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Signal Integrity and PCB Design

Signal integrity is a set of quality measurement of signal parameters at the transmission and receiving end. Signal integrity is applied both on IC chip design and PCB design domain. We talk about signal integrity in PCB domain. In digital systems a signals transmission is in binary from. The signal integrity is the problem that arises in digital electronics domain but is solved in the analog domain. The digital signals can be transmitted and received over small distances without losing shape and level. However, at high frequencies, low voltage signal levels, long distance signals traveling can affect the electrical signal. As the signal frequency increases the effect of parasitic inductance and capacitances become prominent. At high frequencies, other than parasitic, the other factors that affect signal health are PCB material signal absorption, propagation of signals from the signal carrying tracks, rise and fall time of the signal, overshoot and undershoot at signal edges etc. The signal integrity issues are resolved in analog signal analysis, PCB design and fabrication, PCB material selection, filtering circuit analysis components selection and EMC/EMI etc.

For a good PCB design, by the signal analysis point of view on the PCB one should note the maximum frequency and the corresponding rise and fall times of the signal, inputs and outputs at the signals sourcing and sinking ends respectively, single and differential ended signals, analog and clock signals, steady or low frequency and high frequency signals, impedance control, signal termination and propagation delay of signals on the track.

By power supply point of view the number of power supplies and their levels and the required current, VCC sag, types of decoupling capacitors network, return or ground path and the ground bounce, thickness of power supply and ground tracks.

In PCB layout and design, the digital, analog and power supply components should be placed apart and appropriately. The analog and digital grounds should be separately managed and connected at a single thin point. The power supply plane should be split properly. The signal layers should be routed at right angles to each other so that minimum crosstalk between signals would be minimized. Avoid right angle traces to reduce the radiation, separate the analog, high speed digital and clock signals, provide sufficient clearance between two tracks to reduce signal crosstalk and interference, avoid vias whenever possible in the single ended tracks, avoid vias in differential signals, if necessary, add same number of vias in the both signals of the differential ended signals, route differential ended signals side by side.

Signal integrity issues are resolved at the very first level of circuit design and PCB layout. Once the PCB is manufactured the signal integrity issues are very hard to resolve and even may require the next design review and manufacturing.

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