Signal Integrity Analysis for Military PCBs


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Like many companies, until Peralex Electronics began designing high-speed differential serial links, we didn't worry too much about signal integrity (SI). We used a good set of rules of thumb, coupled with a good deal of experience and manual calculations.

This seemed to make all the products work, although we knew that we might be spending extra money and time over-designing so that every circuit worked.

Incorporation of high-speed serial links into many of our products has changed that design "methodology" into something much more formal--and more reliable than rules of thumb.

Military Electronics Requirements

Peralex produces high-performance wideband radio receivers, analog-to-digital converter boards, DSP-based processor boards and signal processing and analysis software to drive this hardware. Together with products from partner companies, these are combined to create wideband radio spectrum surveillance, direction finding and signal analysis equipment.Other products include sonar signal processing hardware and software for use in mining, prospecting and other applications and audio signal processing systems.

Despite the wide array of products, they share one thing in common: They must operate in noisy environments and be as close to 100% reliable in those conditions as possible. Often, the environments are not just noise, but are filled with intentionally-generated noise, specifically designed to purposefully interfere, distort and overwhelm the signals that are of interest. Many of our products find their way into military applications and become the brunt of electronic warfare and intense EMI.

There are many tactics used to eliminate interference and isolate the signal of interest. These include shielding, of course, but use of state-of-the-art digital signal processing and filtering is key in making the products work.

As data rates have increased at the same time that operating voltages have decreased, the problem of keeping signals clean has multiplied. Now, with high-speed serial links dominating designs, those "rules of thumb," mentioned above simply cannot produce the designs required for 100% reliability.

Product Design

With electronic warfare products, and as you might expect, the populated PCBs often push the limits of speed, density and low power consumption. Our products are typically housed in 19" 2U, 4U, and 8U form factors. The PCBs are often very densely populated, and it is not uncommon to have multiple FPGA packages to add to the complexity (Figure 1).peralex fig1.jpg

Figure 1: Densities this tight, running at high speed, require signal integrity analysis to save development time and re-spin costs.

Maintaining signal integrity so that a DSP can work efficiently and correctly is of extremely high priority. With the increasing prevalence and edge rate of high-speed serial links integrating a SI tool into our design process was essential--some of the serial links run as fast as 3.5 Gbps.

A Three-Part SI Strategy

When we begin a PCB design, we first spend a time defining the board stackup with impedance control in mind. The proposed stackup is analyzed for signal integrity and any modifications are made right then, before layout begins. This foundation is what we apply to each PCB layout.

Impedance control is critical in low-level signals that these products typically deal with. Any impedance mismatching can result in reflections, distortions, crosstalk and data errors. And, impedance control is not just critical on the signal paths: The power distribution network (PDN) must also be impedance controlled to ensure that supply voltages remain in-spec at every single IC connection.

We now apply a complex set of design rules prior to the actual design of any PCB. These rules have evolved over time to enable us to produce a PCB on which we can rely for processing the signals within a potentially noisy environment.

Using these rules, the boards are autorouted as much as practical. Once routing of the PCB is completed, a first-pass SI analysis for crosstalk and other coupling is run. We run the SI tool across the entire board and generate a report that identifies problem signals. Using this report, which very specifically identifies signal problems, we then spend time correcting crosstalk problems.

peralex fig2.jpg 

Figure 2: Crosstalk reports allow quick identification of problem areas so that they can be fixed early in the design.

Often, these problem areas require hand-layout to get the precise control needed to ensure the reliability that is required. Then, we re-run the SI analysis. We do this not only to verify that we have solved the problem--and sometimes it takes more than one iteration--but also to ensure that the solution has not generated a new problem elsewhere. It's not unusual to take several iterations to clear all the identified SI problems.

Since we chose SI analysis over straight rules of thumb, our time needed to identify and rectify SI problems has decreased dramatically. We eliminated the secondary problem of generating new SI issues every time we fixed one. Plus, when we finish the analysis, we know for certain that we will not have signal integrity issues with the completed product.

We have found that using rules of thumb often results in one or more of the following outcomes:

1) The board is over-designed, leading to a larger board form factor;2) The design has more layers than necessary;3) The design features randomly placed bypass capacitors, and often more caps than necessary; and4) Any combinations of the above.

From a design efficiency standpoint, using SI analysis wins hands-down. On the other hand, if an SI problem is identified, the rules of thumb provide little clue as to where to look for a solution. With SI analysis tools, the problem areas are precisely identified and the solutions are generally obvious.

The analysis tool of choice should, of course, be fully compatible with the other PCB tools that your company uses. In our case, using Mentor Graphic PADS, it was a natural choice to acquire HyperLynx, although HyperLynx will run with other design tools too. 

Conclusion

Going forward, there just is no choice for those of us in our industry segment: signal integrity analysis is essential. The switching speeds, low-voltage and multiple thresholds, and required data integrity dictate that impedance must be accurately controlled.

Processing and analyzing multiple signals approaching gigabit (or higher) rates requires specific tools that can deliver fast, accurate results. When coupled with other PCB design tools, a methodology similar to the one we have adopted produces working circuit boards quickly and with fewer, if any, respins.

And this three-part method for identifying and correcting problems that can introduce nightmarish effects onto high-speed signals is just as applicable in industry segments other than the military and communications.

 ByGlen Thiele, Peralex Electronics (Pty) Ltd.

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