PCB Tech - High frequency circuit PCB design

Components are developing in the direction of high speed, low consumption, small size and high anti-interference. This development trend puts forward many new requirements for the design of printed circuit boards. PCB design is an important stage in the design of electronic products. After the electrical schematic diagram has been designed, several functional boards are determined according to the structural requirements and according to the functional division, and the external dimensions and installation methods of each functional board’s PCB must be determined at the same time. Consider the convenience of debugging and maintenance, as well as factors such as shielding, heat dissipation, and EMI performance. Engineers are required to determine the layout and wiring plan, determine the details of key circuits and signal lines and wiring methods, as well as the wiring principles that should be followed. Several stages of the PCB design process must be inspected, analyzed, and modified. After the entire wiring is completed, it can go through a comprehensive rule inspection before it can be processed.

1. Introduction

For a long time, designers often spend their energy on the verification of procedures, electrical principles, parameter redundancy, etc., but seldom spend their energy on the review of PCB design, and it is often precisely due to PCB design defects that lead to a large number of Product performance issues. PCB design principles involve many aspects, including basic principles, anti-interference, electromagnetic compatibility, safety protection, and so on. For these aspects, especially in high-frequency circuits (especially in microwave-level high-frequency circuits), the lack of related concepts 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 considerations of structure, process and improving production efficiency." Many engineers have not fully realized 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. As a result, the cost has risen sharply, and the performance improvement is minimal.

2. 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 rules will be treated as general guidelines. Applying the PCB design principles and techniques of high-frequency circuits to the design can greatly increase the design success rate.

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

1. To minimize the logical fan-out, it is best to carry only one load.

2. Avoid the use of through holes as much as possible between the output and the receiving end of the high-speed signal line, and avoid the cross of the pin pattern. 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 turning at right angles.

4. The parallel termination load resistor should be 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 matching terminations) must meet the following formula:

Lopen-open route length (inches)

trise--Signal rise time (ns)

tpd--Line propagation delay (0.188ns/in--according to strip line characteristics).

Typical rise time of several high-speed logic circuits:

6. When the length of the open circuit 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 output pin as much as possible.

7. Ensure that the analog circuit and the digital circuit are separated. AGND and DGND must be connected together through an inductor or a magnetic bead, and should be as close as possible to the A/D converter.

8. Ensure sufficient decoupling of the power supply.

9. It is best to use surface mount resistors and capacitors.

(2) Bypass and decoupling

1. Before choosing a decoupling capacitor, first calculate the resonant frequency requirements for filtering out high-frequency currents.

2. Above the self-resonant frequency, the capacitor will become inductive and lose its decoupling capacitance. It should be noted that some logic circuits have higher spectral energy than the common decoupling capacitor's own resonant frequency.

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

4. It is necessary to 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 special attention. Don't use guesswork or use it according to the previous usual usage.

5. Calculate the resonant frequencies of the ground and power planes. The decoupling capacitor constructed with these two planes can achieve the greatest benefit.

6. For high-speed components and areas with abundant RF bandwidth energy, multiple capacitors should be used in parallel to remove RF energy with a large bandwidth. It should also be noted that when the large capacitor becomes inductive at high frequencies, the small capacitor remains capacitive. At a particular frequency, it will form an LC resonant circuit, resulting in infinite impedance, thus completely losing the bypass function. If this happens, it is more effective to use a single capacitor.

7. Set parallel capacitors on the sides of all power input connectors on the circuit board and 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 of large enough capacity should be used to ensure the current change generated when the circuit is switched. The same consideration should be given to the decoupling capacitors of other circuits. The greater the operating current, the greater the required capacitance. In order to reduce the pulsation of voltage and current, improve the stability of the system. Therefore, the decoupling capacitor shoulders the dual role of decoupling and freewheeling.

9. If too many decoupling capacitors are used, a large amount of current will be drawn from the power supply when it is turned on. Therefore, 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 (make the load impedance equal to the internal resistance of the excitation source). 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 and the excitation source should be conjugate matched. Therefore, the matching has a direct impact on the working performance of the microwave circuit. Visible:

If the terminals are not matched, reflections and standing waves will occur on the signal line, resulting in a drop in load power (high-power standing waves will also cause sparks at the antinodes).

Due to the existence of the reflected wave, it will have an adverse effect 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 and the characteristic impedance of the signal line are not necessarily the same, 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.

2. λ/4 impedance converter

When the signal line length L=λ/4, that is, βL=π/2, we can get: Zin=Zc2/ZL

The above formula shows that after the λ/4PCB transmission line is transformed, its impedance will change significantly. It can be known that when the ZL does not match, the reconfiguration of the PCB transmission line can be used to achieve the matching purpose. For the two PCB transmission lines with characteristic impedances of Z'c and Z"c, the PCB transmission line can be connected to achieve the purpose of matching Z'c and Z"c.

It should be noted that the operating frequency of the λ/4 impedance converter after matching two PCB transmission lines with different impedances is very narrow.

3. Single branch short circuit matching

The impedance of the PCB transmission line can be changed by connecting a short-circuit line with a proper structure at the appropriate position of the PCB transmission line to achieve the matching purpose.

(4) PCB layering

High-frequency circuits tend to have higher integration and higher wiring density. The use of multi-layer boards is not only necessary for wiring, but also an effective means to reduce interference. A reasonable choice of the number of layers can greatly reduce the size of the printed board. It can make full use of the intermediate layer to set up shielding, which can be better realized. The nearby ground can effectively reduce the parasitic inductance, can effectively shorten the signal transmission length, can greatly reduce the cross-interference between signals, etc. All of these are beneficial to the reliable operation of high-frequency circuits. There are data showing that the same material is better than a four-layer board. The noise of the double panel is 20dB lower, but the higher the number of layers, the more complicated the manufacturing process and the higher the cost.

(5) Power isolation and ground wire separation

Circuit wiring with different functions or different requirements often requires power isolation and grounding. For example, analog circuits and digital circuits, weak signal circuits and strong signal circuits, sensitive circuits (PLL, low-jitter trigger, etc.) and other circuits should minimize interference with each other so that the circuits can meet the expected specifications.

Basic requirements:

1. The power layers or ground layers in different areas should be connected together at the power entrance, usually tree-shaped structure or finger-shaped structure, and the ground line division method of different functional circuits, the division gap and the board edge shall not be less than 2mm.

2. Different types of power areas and ground areas cannot cross each other

3. Trenches and bridges. 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 and ground, in addition to the use of transformer isolation (cannot transmit DC signals), optocoupler isolation ( In addition to difficult to transmit high frequencies), bridging methods are commonly used. The "bridge" is actually a gap in the trench, and there is only one place. The signal line, power supply and ground all cross the trench from here as shown in the figure. When using this method, if it is a multi-point grounding system (all high-speed designs are), it is best to connect both sides of the bridge to the chassis ground.

3. Conclusion

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