Detailed analysis of the importance of coupling capacitors in high-speed circuit design (III)

Finally, solve the above two problems, and then analyze the optimal design of this capacitor from the PCB design. Actually in the project, compared to the visible factors such as the location and capacitance of the AC coupling capacitor, the more elusive is the plate itself (including the accuracy of the pad, the uniformity of the copper foil, etc.) and the parasitic capacitance at the pad. The effect of signal integrity. We know that high-frequency signals must travel along a path with uniform characteristic impedances. If impedance mismatch or discontinuity is encountered, some signals will be reflected back to the transmitting end, causing signal attenuation and affecting signal integrity. In the project, this situation usually occurs at the pad or board connector. This problem was often encountered when the author first addressed the high-speed circuit design.

To solve this problem, we must start from two aspects. First of all, in the selection of the plate, we usually use high-performance ROGERS plate in the application. Rogers plate is very precise in the control of the thickness of the copper foil, uniform copper foil coverage greatly reduces the impedance discontinuity; then in the elimination of welding A common solution in the parasitic capacitance of the disc is to perform interlayer processing at the pad (hollowing out the reference plane area directly below the pad, and creating a copper padding on the inner layer) by increasing the pad and its reference plane ( Or is the distance between the return paths, reducing the discontinuity of the capacitance. In the author's project, the use of ROGERS plates with uniform medium and precisely controlled copper foil width also effectively improves the processing accuracy of the pad.

Compare the signal integrity of the ROGERS plate before and after precise spacer treatment by simulation.

It was found that the jump in impedance before treatment was obvious, and the impedance after the interlayer treatment was improved much, with almost no steps or discontinuities.

In summary, if you want to get the “fatal” AC coupling capacitor in high-frequency circuits, not only do you have to do homework on the design of the circuit, but at the same time, choosing a high-frequency PCB board material with better performance will allow you to do more with less.