Printed Circuit Design & Fab - January 2008 - (Page 20)

Effects of Routing High-Speed Traces Close to the PCB Edge To control emissions, high-speed signals should not be routed near the edge of the ground-reference plane. IT IS OFTEN difficult to route all the required traces on a PCB. There is a constant goal to keep costs at a minimum and so the number of layers available for routing is very limited. This can force designers to route traces too close to the edge of the PCB. Routing too close to the edge of the PCB can increase emisDR. BRUCE sions from the edge of the ground-referARCHAMBEAULT ence plane. Controlling these emissions is simply a matter of not routing highspeed signals near the edge of the ground-reference plane. wg = the width of the ground-reference plane (set to 1000 mils always) d = the distance from the trace to the edge of the ground-reference plane in mils h = the height between the trace and the ground-reference plane in mils Equation 1 is plotted in FIGURE 1 for a microstrip with a variety of different trace heights and distances to the edge of the ground-reference plane. The amplitude of the current along the edge of the ground-reference plane decreases as the distance between the trace and the edge increases. Also, when the trace height is further from the reference plane, the current along the edge is higher. Amplitude of Current Along the Edge of the PCB Ground-Reference Plane In the case of both microstrip and stripline, the majority of return current will flow in the plane nearest to the trace. However, some of the return current will spread away from the trace to find the path of least impedance. As the trace is routed further from the edge of the plane, less current is along that edge. However, it can still be significant for EMI/EMC purposes even when too low for signal integrity concerns. EQUATION 1 gives the formula to find the current along the edge of a ground-reference plane for a microstrip transmission line1. The currents on each reference plane for a symmetrical strip line are simply one-half of those in Equation 1. Impact with Real-World Signals Let’s consider a typical frequency spectrum for a clock signal’s current. In this case, we’ll assume the current waveform follows the voltage waveform, as in the case of a resistive termination for this clock net. Of course, if a nonresistive termination is used, the frequency spectrum will be different. FIGURE 2 shows the frequency spectrum for a 200 Mb/s square wave with a 1 ns rise and fall time. For this case, the 5th harmonic occurs at 900 MHz, and its amplitude is approximately 40 dBuA (100 microamps). If the trace is 50 mils from the board edge, and the trace height is 10 mils from the plane, then the edge current will be expected to be approximately 60% of the trace current, or 60 microamps. However, if the trace is moved to 0.5˝ from the edge, the edge current falls to 6% of the trace current, or 6 microamps. I Edge = ISignal π wg − 2 (wg / 2) − d − arctan π 2h 2 {{ [ ] } EQ. 1 where: Isignal = the current amplitude (in linear scale) for each harmonic of the current waveform. How Much is Too Much? It is very difficult to calculate the expected far field radiated emissions without knowing specific information about PCB, including its size, shielding metal enclosures, etc. However we can consider that the edge of the reference plane will be an efficient radiator 100 Current Amplitude (dBuA) 80 60 40 20 0 -20 -40 -60 0 200 400 600 800 1000 1200 1400 1600 1800 2000 Frequency (MHz) 1.0 Current (A) 0.1 3 mils 5 mils 7 mils 10 mils 15 mils 20 mils 25 mils 30 mils 35 mils 0.01 0.001 0 100 200 300 400 500 600 700 800 900 1000 Distance from Edge (mils) FIGURE 1. Edge currents vs. trace distance to edge and trace height (normalized to 1 amp trace current). 20 FIGURE 2. Typical clock current frequency spectrum, 200 Mb/s and 1 ns rise/fall. JANUARY 2008 PRINTED CIRCUIT DESIGN & FAB

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Printed Circuit Design & Fab - January 2008

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