PCB Tech - Several common problems in PCB high frequency board design

With the rapid development of electronic technology and the wide application of wireless communication technology in various fields, high frequency, high speed, and high density have gradually become one of the significant development trends of modern electronic products. High-frequency signal transmission and high-speed digitization have forced PCB high-frequency boards to move towards micro-holes and buried/blind vias, fine wires, and uniform and thin dielectric layers. High-frequency, high-speed, high-density, multi-layer PCB high-frequency board design technology has become a Important research areas. Based on years of experience in hardware design work, the author summarizes some design techniques and precautions for high-frequency circuits for your reference.

1. How to choose PCB high-frequency board?

The choice of PCB high-frequency board material must strike a balance between meeting design requirements and mass production and cost. The design requirements include both electrical and mechanical parts. This material problem is usually more important when designing very high-speed PCB high-frequency boards (frequency greater than GHz). For example, the commonly used FR-4 material, the dielectric loss at a frequency of several GHz will have a great influence on the signal attenuation, and may not be suitable. As far as electricity is concerned, pay attention to whether the dielectric constant and dielectric loss are suitable for the designed frequency.

2. How to solve the signal integrity problem in high-speed design?

Signal integrity is basically a problem of impedance matching. The factors that affect impedance matching include the structure and output impedance of the signal source, the characteristic impedance of the trace, the characteristics of the load end, and the topology of the trace. The solution is to rely on the topology of termination and adjustment of the wiring.

3. How to avoid high frequency interference?

The basic idea to avoid high-frequency (PCB high-frequency board) interference is to minimize the interference of high-frequency (PCB high-frequency board) signal electromagnetic fields, which is the so-called crosstalk (Crosstalk). Can increase the distance between high-speed signal and analog signal, or add groundguard/shunttraces beside the analog signal. Also pay attention to the noise interference from the digital ground to the analog ground.

4. Can a matching resistor be added between the differential line pairs at the receiving end?

The matching resistance between the differential line pairs at the receiving end is usually added, and its value should be equal to the value of the differential impedance. This way the signal quality will be better.

5. How to implement differential wiring for a clock signal line with only one output terminal?

To use differential wiring, it makes sense that both the signal source and the receiving end are differential signals. Therefore, it is impossible to use differential wiring for a clock signal with only one output terminal.

6. How is the differential wiring method realized?

There are two points to pay attention to in the layout of the differential pair. One is that the length of the two wires should be as long as possible, and the other is that the distance between the two wires (this distance is determined by the differential impedance) has to be kept constant, that is, to keep parallel. There are two parallel ways, one is that the two wires run on the same side-by-side, and the other is that the two wires run on two adjacent layers above and below (over-under). Generally, the former side-by-side (side by side, side by side) is implemented in more ways.

7. Why should the wiring of the differential pair be close and parallel?

The wiring of the differential pair should be appropriately close and parallel. The so-called proper close is because the distance will affect the value of differential impedance, which is an important parameter for designing a differential pair. The need for parallelism is also to maintain the consistency of the differential impedance. If the two lines are suddenly far and near, the differential impedance will be inconsistent, which will affect the signal integrity

(signalintegrity) and time delay (timingdelay).

8. How to solve the contradiction between manual wiring and automatic wiring of high-speed signals?

Most of the automatic routers of strong wiring software now have set constraints to control the winding method and the number of vias. The winding engine capabilities and constraint setting items of various EDA companies sometimes differ greatly. For example, whether there are enough constraints to control the way of serpentine winding, whether to control the trace spacing of the differential pair, and so on. This will affect whether the routing method of the automatic routing can meet the designer's idea. In addition, the difficulty of manually adjusting the wiring is also absolutely related to the ability of the winding engine. For example, the pushing ability of the trace, the pushing ability of the via, and even the pushing ability of the trace to the copper coating, etc. Therefore, choosing a router with strong winding engine capability is the solution.

9. How to deal with some theoretical conflicts in actual wiring?

Basically, it is right to divide and isolate the analog/digital ground. It should be noted that the signal trace should not cross the divided place (moat) as much as possible, and the return current path of the power supply and signal should not be too large.

The crystal oscillator is an analog positive feedback oscillation circuit. To have a stable oscillation signal, it must meet the loopgain and phase specifications. The oscillation specifications of this analog signal are easily disturbed. Even with groundguardtraces, it may not be able to completely isolate the interference. And if it is too far away, the noise on the ground plane will also affect the positive feedback oscillation circuit. Therefore, the distance between the crystal oscillator and the chip must be as close as possible.

Indeed, there are many conflicts between high-speed wiring and EMI requirements. But the basic principle is that the resistance and capacitance or ferritebead added by EMI cannot cause some electrical characteristics of the signal to fail to meet the specifications. Therefore, it is best to use the skills of arranging the traces and the PCB high-frequency board to solve or reduce the EMI problem, such as the high-speed signal going to the inner layer. Finally, the resistance capacitor or ferritebead method is used to reduce the damage to the signal.

10. Is it possible to use the microstrip line model to calculate the characteristic impedance of the signal line on the power plane? Can the signal between the power supply and the ground plane be calculated using the stripline model?

Yes, when calculating the characteristic impedance, both the power plane and the ground plane must be regarded as reference planes. For example, a four-layer board: top layer-power layer-ground layer-bottom layer. At this time, the characteristic impedance model of the top layer is a microstrip line model with the power plane as the reference plane.