High frequency circuit PCB design

For a long time, designers often spend their energy on the verification of programs, electrical principles, parameter redundancy, etc., but seldom spend their energy on the review of PCB design, and it is often because of PCB design defects that lead to a large number of product performance issues. PCB design principles involve many aspects, including various basic principles, anti-interference, electromagnetic compatibility, safety protection, and so on. For these aspects, especially in high-frequency circuits (especially microwave-level high-frequency circuits), the lack of relevant ideas often leads to the failure of the entire R&D project. Many people still stay on the basis of "connecting electrical principles with conductors to play a predetermined role", and even think that "PCB design belongs to the consideration of structure, process and production efficiency". Many engineers also do not fully realize that this link should be the special focus of the entire design work in product design, and mistakenly spend their energy on selecting high-performance components, resulting in a significant increase in cost and little improvement in performance.

1. High-speed PCB design

In product engineering, PCB design occupies a very important position, especially in high-frequency electrical design. There are some general rules and these are to be treated as general guidelines. Applying the design principles and skills of high-frequency circuit PCB to the design can greatly improve the success rate of the design.

(1) Wiring design principles of high-speed circuit PCB

1. Minimize logic fanout, preferably with only one load.

2. Avoid using through-holes as much as possible between the output and receiving ends of high-speed signal lines, and avoid crossing pin patterns. Especially the clock signal line needs special attention.

3. The signal lines of the upper and lower adjacent layers should be perpendicular to each other to avoid right-angle turns.

4. The parallel terminated load resistor should be placed as close as possible to the receiving end.

5. In order to ensure the minimum reflection, the length of all open lines (or lines without termination matching) must satisfy the following formula: Lopen——open line length (inches); trise——signal rise time (ns); tpd——line propagation Latency (0.188ns/in - per stripline characteristic).

6. When the length of the open line exceeds the value required by the above formula, a series damping resistor should be used, and the series termination resistor should be connected to the pin of the output terminal as much as possible.

7. To ensure that the analog and digital circuits are separated, AGND and DGND must be connected together through an inductor or ferrite beads, as close as possible to the A/D converter. 8. Ensure adequate decoupling of the power supply. 9. It is best to use surface mount resistors and capacitors.

(2) Bypass and decoupling

1. Before selecting a decoupling capacitor, calculate the resonant frequency requirement required to filter high-frequency currents.

2. Above the self-resonant frequency, the capacitor will become inductive, thus losing the role of decoupling capacitor. It should be noted that some logic circuits have spectral energy higher than the self-resonant frequency of common decoupling capacitors.

3. The resonant frequency of the container itself is called self-resonant frequency. If you want to filter out high frequencies.

4. Calculate the required capacitance value based on the RF energy contained in the circuit, the rise time of the switching circuit, and the frequency range of particular interest, rather than guesswork or conventional usage.

5. Calculate the resonant frequency of the ground and power planes. The decoupling capacitor constructed on such two planes can achieve maximum benefit.

6. For high-speed components and areas with rich RF bandwidth energy, multiple capacitors should be connected in parallel to remove RF energy with a large bandwidth. Also note: when the large capacitor becomes inductive at high frequencies, the small capacitor remains capacitive, and an LC resonant circuit will be formed at a special frequency, resulting in infinite impedance, thus completely losing the bypass function. When this happens, it is more efficient to use a single capacitor.

7. Set parallel capacitors on all power input connectors of the circuit board and on the power pins of components whose rise time is faster than 3ns.

8. At the diagonal direction of the PCB power input terminal and the wrench, a capacitor with a large enough capacity should be used to ensure the current change when the circuit switches states. The same consideration should be given to the decoupling capacitors of other circuits, the greater the operating current, the greater the required capacitance. To reduce voltage and current pulsation and improve system stability. Therefore, the decoupling capacitor shoulders the dual functions of decoupling and freewheeling.

9. If you use too many decoupling capacitors, it will draw a lot of current from the power supply when it is turned on, so a group of large capacitors should be placed at the output of the power supply to provide a large amount of current.

