Digital Circuit Boards: Mach 1 GHz

2.23 Measuring Characteristic Impedance

A time domain reflectometer can be used to measure the characteristic impedance of a transmission line. The line is driven with a step function through a known matching resistor. If the transmitted voltage is reduced by a factor of 2, the characteristic impedance is equal to the resistor value. The voltage is sensed after the initial rise time. Obviously, measurements made on actual traces allows for very little experimentation.

An analog method can be used to measure the characteristic impedance of many transmission line geometries. The method makes use of the fact that characteristic impedance involves the ratio of conductor dimensions. For example, two 5-mil wide traces that are 2 mil thick that are spaced 5 mil apart has the same characteristic impedance as two bars of metal 0.5 in wide and 1/16 in thick, which are spaced 0.5 in apart. The bars of metal can be laid out on the surface of a test bench. The capacitance of this model can be measured by using simple test equipment. The characteristic impedance is related to the capacitance by Equations 2.3 and 2.4. The capacitance of the model is the capacitance of the traces multiplied by the model scale factor. In the example above, this factor is 100.

The conductors used in a model must be long enough to provide a useful capacitance. In the example above, a length of 1 m will yield a capacitance of about 50 pF. To measure this capacitance, a function generator that supplies triangle waves ...