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PCB’s top ground plane and its effect on a microstrip line’s characteristic impedance

-February 17, 2013

Introduction

When designing microstrip circuits, several precautions have to be taken to ensure proper grounding and transmission line impedance.  The structure shown in Figure 1 was analyzed while varying the substrate thickness, h, the gap between the microstrip line and top layer ground plane, G, and the relative dielectric constant of the substrate, εr .  Two questions are addressed in this study; does the impedance stay fairly constant from IF to RF frequencies, and what effect does the gap between the microstrip line and the top ground plane have on the transmission line’s characteristic impedance, Z0?

Figure 1  A typical microstrip line scenario.

 

Frequency dependence of a microstrip line with a top ground plane 

The first case investigated had the following parameters:                                     

 

The parameters given above should correspond to a microstrip line with characteristic impedance, Z0 of 50 Ω. Figure 2 shows the relationship between the transmission line’s characteristic impedance, Z0 , and the operating frequency.  The lowest frequency analyzed in this study was 100 MHz.  The variation of Z0 between 100 MHz and 2 GHz is only 0.7 Ω, which is negligible for most practical applications.  The 0.7 W change in the transmission line’s impedance would result in a VSWR of 1.014, or a reflection coefficient S11 of -43.2 dB.  However, when the gap G in Figure 1 is varied from 10 mils to 40 mils (0.5 to 2 times the microstrip line width, W) the change in characteristic impedance Z0 is 1.9 Ω.  The largest variation in Z0 occurs when the gap, G, becomes significantly smaller than the line’s width, hence resulting in coplanar waveguide mode.


Figure 2. Microstrip impedance as a function of frequency for different PCB top-layer ground gaps for a thin PCB substrate (10 mils).

 

The second case analyzed had the following parameters:



Here, the substrate’s thickness, h, was increased from 10 mils to 60 mils.  This case had higher transmission line impedance since more of the field lines are concentrated in the substrate and the effective dielectric constant is somewhat higher, which is shown in Figure 3. 

The characteristic impedance varies from 78 Ω to 104 Ω when the gap G is varied from 10 mils to 40 mils.  Therefore, varying the gap, G, between the microstrip line and the top ground plane has a greater effect on varying the desired transmission line impedance than the variation caused by increasing frequency of operation. 

For example, the characteristic impedance Z0 of the transmission line differs by 26.5 Ω at 2 GHz when a gap of 10 mils is compared to a gap of 40 mils.   This great variation will result in a substantial VSWR and correspondingly bad return loss.


Figure 3. Microstrip impedance as a function of frequency for different PCB top-layer ground gaps for a thin PCB substrate (60 mils).

 


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