(3) Impedance transformation and matching

1. In low-frequency circuits, the concept of matching is very important (making the load impedance equal to the conjugate of the excitation source internal resistance). In high-frequency circuits, the matching of signal line terminals is more important:

On the one hand, ZL=Zc is required to ensure that there is no standing wave along the line; on the other hand, in order to obtain the maximum power, it is required that the input end of the signal line should be conjugate matched when connected to the excitation source. Therefore, matching has a direct impact on the performance of microwave circuits. It can be seen that if the terminals do not match, reflections and standing waves will occur on the signal line, resulting in a drop in load power (high-power standing waves will also cause ignition at the antinode).

Due to the existence of reflected waves, it will have adverse effects on the excitation source, resulting in a decrease in the stability of the operating frequency and output power. However, in practice, the given load impedance is not necessarily the same as the characteristic impedance of the signal line, and the impedance of the signal line and the excitation source is not necessarily conjugate, so it is necessary to understand and apply impedance matching technology.

(4) PCB layering

High-frequency circuits often have high integration and high wiring density. The use of multi-layer boards is not only necessary for wiring, but also an effective means to reduce interference. Reasonable selection of the number of layers can greatly reduce the size of the printed board. It can make full use of the middle layer to set up shielding, which can be better realized. Nearby grounding can effectively reduce parasitic inductance, can effectively shorten the transmission length of signals, can greatly reduce cross-interference between signals, etc. All of these are beneficial to the reliable operation of high-frequency circuits. There are data showing that four-layer boards of the same material are more expensive than The noise of double-sided panels is 20dB lower, but the higher the number of layers, the more complex the manufacturing process and the higher the cost.

(5) Power isolation and ground split

Circuit wiring with different functions or different requirements often requires power isolation and ground wire splitting. For example, analog circuits and digital circuits, weak signal circuits and strong signal circuits, sensitive circuits (PLL, low jitter trigger, etc.)

Basic requirements: 1. The power layer or ground layer in different areas should be connected together at the power inlet, usually in a tree structure or finger structure. The ground wire division method of different functional circuits, the division gap and the edge of the board should not be less than 2mm. 2. Power zones and ground zones of different types cannot cross each other. 3. Trench and bridge. Due to the division of the ground plane, the signal transmission return loop between the various functional circuits is often discontinuous. In order to ensure the connection of the signal, power supply and ground, in addition to using transformer isolation (can not transmit DC signals), optocoupler isolation ( Difficult to transmit high frequency), the commonly used bridging method. The "bridge" is actually a gap in the trench, and there is only one. When using this method, it is best to connect both sides of the bridge to chassis ground if it is a multi-point grounding system (as all high-speed designs are).

Summarize

In product engineering, PCB design occupies a very important position, especially in high-frequency electrical design. The same principle design, the same components, PCBs made by different people have different results. There are many things that are feasible in principle but difficult to achieve in engineering, or things that others can achieve but others cannot. Therefore, it is not difficult to make a PCB board, but it is not easy to make a good PCB board. things.

The above is the high-frequency circuit PCB design technology introduced by Shenzhen Zuchuang Microelectronics Co., Ltd. for you. If you have software and hardware function development needs for smart electronic products, you can rest assured to entrust them to us. We have rich experience in customized development of electronic products, and can evaluate the development cycle and IC price as soon as possible, and can also calculate PCBA quotations. We are a number of chip agents at home and abroad: Songhan, Yingguang, Jieli, Ankai, Quanzhi, realtek, with MCU, voice IC, Bluetooth IC and module, wifi module. We have hardware design and software development capabilities. Covering circuit design, PCB design, single-chip microcomputer development, software custom development, APP custom development, WeChat official account development, voice recognition technology, Bluetooth wifi development, etc. It can also undertake the research and development of smart electronic products, the design of household appliances, the development of beauty equipment, the development of Internet of Things applications, the design of smart home solutions, the development of TWS earphones, the development of Bluetooth earphone speakers, the development of children's toys, and the research and development of electronic education products.