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TDR What’s the big deal? by Tony Drummond-Murray So, what exactly is this TDR?, and what does it imply? What do we mean when we use these three letters, which for some may be a new mnemonic. T DR stands for Time Domain Reflectometry. At its simplest, the concept has similarities with Radar (Radio Detection and Ranging) the basic principles of which are understood by most folk. Essentially, a pulse of energy is emitted, and a sensitive detector then waits for a returning echo, and displays it’s presence on a screen. The longer the period between the transmission of the pulse and the echo’s eventual return, the greater the distance to the object causing the echo. Knowing the pulse’s speed of travel through the medium, and accurately measuring the total delay period, enables precise calculation of the separating distance. Conventionally, most TV signals are carried in coaxial cables. These generally work very well, and the construction of the cable means it’s inner conductor is totally surrounded by a (usually) copper braid. This elegant construction provides a high level of protection for the 50R Termination 100R Termination inner core from sources of external interference. However, even the shortest length of a cable will experience the simultaneous effects of series inductance and shunt capacitance. It is sufficient for this explanation to accept that this will result in the coaxial cable exhibiting a ‘Characteristic Impedance’. The effect of this is for the cable to electrically emulate a fixed resistor of known value. Popular values for this resistance in coaxial cables are 50 ohms and 75 ohms, with the latter almost exclusively used in TV applications. So, what are the implications of this ‘iterative impedance’? Coaxial cables are used to transport signals (packets of energy) around a TV Station, usually from the output of one amplifier, via the cable, to the input of the distant receiver. Mindful of that echo we mentioned earlier, there would be nothing to stop those pulses of energy bouncing to and fro along the length of coaxial cable, until they decay to zero. Knowing the iterative impedance of the coaxial cable enables Engineers to precisely ‘match’ the amplifiers to the cable, which is usually achieved using fixed resistors equal to the cable’s impedance of 75W. This has important benefits. By carefully matching to both ends of the cable, the signal is unaffected when it makes its transition from TX amplifier to cable, and eventually from cable to RX amplifier at the distant point. In its simplest terms, the energy packets are unaware of any difference between being launched into the 75W coaxial cable, travelling down the cable, or being absorbed by the 75W termination resistor at the far end! Effectively, the cable performs as an extension to the 75W resistor; this is shown in Drawing1. The only minor disadvantage is a 2:1 potential divider action that reduces the amplitude of the signal by 1/2 ( = -6dB). Altering the termination resistor from 75W to perhaps 74W effects a mismatch at the far end of the cable, and this is diagrammatically shown in Drawing 2, where a small amount of energy will be reflected as an echo. This ‘echo’ is what the Test Chest TDR display will show on its LCD, revealing both the distance down the cable to where the mismatch is, and it’s severity. The major advantage TDR has as a technique is that it uses the equipment that is normally connected to that line for termination purposes. This is of fundamental importance if there was a fault due to a poor BNC connection that ‘clears’ when the BNC at the remote location is reseated. The TDR Utility offers a choice of 3 different ‘Test Pulse’ widths 16, 32 and 64nS to optimise performance with different cable types and cable lengths. The first two photographs show the launch of the 32nS Test Pulse and the associated reflections from a 100M cable with crass termination errors (caused by the substitution of the correct 75W termination resistor by 50W and 110W versions. The Murraypro Test Chest is certainly capable of detecting very much smaller mismatches, and Photo 3 demonstrates the effect of adding a tiny ‘stub’ of open circuit coaxial cable less than 100mm long in parallel with a 75 termination, at the end of a 40M cable. TDR techniques are indispensable where problems are due to cable faults, as the reflected energy (from a missing 70 | TV-BAY MAGAZINE TV-BAY067JUL12.indd 70 05/07/2012 22:31