博文 | USB 4 高速信号布局关键因子 (三 ) : Via Stub

 

Via Stub

Via 本身有电容(Capacitive)及电感(Inductive)效应. 倘若用 PATH (Plated Through-Hole) 贯串性Via 会产生stub (残端)效应, 产生反射波,影响SI(Signal Integrity).

General Rules :

         10GBd NRZ (Nyquist Freq. <= 5GHz)   allow  < 30 mils stub

         20GBd NRZ (Nyquist Freq. <= 10GHz)  allow < 15 mils stub

         20GBd NRZ or PAM4 (Nyquist Freq. > 10GHz)   allow < 10 mils stub

 

Via with long stub

 

Via with short stub

 

Via Stub- Back Drill

利用Back-Drill (背钻)来移除stub，使得差动阻抗不会有所偏差，因此可以减除信号的反射波，由此方式大大改善SI 的品质。