Analysis of transmission characteristic of a microstrip line placed above a ground slot

Recently, electric circuits may be become small, and also high-density electrical technologies have advanced. In order to place many elements on the small printed circuit board (PCB), a high-density multi-layered PCB may be used. Many vias are used in such multi-layered PCB to connect lines on each layer. If many vias are placed in narrow area, they make ground slots. If a ground slot exists near a transmission line, the distortion of the signal may be occurred. Then, existence of slots is issue for high-speed circuit designs. In this paper, we analyzed the transmission characteristic of a microstrip line above a ground slot on a PCB using multiconductor transmission-line (MTL) method. First, we measured it for various slot width. And also, we analyzed it using FDTD method. Comparing the results of measurement value and analysis value, a validity of the model of the FDTD method was confirmed. Next, we derived the characteristic impedance of the microstrip line using spectral domain approach (SDA). We calculated the transmission characteristics of the microstrip line above the slot by applying MTL method. And, the analysis values of MTL and FDTD are almost same in the case of small slot up to high frequency.