PCB News - Common sense of high frequency PCB wiring (1)

Common sense of high frequency PCB wiring (1)

1. Choose PCB board

The choice of PCB board must strike a balance between meeting design requirements and mass production and cost. The design requirements include both electrical and mechanical parts. This material issue is usually more important when designing very high-speed PCB boards(frequency greater than GHz). For example, the commonly used FR-4 material now has a dielectric loss (dielectric loss) at a frequency of several GHz, which 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. Avoid high frequency interference

The basic idea of avoiding high-frequency interference is to minimize the interference of the electromagnetic field of high-frequency signals, which is the so-called crosstalk (Crosstalk). You can increase the distance between the high-speed signal and the analog signal, or add ground guard/shunt traces next to the analog signal. Also pay attention to the noise interference from the digital ground to the analog ground.

3. Solve the integrity of the signal

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.

4. Differential wiring

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.

5. How to realize 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. The wiring of the differential pair should be close and parallel

The wiring method of the differential pair should be close and parallel appropriately. The so-called appropriate proximity is because the distance will affect the value of differential impedance, which is an important parameter for designing differential pairs. 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 and timing delay.

7. 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.

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 it is possible to control the trace spacing of the differential pair, etc. 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, and so on. 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 loop gain and phase specifications. The oscillation specifications of this analog signal are easily disturbed. Even if ground guard traces are added, it may not be able to completely isolate the interference. Moreover, 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 ferrite bead 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 traces and PCB stacking to solve or reduce EMI problems, such as high-speed signals going to the inner layer. Finally, resistance capacitance or ferrite bead is used to reduce the damage to the signal.

10. About the test coupon.

The test coupon is used to measure whether the characteristic impedance of the produced PCB board meets the design requirements with TDR (Time Domain Reflectometer). Generally, the impedance to be controlled has two cases: a single line and a differential pair. Therefore, the line width and line spacing on the test coupon (when there is a differential pair) should be the same as the line to be controlled. The most important thing is the location of the grounding point during measurement. In order to reduce the inductance value of the ground lead, the grounding place of the TDR probe is usually very close to the probe tip. Therefore, the distance and method between the signal measurement point and the ground point on the test coupon Must match the probe used.