Akber Roy

To increase signal quality, match the impedance, capacitance and inductance of all areas of the signal path.

In PCB design and fabrication, possibly the most frequently used, yet least understood, term is impedance.

Most of us have seen the buzzwords that accompany it: impedance Zo, 50Ω, 10%, balanced lines, microstrip, stripline ground plane, dielectric loss, dielectric constant, and others. What do they mean?

In this first of a two-part column, we’ll start by defining them in common terms for the novice. By the end, a few more people might make better sense of what is happening inside the circuit board. The second half will take a more thorough look at impedance.

First, why do we need impedance specifications? When a fast rise time signal (i.e., a 100MHz to 10GHz or higher pulse) travels down a PCB trace without impedance matching, not much of any use comes out the other end of the trace. What does come out is only a percentage of the original signal, with rounded-off edges and ringing on the trailing edge.

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