Reverse Engineering PCB Board with Controlled Impedances Trace

Reverse Engineering PCB Board with Controlled Impedances Trace

A variety of trace geometries are possible with controlled impedance designs from Reverse Engineering PCB Board, and they may be either integral to or allied to the PCB pattern. In the discussions below, the basic patterns follow those of the IPC, as described in standard 2141.

Note that the figures below use the term “ground plane.” It should be understood that this plane is in fact a large area, low impedance reference plane. In practice it may actually be either a ground plane or a power plane, both of which are assumed to be at zero ac potential.

A Wire Microstrip Transmission Line With Defined Impedance is Formed by an Insulated Wire Spaced From a Ground Plane

The first of these is the simple wire-over-a-plane form of transmission line, also called a wire microstrip. A cross-sectional view is shown in Figure 12.35. This type of transmission line might be a signal wire used within a breadboard, for example. It is composed simply of a discrete insulated wire spaced a fixed distance over a ground plane. The dielectric would be either the insulation wall of the wire, or a combination of this insulation and air.

The impedance of this line in ohms can be estimated with below Equation.

The impedance of this line in ohms can be estimated

For patterns integral to the PCB, there are a variety of geometric models from which to choose, single-ended and differential. These are covered in some detail within IPC standard 2141 (see Reference 16), but information on two popular examples is shown here.

Before beginning any PCB-based transmission line design, it should be understood that there are abundant equations, all claiming to cover such designs. In this context, “Which of these are accurate?” is an extremely pertinent question. The unfortunate answer is, none are perfectly so!

All of the existing equations are approximations, and thus accurate to varying degrees, depending upon specifics. The best known and most widely quoted equations are those of Reference 16, but even these come with application caveats.

Reference 17 has evaluated the Reference 16 equations for various geometric patterns against test PCB samples, finding that predicted accuracy varies according to target impedance. Reference 18 also evaluates the Reference 16 equations, offering an alternative and even more complex set (see Reference 19).

The equations quoted below are from Reference 16, and are offered here as a starting point for a design, subject to further analysis, testing and design verification. The bottom line is, study carefully, and take PCB trace impedance equations with a proper dose of salt when Clone PCB.