Putting Signal Integrity in its Place

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In the past, tracks on a circuit board could be effectively considered as simple connections. Newer logic families and faster clock speeds have undermined this basic assumption.

Fast signal edge rates in modern digital designs and the introduction of electromagnetic compliance (EMC) regulations in many countries have been increasingly focusing the spotlight on signal integrity issues in board design. Where fast rise times are present, signal reflections along the board's tracks need to be carefully monitored and controlled to ensure both the proper functioning of the circuit and the minimization of electromagnetic interference (EMI). What's more, signal integrity issues need be addressed early in the design process to avoid significant rework of the final board.

In high-speed digital designs, board tracks cannot be considered as simple interconnections. When signal edge rates become comparable to the round trip delay time of the signal traveling along a track, transmission line effects come into play that can significantly alter a circuit's behavior. As a rule-of-thumb, transmission line behavior becomes important in a digital circuit when the track connection length in centimeters is greater than about eight-times the maximum rise (or fall) time in nanoseconds of the signal.
For typical TTL circuits, which have rise times of around 3ns, transmission line effects really don't come into play for track length under 24cm. However, the situation changes dramatically when we look at designs which use some of the fast logic families available today.

Consider a design that uses Emitter-Coupled Logic (ECL) devices. Regardless of clock speed, these devices can produce edge rise times of below one nanosecond. With these transients, track lengths of as little as 8cm can produce transmission line effects. GaAs devices can exhibit problems on track lengths less than 1cm!
As higher speed devices become more common, the need for distributed circuit analysis at the board design stage becomes crucial. Where fast edge rates are present in a design, careful analysis of board impedances is necessary to ensure proper termination of signal lines in order to minimize reflections and EMI, and to ensure signal integrity throughout the board.

Addressing Signal Integrity Early in the Design Process

To help designers cope with the added complexity of transmission line analysis on a PCB, EDA vendors are starting to include signal integrity analysis software as part of the standard board design toolset. Unlike circuit simulation, signal integrity analysis does not concern itself with the functional operation of the circuit - components are only modeled in terms of the I/O characteristics of their pins, without regard to the component's function. Connections between component pins are modeled using transmission line techniques that factor in the length of the trace, the characteristic impedance of the trace at the stimulus frequency, and the termination characteristics at each end of the connection.
Traditionally, signal integrity tools are designed to work with a fully routed board. While this gives accurate results because each individual trace length is known, it does mean that the analysis is performed quite late in the design cycle. One of the most common signal integrity issues that arises is signal degradation resulting from reflection caused by mismatched impedances of the pins at either end of a connection. The solution to this problem usually involves the addition of a termination resistor or R/C network to match the termination impedances and minimize reflections.

Figure 1. Output from a reflection simulation for a problem net on a board.

Figure 2. Same net after the insertion of an appropriate series termination resistor.

Using signal integrity simulation to uncover problems in a completed board design before any prototyping is undertaken can reduce the number of prototype iterations needed to complete a project. However, the addition of new components late in the board design stage can be a major problem, particularly on dense boards. The rework involved can be time consuming, partially negating the time saved in the prototype/test phase.
What's needed is the ability to detect and rectify potential signal integrity problems, particularly impedance mismatches, early in the design cycle.

Pre-layout Signal Integrity Analysis

Altium Designer includes a signal integrity simulator that can be accessed during both the design capture and board layout phases of a design. This allows both pre- and post-layout signal integrity analysis to be performed.
The signal integrity simulator models the behavior of the routed board by using the calculated characteristic impedance of the traces combined with I/O buffer macro-model information as input for the simulations. The simulator is based on a Fast Reflection and Crosstalk Simulator, which produces very accurate simulations using industry-proven algorithms.

Because both design capture and board-level design environments use an integrated component library system that links schematic symbols to relevant PCB footprints, SPICE simulation models and signal integrity macro-models, signal integrity analysis can be run at the schematic capture stage prior to the creation of the board design. When no board design is present, the system allows you set up the physical characteristics of the design, such as the desired characteristic trace impedance, from within the signal integrity simulator.
At this stage of the design process the signal integrity simulator cannot determine the actual length of particular connections, so it uses a user-definable average connection length to make its transmission line calculations.
By carefully choosing this default length to reflect the dimensions of the intended board, you can gain a fairly accurate picture of the likely signal integrity performance of the design. Potential reflection problems can then be identified and any additional termination components added to the schematic before proceeding to board layout.
As signal integrity issues become more prominent in board design, the ability to detect and correct potential problems before committing to board layout is becoming essential to minimize rework time and keep up with increasing "time-to-market" pressures.

Integrating Signal Integrity into Board Design Flows

Picking up potential signal integrity problems before layout is important in minimizing overall design time, but it needs to be used in conjunction with post-layout analysis to make sure a board is ready for manufacture. Extensive checking must be done on the finished board to ensure that the physical layout has not introduced problems of its own. Because exact trace lengths are known and the connection path, including vias and direction changes, can be analyzed in detail, post-layout signal integrity analysis can be used to not only examine reflections due to impedance mismatches, but also the interaction, or crosstalk, between physically adjacent tracks.

Figure 4. Interference from an adjacent net - crosstalk.

To be truly useful to a board designer, however, post-layout signal integrity must be integrated into the board design environment. If substantial time and effort is needed to export a design to a separate analysis tool, then the cost of the analysis needs to be weighed against the benefits. To reduce the costs, signal integrity needs to made part of the natural design and verification process.
With respect to board layout, signal integrity analysis is integrated directly into the PCB editor as an addition to the standard set of design rules. A board design can also be viewed and analyzed from within the design capture environment. Designers can set threshold limits for parameters such as undershoot and overshoot, edge slope, signal levels, and impedance values. Potential problem nets are then highlighted as part of a normal design rule check. If a signal integrity problem is detected, the designer can examine problem nets in more detail by performing reflection or crosstalk analysis to produce accurate waveform modeling of the problem nets.
In this way, setting up acceptable signal integrity parameters becomes part of the normal board definition process, much the same as defining minimum track clearances and widths. Identifying signal integrity problems caused by the physical layout then becomes a natural part of performing a complete design rule check on the finished board. This level of integration is necessary if the full benefits offered by signal integrity analysis are to be effectively and practically realized.

Realizing the Benefits

In designs that include high-speed logic elements, thorough signal integrity checking both before and after board layout can save significant time and money in the overall product design cycle. The benefits are twofold. Firstly, taking proper account of transmission line effects in signal traces at both the design capture and the board layout stage can increase circuit reliability by minimizing distortion along the signal path, thereby minimizing potential signal "glitches". Tracking down such problems after a board has been prototyped can be difficult and time consuming, and problems not detected during design could lead to low yield rates during construction.
A second benefit is that correct termination of high-speed signal lines not only ensures correct operation of the circuit, but it also minimizes the potential for board-level EMI and susceptibility problems. Apart from the direct costs associated with EMC testing and certification, the time and money costs of redesigning products that fail initial testing can be devastating, particularly if the failure results from poor board design.
Time-to-market pressures, stricter EMC rules and tighter circuit requirements are increasing the demands on board designers, making signal integrity analysis a necessity rather than a luxury in board design today. To fully realize the benefits that signal integrity analysis has to offer, however, the tools used and the process of performing the analysis must be fully integrated with and in line with existing board design flows, rather than being seen as separate additional stages to the current board design process.

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