobile phones, where most circuits have fairly low DC currents, the width of the antique circuit is usually not a problem, but a separate high-current circuit for the power supply of the high-power amplifiers has to be designed as wide as possible in order to minimise the voltage drop during emission. To avoid too much current loss, multiple vias are used to transfer current from one layer to another. In addition, if a high power amplifier is not adequately decoupled at its power supply pins, the high power noise will radiate throughout the board and cause a variety of problems. The grounding of high power amplifiers is very important and often requires a metal shroud to be designed for them.
7.High-frequency outputs must be kept away from high-frequency inputs.
In most cases, the high-frequency output must be kept away from the high-frequency input. This principle also applies to amplifiers, buffers and filters. Buffers and filters. In the worst case, amplifiers and buffers may generate self-excited oscillations if their outputs echo their inputs with the proper phase and amplitude. They may become unstable and add noise and intermodulation multiplication to the high frequency signal.
If the high-frequency signal line is looped back from the input of the filter to the output, this may seriously impair the bandpass characteristics of the filter. For good separation of input and output, there must be a main ground area around the filter, and a ground area in the lower area of the filter that is connected to the main ground surrounding the filter. It is also a good idea to route the signal lines that need to pass through the filter as far away from the filter pins as possible. In addition, great care must be taken with grounding everywhere on the entire board, otherwise an unwanted coupling path may be introduced unintentionally. This grounding method is described in detail.
Sometimes it is possible to run single-ended or balanced HF traces, and the same principles of crosstalk and EMC/EMI apply here. Balanced HF traces can reduce noise and crosstalk if aligned correctly, but their impedance is usually higher. And in order to get an impedance match on the source. In order to get an impedance match between the source and the load, it is necessary to maintain a reasonable line width, which may be difficult in the actual wiring.
8.High Frequency Circuit Board Design Buffers
buffer can be used to improve isolation because it can split the same signal into two parts and be used to drive different circuits. In particular, a local oscillator may require a buffer to drive multiple mixers. When a mixer reaches common mode isolation at high frequency, it will operate normally. The buffer is a good way to isolate impedance variations at different frequencies so that the circuits do 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 line is very short, because the input signal level of the buffer is relatively low, so they are not easy to cause interference to other circuits on the board.
9.Voltage Controlled Oscillators for High Frequency Board Designs
Voltage controlled oscillators (VCOs) convert changing voltages into changing frequencies, a feature used in high speed channel switching, but they also convert the slight noise in the control voltage into a small change in frequency, which adds noise to the high frequency signal. In short, there is no way to remove the noise from the HF output signal after it has been processed by the voltage controlled oscillator. The difficulty is that the desired bandwidth of the VCO control line can range from DC to 2MHz and it is almost impossible to remove noise from such a wide band by filtering. Secondly, the VCO control line is usually part of a frequency controlled reverberation loop, which has the potential to introduce noise in a number of places, so VCO control lines must be handled with great care.
High Frequency Circuit Board
10.High Frequency Circuit Board Design for Resonant Circuits
Resonant circuits (tank circuits) are used in transmitters and receivers, which are related to VCOs but have their own characteristics. Simply put, a tank circuit is a series of inductive-capacitive diodes connected in parallel to a resonant circuit, which helps to set the operating frequency of the VCO and modulate voice or data onto the HF carrier.
All VCO design principles apply to resonant circuits as well. Since resonant circuits contain a considerable number of components, they are often used as a resonator. They occupy a large area. Since resonant circuits contain a considerable number of components and occupy a large area, they usually run at a very high frequency, and antiques are usually very sensitive to noise. The signals are usually arranged on neighbouring pins on the chip, but these pins need to be coupled with large inductors and capacitors in order to operate, which in turn needs to be positioned as close as possible to the signal pins and connected back to a noise sensitive control loop, but with as little noise interference as possible. This is not easy to do.
11.Automatic Gain Control Amplifiers for High Frequency Board Designs
Automatic Gain Control (AGC) amplifiers are also a problematic area, both in the transmitter and receiver circuits. AGC amplifiers are generally effective at filtering out noise, but because mobile phones have the ability to handle rapid changes in transmit and receive signal strength, antiques require a large bandwidth for AGC circuits, which makes AGC amplifiers susceptible to noise introduction.
AGC circuits must be designed to follow the principles of analogue circuit design, i.e. using short input pins and short echo paths, both of which must be well away from the HF.IF or high-speed digital signal lines. Likewise, good grounding is essential, and the chip's power supply must be well decoupled. If it is necessary to design a long line on either the input or the output, it is best to realise it on the output, as the impedance at the output is usually much lower than at the input, and it is less likely to introduce noise. Usually the higher the signal level, the easier it is to introduce noise into other circuits.
12.High Frequency Circuit Board Design Grounding
Ensure that the ground under the high-frequency circuitry is solid, and that all components are securely connected to the main ground and isolated from other circuits that may introduce noise. In addition, make sure that the power supply to the VCO is adequately decoupled. Since the high-frequency output of the VCO is often at a very 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 high-frequency region, and sometimes it also needs a metal
13.Masks for high-frequency board design
It is a general principle in all PCB designs to keep digital circuits as far away from analogue circuits as possible, and this also applies to high frequency PCB designs. Common analogue ground and ground for masking and isolating signal lines are usually equally important. Similarly, high frequency circuits should be kept away from analogue circuits and some critical digital signals, and all high frequency alignments. The pads and components should be surrounded by a grounded copper skin and connected to the main earth as far as possible. Microvia structured boards are useful in the development phase of high frequency circuits, where many vias can be used at will without any overhead, otherwise drilling holes in a normal PCB would add development costs which would be uneconomical in high volume production.
The best isolation is achieved by placing a solid piece of ground plane directly under the surface of the first layer. Separate the ground plane into several pieces for isolation analogy. This does not work well for digital and high frequency circuits because eventually there will always be some high speed signal lines going through these separate grounding surfaces, which is not good design.
Regardless of whether HF circuit board design is a ‘black art’ or not, observing some basic HF design rules and referring to some excellent HF circuit board design examples will help to complete the HF design work. Successful high-frequency design requires careful attention to every step and every detail of the entire design process, which means that thorough planning must be done at the beginning of the design process. This means that thorough planning must be done at the beginning of the design process, and the progress of each HF circuit board design step must be thoroughly and continuously evaluated.