背接金屬共平面波導之共振頻率電磁波輻射分析及改善

Abstract

在EMC設計規則之中，有一在訊號線之相鄰的Ground Plane上依照最高頻率的二十分之一波長打貫穿孔(via)，這可以預防訊號線的頻率耦合至共平面的地所產生的EMI問題。另外，若訊號剛好為PCB結構之共振頻率時，訊號會被這樣共振的機制帶走部分能量而造成訊號品質下降，而被帶走的能量也因共振機制輻射出更大的能量，造成更嚴重的EMI問題。 本文將用一背接金屬共平面波導結構來探討此問題，背接金屬共平面波導在上下地面所形成的平行平板結構激發TEM波，而使得印刷電路板裡產生不必要的耦合及串音。為了分析此問題，本文利用HFSS來模擬並實際做出一PCB板來量測EMI輻射。由模擬與實作中可發現當慢慢的增加兩旁地平面的貫穿孔，介電層中的電場與電流密度愈不易擴散至兩邊板邊開路形成輻射，大部分的電場能量會被鎖在訊號線與貫穿孔之間；而貫穿孔的增加也改善S21使得共振頻率往更高頻移動、插入損耗亦更低。

另外，由實作中發現在共振頻率上的輻射能量會大於非共振頻率約4-8dB，最大可達20dB；而當貫穿孔間距小於或等於等效波長之五分之一至十分之一時，即能達到減少輻射能量約5-10dB，這可避免λ/20 rule造成內層電源區塊破碎並擁有更大的走線空間。

There is one EMC design rule that the ground plane on the same plane as signal traces must be connected to the ground through vias and the distance between adjacent vias should not be longer than 1/20 the wavelength of the highest frequency of concern to prevent EMI problem caused by signal coupling. On the other hand, signal quality will be reduced while a lot of signal energy are taken away when the signal frequency is the same as the resonant frequency of PCB structure, and this lost energy will radiate into the air and cause more serious EMI problem.

The conductor-backed coplanar waveguide (CBCPW) structure was used to discuss this issue, CBCPW could excite TEM wave in the parallel-plate waveguide formed by the upper and lower ground planes and induce coupling noise and cross-talk. To analyze this problem, we do the simulation and use real PCB to measure EMI noise. We find that when the vias were added sequentially, more and more electric field and surface current density in the dielectric layer will be confined between signal trace and via and improve the S21curve and the resonant frequency will be higher, and the insertion loss will be lower.

By the way, we found the emission energy of resonant frequency was larger about 4-8dB than the frequency of non-resonant, the biggest up to 20dB. With via adding, it can improve 5-10dB emission noise at the same frequency. Finally, we found that if the via distance is less than 1/5-1/10 effective wavelength of the highest frequency, it can reduce 5-10dB emission noise, that can save the space for layout and block/area of inner layer.