The Magic of Impedance Paper

EMC engineers are constantly using passive components. Calculating a RC corner frequency, L-C resonant frequency or characteristic impedance, though typical first-year student tasks, are nonetheless a necessary part of the EMC engineer’s responsibilities. Very quickly, the tedium becomes mind numbing to the point of irritation!

Alas, there is a quick and easy solution, impedance paper. The exact origin of impedance paper is long lost to history, but sometime in the past an inventive soul endeavored to plot on log paper impedance, capacitive reactance, and inductive reactance. A section of the result is shown in Figure 1. The convenience of reading values from a graph is truly a liberation from tedium; the resonant frequency of an arbitrary inductance and capacitance is found from the graph at the intersection of their curves.

Figure 1

For example, 10µH and 10pF resonates at about 14 MHz, with a characteristic impedance of 1,000 Ω. Need a lower resonant frequency? Slide down to the 100 pF curve and find the resonance of 5 MHz, with a characteristic impedance of about 300 Ohm (shown in Figure 2).

Figure 2

Even greater insight is given into the synthesis problem. Need a filter with a 100 Ω characteristic impedance and 100 kHz resonance? The closest L and C values are easily read by inspection. Even better, combinations of values closest to standard values can be evaluated for the closest solution. This can save the effort of many calculations.

Subsequent articles will address using impedance paper for capacitive and inductive filter attenuation calculations. Graphical calculations are faster and easier, often generating the intuitive insights needed to develop a trial solution for subsequent computer analysis.

1. Powerline Filter Design for Switched Mode Power Supplies, 2nd Ed. 2010. Mark Nave Consultants.

Mark Nave Mark Nave entered EMC at Don White Consultants.  Since then, he was worked at Raytheon, E-Systems, Network Appliance, Cisco, and was the principal at EMC Services, Inc., a training and consulting company focusing on SMPS EMC. He is currently performing contract work, and can be reached at (352) 562-5000.