Poor Man’s Homebrew TDR With 4cm Resolution – Part 1

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.

stub1929

Figure 1: 19cm & 29cm Stubs

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.

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Images of Late-70s Burr-Brown Thick-film Hybrid ICs

In the late 70s Burr-Brown made some of the most advanced analogue ICs on the open market. Technology limitations prevented the full integration of many components, so the “thick-film hybrid” IC was used instead. As a publicity gimmick Burr-Brown distributed a calendar of the some of the ICs internals.

For no reason other than the pictures are pretty in themselves, I kept them. Now I’ve finally got round to crudely digitising them. Here they are…

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