The standard equipment for measuring impedance variations is a Time Domain Reflectometer, TDR. TDRs are very effective but resolving small elements requires a wide bandwidth, which implies the TDR will be very expensive. This note explores a £35/$55 alternative based on SDR dongles and noise sources, to see what can and cannot be achieved.
Although there are limitations, initial results are surprisingly good and useful. For example, Figure 1 shows reflections in two different transmission lines with an open-circuit stub 3.1m from the TDR. The first stub is 19cm long, and the second is 29cm long.
The stubs’ differing lengths are clearly distinguishable.
Why do such impedance variations matter? Because with RF circuits and medium/high speed digital circuits, connections must be uniform-impedance correctly terminated transmission lines. Impedance discontinuities in RF circuits causes peaks and troughs in the frequency response, leading to poor performance and/or link failure. Impedance discontinuities in digital circuits cause signal integrity problems, leading to marginal operation and/or pattern-sensitive errors.