Specialist TDRs are not always needed
When signals are applied to a cable you had better match the impedance or things will go wrong! Well, that's generally correct and it's how a TDR (Time Domain Reflectometer) works. Pulses or steps are applied to a cable and the reflections from the cable are displayed and analysed. Reflections will appear along a trace of amplitude versus time wherever energy is reflected; reflections occur where impedance of the cable (or optical fibre) changes.
But all cables are not made the same. Can I use the same TDR?
In most cases the practical answer is "yes". You can use a "copper" TDR on most electrical cables. There are some "outlier" types of cable, but generally a TDR can be use on twisted-pair, telephone or Ethernet standard cables, coaxial cables for CATV or antenna feeds or even (isolated) power cables.
Yes there are a few things to be aware of, but once you know these you will find that our new CableScout CS90 is useful in many applications. For those who want a little more background information I have written a detailed article which you will find on our website here.
Summary
Once you are aware of how a TDR works, provided you understand that there will be a small amount of energy lost and reflected at the junction between the different cable types then you can continue testing beyond that junction with full fidelity and accuracy. For example, when using the new CS90 which is primarily a TDR intended for use with 75 ohm coaxial cables, typically used in the CATV industry, with CAT6 "Ethernet" cable, which has an impedance of roughly 100 ohms at higher frequencies. Then you will see a little "blip" where a fraction of a decibel of "return loss" occurs as the coaxial patch cord connects to the twisted pair, but beyond that you can see a perfect TDR trace to the end of the cable.
Earlier TDRs may have struggled to do this because of lower specification amplifiers which suffered with more noise etc. But this new generation of TDR, built using the latest technology, can cope admirably with these small "interface" losses.
Example
Alongside is a screen shot from CS90 showing the end of cable clearly for a drum of CAT5e. If you examine the launch time, near zero, yes there is some discontinuity but beyond that all is good and clearly readable.
Conclusions
You can use modern TDRs like the CS90 for many purposes. There are minor shortcomings when testing cables of different impedances, but when you are aware of and expect these you can "test through" the discontinuity as there will be only a tiny loss of signal, as the impedance changes from cable to cable, and "fault" signatures in the cable tested beyond that point will remain clearly visible.
For more information on this or our other products please visit: