Signal Integrity for PCB Designers
CHAPTER 11.1 Integrity of Point to Point Signal
When a driver launches a signal, it travels along the traces of the PCB. The traces that travel on the PCB must follow certain rules for proper operation at high speed. One of the rules they usually need to follow is that their trace width and their separation above the ground or power plane has a well defined value. These well defined values of the trace width and its separation from power plane make the trace a transmission line. A transmission line is called a microstrip if it is on outer layer. The transmission line is called a stripline, if it is lies on an inner layer sandwiched between metallic layers that can be power or ground layers. If the width of the trace is fixed, and its separation from the ground plane is fixed throughput the duration of the trace, its characteristic impedance stays fixed for the complete length of the trace. Keeping the characteristic impedance constant through out the duration of the trace is the key to preserving the distortion less signal transmission. Any discontinuity in the characteristic impedance makes a part of the signal reflect at the point of discontinuity. The addition of via, layer changes, breakout from BGA, change in trace thickness, addition of test point are some reasons that could lead to change in the trace characteristics leading to change in the transmission line impedance. These impedance discontinuities degrade and distort the signal received at the receiver. During it travel on the PCB the signal may also come across the discrete components, which used to ensure proper signal quality. It is the responsibility of the PCB designer to ensure proper placement of these component and proper routing of the traces connecting these components.
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