Signal Integrity for PCB Designers - Introduction
CHAPTER 1Hiring a signal integrity engineer is expensive. Not doing signal integrity analysis can be more expensive.
1 Introduction
Signal integrity refers to the quality of the signal that needs to be maintained for the receiver in an electronic design to deliver its intended goal. As the speed of the data signal increases a number of reasons lead to the degradation of the high speed signals. High speed PCB design refers to the techniques that must be followed in order for a circuit on a Printed Circuit Board to function properly when the edge rate and data rate of the signal propagating on the PCB is high. A high edge rate also, in general, implies a high frequency. The failures in high speed digital circuit are often not readily reproducible. They are often difficult to diagnose, reproduce and fix. This is unlike the errors in the schematics and layout stage, where, it is possible to check the design using simple rules and instruments. High speed design failures show up as failures at higher operating frequency, data error rates, cross talk errors and EMI errors. The debugging of high speed related errors may need expensive instruments, e.g., high bandwidth oscilloscope, spectrum analyzers, time domain reflectometer, to detect and understand the failure mechanism. Therefore, care must be taken at the design stage itself to ensure that the design is in accordance to high speed design rules.
Modern day examples of the high speed signals include DDR Bus, HyperTransport Bus, DDR2 Bus, USB 2.0, SATA, PCI Express, PCI, Gigabit Ethernet etc. High speed design techniques must be applied to the printed circuit boards containing these and other high speed signals to ensure proper and reliable operation.
During the last two decades we have seen a lot of obsolescence in electronic consumer products for want of more processing power, higher speed and better specifications. Whether these are computers, servers, PDAs, professional cameras, or other handheld devices– they have planned obsolescence either due to competitive market requirement or due to the availability of the newer products at similar price. A product can fetch a good price only if it uses state of the art technology. The time duration for which a product can fetch higher price is becoming shorter. From designer point of view this reduces time available to them to bring new design. These two factors – small span of time available and the ever increasing data and clock rate demands that the PCB designer as well as manager must understand the risk involved due to failure in high speed design area.
A single mistake from a PCB design engineer can render a design useless. A single short in power and ground plane will not only render the design useless, it will shift the schedule and derail the business prediction. Not following high speed design rule may also render a design useless with one additional disadvantage – it is difficult to diagnose, difficult to reproduce and on the top of that, difficult to fix. An understanding of the design rules will help PCB designer get a head start even while signal integrity expert prepares a detailed design rules and checklist.
The geometry of normal PCB trace has no substantial consequence on the signal quality if the signal speed is arbitrarily low. At high speeds of signal – the values of resistance, capacitance and inductance of PCB traces need to be considered for analyzing the signals. We can no longer treat capacitors used to filter power supply as pure capacitors. Their series inductance as well as the inductance of the trace connecting the capacitor play important role in shaping the integrity of the signal.
PCB designers have to consider five different aspects of design when dealing with a high speed PCB design. These five broad aspects are
1. Integrity of point to point signal
2. Timing relationship between signals
3. Coupling of traces
4. Noiseless power supply
5. Electromagnetic radiation
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