New Column: The Pulse


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Heart rate monitors arrive for refurbishment in Polar's office on an almost weekly basis, despite our having no connection with the Finnish company well known for its fitness monitors.

Ralph, our IT Manager, technical author and Webmaster, is quite concerned. "Think of all those people with heart problems who sent their monitor to the wrong place for repair." A colleague replies, "No, Ralph, you misunderstand, only fit and healthy people use heart rate monitors."

Where is this story going? Well, the same could be said for signal integrity. And this new column aims to highlight the smoothest way to transition quality designs from prototype through to production from a signal integrity perspective. In the world of signal integrity, only the best designers and fabricators measure, model, then measure again to ensure the highest quality predictable signal integrity in their designs and products.

I'll be exploring a number of themes over the coming months: For example, a major topic that is set to grow and grow is the emergence of new silicon families designed to push traditional materials into the multi-gigahertz arena. These new chipsets lift transmission speeds up to a point where signal losses rather than reflections become the predominant concern from an SI perspective.

Another of my favourite subjects is digging for the root cause when analysing measurement and modeling conflicts, something that can be applied to any engineering challenge. In addition, I'll be introducing a range of practical application notes and stories that ensure that the designer's original intentions do not get lost as the design flows from design to prototype and onwards, possibly through a broker, possibly out to a volume fabricator in Asia.

Expanding a little on the topic of transmission line losses, it is worth thinking back to a previous generational shift in high-speed considerations for PCBs; way back in the early 1990s I can vividly recall the emerging need to consider controlling the characteristic impedance of PCB tracks, something we now casually call "controlled impedance" or "impedance controlled." What happened at that time to make PCB transmission line impedance a driving factor in delivering the high-speed boards of that era?

The driving force was the arrival of chipsets with sub-nanosecond switching times. Any interconnect that is long enough at a given speed will exhibit transmission line characteristics and the predominant characteristic of interest will depend on a mix of factors: The line length, the switching speed, the conductor characteristics and the dielectric properties of the insulator carrying the conductor. In 1990s technology, speeds were high enough and traces long enough that without correct impedance matching, reflections could become a significant hindrance to error-free high-speed performance.

Were there losses in 1990s era PCB signal paths? Yes, but in the majority of cases the losses were small enough to be ignored. Roll the clock forward to the "noughties" and a whole variety of logic families is succeeding in running at much higher speeds on the same basic interconnect.

Now, though impedance matching is still important to ensure maximum energy transfer into the trace, designers need to be interested not so much in how much energy will reflect at the receiving end, but if there will be any energy left to convey the signal at all!

At multi-gigahertz speeds, the transmission line materials sap away energy, and the core and prepreg will lose energy as heat, with the loss tangent being the measure of how much heat energy is wasted in the dielectric. From the conductor perspective, the skin effect results in the available copper conductor area for signal transmission being significantly reduced; again, the resistance of this small area of copper for signal transmission results in heat losses.

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Figure 1: Loss graph for 30-inch line.

The fundamental construction of the transmission line and its characteristics have not changed; simply, at these higher speeds some characteristics that had a second-order effect at lower speeds now have first-order effects, while other first-order characteristics become second order. Designers and fabricators need to work together to produce the most cost-effective and reproducible designs by stirring all the driving factors into the optimal mix for both profitability through the supply chain and fitness for the end-user application.

An intimate knowledge of the interplay between the driving factors and a willingness to communicate along the supply chain will ultimately result in more predictable and repeatable yet cost-effective design. It goes without saying that careful modeling and measurement are key players in ensuring the health of your finished product.

And finally, if you are about to ship us a fitness monitor for repair--please, please send it to the "other" Polar!

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