Poor Man’s Time Domain Reflectometer

A time domain reflectometer, or TDR, is an essential piece of test gear when working on long cables. The idea is simple: send a pulse down the cable and listen for the reflection from the far end. The catch is that pesky universal constant, the speed of light.

The reason the speed of light is an issue is that, in a traditional system, the pulse needs to be complete before the reflection. Also, time is resolution, so a 1 GHz sampling rate provides a resolution of about 10 centimeters. [Krampmeier] has a different design. He sends variable length pulses and measures the overlap between the outgoing and reflected pulses. The approach allows a much simpler design compared to the traditional method.

There is one exotic part: an ECL XOR gate. ECL is a logic family that uses transistors in their active region to achieve very fast switching rates. By using an RC low pass filter on one input of the XOR gate and driving it with a pulse, the device can generate a variety of fast pulse lengths.

[Krampmeier] submitted the design for a contest, but will provide more details after the contest is done (and there is more detail further down in the same discussion thread). In addition, others mentioned links to other resources, including a cheap TDR with a Microchip PIC (the [Krampmeier] project uses an ST ARM board) and  the obligatory video from [w2aew].

We’ve talked TDRs before, of course. We’ve even looked at a tricky case where it didn’t really help much.

Filed under: ARM, tool hacks

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