| 摘要: | 多層印刷電路板中的傳輸線相當多，有些傳輸線無法直接到達接收端，必須經過連通柱(via)來連結位於不同層的傳輸線以避開其它的傳輸線，這種via換層對高頻信號而言，將產生電感和電容的不連續，進而造成電磁干擾(Electromagnetic interference, EMI) 增加的問題。
本論文針對四層及三層電路板使用via換層時所造成的EMI電磁輻射進行探討，這包括一傳輸線具有兩個連通柱的三種情況以及一傳輸線具有六個連通柱的兩種情況。主要的目的在研究via的間距對S參數與EMI電磁輻射的影響。
理論方面，我們可以傳輸線上的差模/共模的概念來看待一對連通柱的效應，當兩個相鄰連通柱的間距很近時，因電流大小相同但方向相反，使其對電磁場的激發有抵消的效果，進而降低EMI輻射，當兩個相鄰連通柱的間距剛好等於半波長時，其激發後的電磁場，疊加後將變大。
實驗部份，我們使用長15cm、寬10cm的FR4板，用堆疊的方式製做四層印刷電路板的樣品，並在板上的傳輸線上引進兩個via或六個via，其via與via的間距離各不相同，接著使用網路分析儀分別對上述電路板樣品進行S參數（含S11與S21）的量測，並與模擬軟體HFSS的S參數模擬結果比較，兩者非常的接近。
吾人發現在30MHz~1GHz的頻率範圍，當via與via的間距越小時，S11顯示將比較沒有反射，而S21的穿透性也比較好；而當via與via的間距越大時，結果則相反。另外，將這些樣品置於3米全電波暗室內進行EMI電磁輻射量測，結果顯示兩via之間的距離越近時其電磁輻射量是確實是相對較低的，這和前面的理論預測一致。
一般四層印刷電路板的參考面為兩層，一為電源層，一為接地層，當via換層通過時，連通柱所形成的短偶極(short dipole)天線陣列將激發板間共振腔發生共振，進而對EMI電磁輻射產生貢獻，吾人發現，這種來自於板間共振腔所產生的板邊輻射洩漏為EMI的主要來源，這可經由製作具相同傳輸線layout的結構與總厚度相同的三層板樣品（只具有一個參考金屬平面），並進行量測比較予以驗證。最後，本論文亦探討了將樣品置於屏蔽盒後，並外接以電源線的輻射效應，與使用via wall於電路板樣品的四周邊緣以降低電磁輻射干擾的效果。
There are considerable transmission lines in a multi-layer printed circuit boards , while usually some transmission lines can not be connnected to the receiving end directly. Conversely, vias have to be employed to connect the transmission lines located in different layers in orderto avoid other transmission lines. These via transitions exhibit extra inductance and capacitance, especially for the high-frequency signals, which cause discontinuity for the transmission line, and furthermore cause electromagnetic interference (EMI) radiation to increase.
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In this thesis, the EMI radiation of several four- and three-layer circuit boards are investigated, in which there exist some via transitions through different layer. The samples tested includes one type of microstriptransmission line with two via transitions (three samples are tested), in addition to the other type of microstrip transmission line with six via transitions (two samples are tested). The main purpose of this study is
emphasized on the effects of the via distance upon the S parameters and the EMI electromagnetic radiation.
Theoretically, the effect of a pair of vias can be viewed in terms of the concepts of differential mode and/or common mode. Thus, when the spacing between two adjacent vias is small, the associated currents are nearly the same, but in the opposite direction, which in turn make the excited electromagnetic fields tend to cancel each other, thereby lowering EMI radiation. On ther other hand, when the spacing between two adjacent vias is exactly equal to half wavelength, their excited electromagnetic field would superimpose and enhance each other.
Experimentally, we pile up several FR4 boards of length 15cm and width 10cm to form four-layer printed circuit boards, on which two vias or six vias are introduced with different distances. The network analyzer measurements are carried out to measure the S parameters (including the S11 and S21) of the samples. The measurement results are then compared to the simulation results that are obtained by using acommertial EM software. The results compare quite well.
It is experimentally found that for the frequency range of 30MHz ~ 1GHz when the via spacing gets smaller the measured S11 shows less reflection, while the S21 results show more penetration. Conversely, when
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the via spacing get larger, the results show the opposite. In addition, when these samples are placed in a 3-meter anechoic chamber for the EMI test of electromagnetic radiation, the results showed that as the via distance get smaller, the electromagnetic radiation indeed get relatively lower, which is consistent with the theoretical preiction.
In general a practical four-layer printed circuit board would have two reference planes, one for the power plane, the other for the ground plane. When the vias pass through these planes the effective short dipole antenna array would excite the resonant cavity formed between the two referenec planes, thus the leakeage of electromagnetic energy contribute to the EMI radiation. It is found that thoseenergy leakage arising from the board edge of the resonant cavity is the the main source of EMI, which is verified experimentally in this thesis by the measurement tests of the three-layer boards with the same the transmission line layout and total thickness (only with a reference metal plane). Finally, the thesis also examine the radiation effects of the power cord as the previou PCB samples are put into a shielding box, while the the power cord is connected to the edge of the sample. The effects of usingthe via wall to reduce the electromagnetic radiation interference are also examined. |
