Signal Integrity-How To Do It In 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 become prominent. At high frequencies the other factors that affect signal health. PCB material signal absorption. Extension of signals from the signal carrying tracks, rise and fall time of the signal, overshoot and undershoot at signal edges etc.


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1.Signal integrity For a good PCB design

2.Signal integrity In PCB layout and design

3. Signal integrity issues are resolved at the very first level of circuit design and PCB layout.


1.Signal integrity For a good PCB design

By the signal analysis point of view on the PCB, one should note the maximum frequency. The corresponding rise and fall times of the signal. Inputs and outputs at the signals sourcing and sinking ends respectively. And single and differential ended signals, analog and clock signals.


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. Make thickness of power supply and ground tracks.


2.Signal integrity 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 both signals of the differential ended signals, route differential ended signals side by side.


3. 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.


An IC package or PCB designer removes signal integrity problems through these techniques:

• Placing a solid reference plane adjacent to the signal traces to control crosstalk

• Controlling the trace width spacing to the reference plane to create consistent trace impedance

• Route traces perpendicular on adjacent layers to reduce crosstalk

• Providing sufficient ground (and power) connections to limit ground bounce (this subdiscipline of signal integrity is sometimes called out separately as power integrity)

• Adding a preemphasis filter to the transmitter driving cell

• Improved clock and data recovery (CDR) circuitry with low jitter/phase noise

Each of these fixes may possibly cause other problems.


This type of issue must be addressed as part of design flows and design closure.



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