Measuring PCB, Cable and Interconnect Impedance, Dielectric Constant, Velocity Factor, and Lengths

Controlled impedance printed circuit boards (PCBs) often include a measurement “coupon”, which typically includes sample traces, 6 inches long and constructed as part of the printed circuit board panel.  They are measured to assure the PCB stack-up and the accuracy of the PCB transmission lines. These coupons, as well as the actual PCB signal traces, interconnects and cables are typically measured using a Time Domain Reflectometer (TDR).  A TDR is generally a large, expensive instrument that includes a high-speed edge pulse and a sampling oscilloscope.

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Geoffrey Hazelett – Impedance and Polar Instruments Tools

aDuring PCB West, we met with Geoffrey Hazelett to discuss one of the most important PCB topics: impedance. Vice President Sales at Polar Instruments, he also explained the importance of understanding electromagnetic models, solutions of Maxwell’s equations in a two or three-dimensional environment, etc., and shared some insight on PI’s tools.

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How to Analyze a PCB Transmission Line?

After establishing what a PCB transmission line is and talking about signal speed and propagation delay, and the critical length, the controlled impedance, and the rise/fall time of a PCB transmission line, we discussed impedance discontinuity and signal reflection. Today, we are going to explain how to analyze a PCB transmission line using quantities, such as voltages and currents, and line parameters, such as resistance, inductance, capacitance, and conductance.

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PCB Transmission Line: Critical Length, Controlled Impedance and Rise/Fall Time

After explaining the importance of the signal speed and propagation delay in a PCB transmission line, we are now going to talk about the transmission line effects, the critical length, the rise/fall time and when the length of an interconnection is to be considered as a controlled impedance transmission line.

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