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I3: Incident, Instantaneous, Impedance

In my December 2013 column, I discussed “rooting out the root cause” and how sometimes, the real root cause is hidden when digging for the solution to a problem. In that column, I described how sometimes in an attempt to better correlate measured impedance with modelled impedance, fabricators were tempted to “goal seek” the dielectric constant to reduce the gap between predicted and measured impedance. 

To cite a common English saying, “There is an elephant in the room,” (i.e., something else that is contributing to the error that should be obvious but has perhaps been overlooked). And why is that? 

Well, maybe the elephant has been in the room for a while, but it started young and has now grown to a size that cannot be ignored. What is that elephant I am referring to, you ask? DC and AC resistance of the trace is a good possible candidate for an answer here, something that started very small and has grown gradually and perhaps imperceptibly, until now it is often too big to ignore.

R is the resistance per unit length, a combination of the DC and AC resistance, and the G is the conductance per unit length. Unlike the L and C, unfortunately, the R and G cannot be seen as dimensionless and the R piles up along the trace whilst the G stays close to 0 and the two do not divide out. This means that the TDR trace shows the effect of trace resistance presenting an upward slope as ohm upon ohm of combined DC and AC resistance “pile up” on top of the instantaneous impedance.

Without this knowledge, someone trying to analyze why the measured impedance is somewhat higher than the prediction may seek the source of error. The dimensions are hard to argue with if there is access to precision microsections, especially if three or four sections are taken along the trace and all correlate. So, it may be deduced that, given the dimensions are known and provided the impedance measurement system is calibrated and air line verified, that the only unknown is the dielectric constant. 

To read this column, click here.