Measuring Input Protection Components’ Effects With a TDR

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.

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A Traditional TDR Cable Tester With <2cm Resolution

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.

Tektronix 1502 TDR Cable Tester

Tektronix 1502 TDR Cable Tester

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Measuring Digital Signal Edge Speeds Without An Oscilloscope

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.

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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.


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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