My previous post used a Tektronix 1502 to examine discontinuities in cables. This post examines the discontinuity introduced by a “nominally invisible” protection diode on a PCB; it is clearly visible with the TDR, but probably won’t affect the final application.
What can you see, test and measure with a traditional time domain reflectometer (TDR)? The answer is “more than you might expect”:
- measure impedance variations in connectors/filters/antennas/PCBs
- locate short/open circuits and damage in cables
- locate intermittent faults in cables and connectors
- locate connectors in cables
and can resolve discontinuities around 2cm apart. That resolution is at least 10 times better than can be obtained with the typical homebrew logic pulse + oscilloscope combination.
I recently bought a couple of cheap 1970s Tektronix 1502s in the hope that I could make a single working frankenmachine. My initial assessment was depressing: one had a cracked and broken case (so I assumed the CRT was also broken), the other’s electrolytic caps had spewed acid across the PSU and had a faulty 2kV PSU, and both had defective NiCd batteries – and it won’t even start without a working battery. But eventually I managed to get both working: I recapped the PSUs, rewelded the case with methylene chloride, used my “new” 12kV scope probe and 40kV meter to repair the HV PSU, created a “NiCd emulator”, and the CRT wasn’t damaged after all. Later reading of a TekScope magazine indicates it isn’t surprising the CRT survived: it is mechanically completely isolated from the chassis to protect against up to 26 12″ drops.
So I am now the proud possessor of two nice little portable waterproof instruments, literally designed for field use – one of the service manuals indicates they were used with Patriot missile defence systems.
This note shows that measuring a digital signal’s risetimes and falltimes does not require multi-GHz oscilloscopes; with imagination, very cheap test equipment is sufficient. Measurements show that even common-or-garden 74LVC gates can have 10%-90% transition times of around 625ps.
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.