We often give even less thought to how we connect our circuits and systems to the test equipment that we use to power and measure the performance than should. We know this because we frequently see the ramifications in the worst-case circuit analyses that we perform. Here, we will take a look at how the lack of attention to interconnect detail may impact the resulting test data that we obtain.
Every connection can be represented by both coupled and uncoupled inductance, resistance and capacitance. These connections can be represented by a characteristic impedance, ZO, defined by:
Where: C = Capacitance of the connection pair L = Uncoupled inductance of the connection pair.
We can reduce the uncoupled inductance by twisting the wire pair at the expense of increased capacitance. A connection pair that is terminated into its characteristic impedance appears only resistive and neither inductive or capacitance, resulting in optimum frequency response. Some of the more common controlled impedance connection pairs include Ethernet cable and USB cables (100?), cable TV coaxial wire (75?), and most RF equipment, including RF test equipment (50?).