“Which is correct—the modelled or the measured result?” A colleague posed this question to Dr. Eric Bogatin at a Polar conference many years ago. To my colleague’s evident surprise, Dr. Bogatin replied, “Neither.” Read on to find out more.
To a mechanical engineer used to laser precision in mechanical measurements, the world of high-speed electronics can seem somewhat alien. Likewise, to an electronics engineer who has inhabited a low-frequency world in a previous life and is suddenly exposed to high-speed digital, the world of ultra-high-speed serial communications can seem uncomfortably imprecise. DC voltages can be measured to many significant digits with a high degree of precision. Mechanical dimensioning in the laser age seems, and is, incredibly precise. But the world of high-speed digital signalling is less about absolute ones and zeros and more about massaging pulse shapes to squeeze them at the highest possible data rate down channels determined to squash and erode their carefully shaped waveforms.
In this woolly world where high-speed signals enter a transmission line with a well-defined shape and emerge at the receiving end eroded and distorted—and at the limits of interpretation by the receiver—it is well worth running simulation to look at the various levers that can be figuratively pulled to help the pulse arrive in a reasonable shape. At speeds up to 2 or 3 GHz, it usually suffices to ensure the transmission line impedance matches the driver and receiver. And a field solver makes light work of the calculation—a little juggling with line width and dielectric substrate height will have your signals arriving in good shape. But push the frequency higher, and other factors come into play. At this point, it makes good sense to run multiple simulations and ultimately test against measurements.
Whilst on the subject of loss tangent, for many PCB fabricators, it is one of those “mystery” characteristics that isn’t easy to visualise or measure. The simplest way of thinking of loss tangent is to look at it as the ability (albeit undesired) to turn precious RF energy into heat. It’s excellent if you are designing microwave ovens, but not so helpful if you are attempting to transmit small amplitude high-speed signals from point A to B along a PCB transmission line. Because it is a tricky thing to measure, it’s no surprise that there are a variety of measurement methods and some more appropriate to some applications than others.
Split post resonator methods, for example, are ideal for bulk measurement of loss tangent when manufacturing base materials. When choosing a value of loss tangent for use in signal integrity applications, you will get best results if you use a value derived by using transmission line techniques. The loss tangent in a data sheet may have been measured in a variety of ways depending on application and frequency of measurement (most data sheets note this), but if in doubt, ask.
To read this entire column, which appeared in the June 2019 issue of Design007 Magazine, click here.