When handling the wiring density in attention to what issues?

[sofiatian]

Q: At a fixed board size, if the design need to accommodate more functionality, they often need to increase the density of PCB traces, but this can lead to mutual interference traces enhanced while the traces are too small to make impedance can not be reduced, may I ask what skill at high speed (> 100MHz) high density PCB design?

 

A: in the high-speed high-density PCB design, the crosstalk (crosstalkinterference) really should pay special attention because of its timing (timing) and signal integrity sex (signalintegrity) has a great influence. Here are a few caveats: 1. Continuous control go and match the characteristic impedance of the line. 2. trace spacing size. General often see a pitch twice the width. Can Through simulation to know trace spacing on timing and signal integrity, and to find the minimum distance can be tolerated. Results of different signals may be different chips. 3. Select the appropriate termination method. 4. Avoid vertically adjacent layers of the same alignment direction, even just walking up and down the line overlap together, because this case crosstalk traces larger than the layer adjacent to the same. 5. The use of blind and buried vias (blind / buriedvia) to increase the alignment area. However, pcb board production costs will increase. In the actual implementation does not perfectly parallel to the same length, but still want to try to do. In addition, you can reserve a differential and common mode termination termination, to ease the impact on the timing and signal integrity.

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On the selection and wiring hybrid circuit PCB material ：

 

Q: In today's wireless communication device, a radio frequency part often using small outdoor unit structures, while the RF part of the outdoor unit, the IF section, and the outdoor unit to monitor low-frequency circuit section often deployed on the same PCB. Will such a PCB layout on the material requirements? How to prevent interference with radio frequency, intermediate frequency and low frequency circuits between each other?

 

A: hybrid circuit design is a big problem, it is difficult to have a perfect solution. Usually RF circuits in the system are carried out as an independent board layout, and even have a special shield cavity. And RF circuits are generally single-sided or double-sided, the circuit is simple, all of which is to reduce the impact on the distribution parameters of the RF circuit, improve the consistency of RF systems. With respect to the general FR4 material, RF board and the substrate tends to use high Q value of the dielectric constant of this material is relatively small, distributed capacitance smaller transmission line impedance is high, small-signal transmission delay.

 

In the hybrid circuit design, although the RF, digital circuitry to do in the same piece PCB, but are generally divided into the RF circuit area and the digital circuit region, respectively layout. Between grounding and shielding box via tape shield.