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.
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.
For a while my best probes have been HP10020A 1.5GHz passive probes, but only the 10:1 variant. They are really pleasant to use because they are light, robust and cheap (unlike active probes), have very convenient spear tips, and most importantly, don’t distort the signal (unlike the common 10:1 high impedance probes). For the latter reason, these poorly named “low impedance Z0” probes have been a preferred way of looking at high-speed signals. For more information on their characteristics, see my scope probe reference material.
The only problem has been that I only have two, and so have been reluctant to use them in case they were damaged.
Well, that’s changed thanks to Ebay and an Australian vendor: I was able to buy a new-old HP10020A with the 1:1, 5:1, 10:1, 20:1, 100:1 tips, and all accessories in a case remarkably cheaply. That’s the definition of a good transaction: both parties are pleased.
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…