Time Domain Reflectometers have been in existence for many years and are the fastest, most accurate method of locating faults in metallic cabling. The instrument is generally referred to by its abbreviated title of TDR. Other countries have adopted names such as cable radar and Pulse Echo Meter.
If a cable is metallic and has a minimum of two conductors separated by a dielectric (i.e. twisted pair or multi core cable) or a conductor and screen (i.e. Coaxial cable) it may be tested by a TDR, TDR’s will measure cable length and identify faults in the cable i.e. Manufacture process, broken conductors, water damage, loose connectors, and sheathing faults. In addition TDR’s can be used to test cable on reels for shipping damage, cable shortages, usage and inventory management.
Principles of Operation
The basic operating principle of the TDR is similar to that of RADAR, Pulses of energy are sent out upon hitting the target the pulse is reflected back, the time taken for the return is measured and the distance, size and direction of the target is calculated. The TDR works on the same principle, but the TDR requires the input of certain information on the characteristics of the cable being tested such as the cable impedance and the velocity of propagation (VoP). A series of pulses of a pre determined pulse width and voltage referred to as the output pulse or launch post are sent down the cable under test and the time it takes for the pulse to travel to the end of the cable or fault and be reflected back is then multiplied by a constant which is specific to the type of cable under test known as the velocity of propagation (VoP). The returned pulse is displayed as part of the wave form on the LCD. This wave form is known as the signature of the cable. The cursor is aligned at the base of the rising edge of the pulse, indicating either a fault or the end of the cable run and the distance read from the LCD in either feet or meters. Whilst with RADAR the transmission medium of the pulse is through air the characteristics of which remain constant, with cable however the cable characteristics will vary in terms of velocity of propagation and cable impedance dependent upon the materials used in the construction and the purpose for which the cable has been designed, i.e. power cable would normally have an impedance of around 25 ohms and a VoP of around 50%, Data cabling 100 ohms and VoP of 70>80%. In order for the TDR to deliver accurate results these variables will need to be determined and entered into the TDR before cable testing commences. For certain telecommunication and data cables VoP and impedance values may be quoted by the cable manufacturer, for some cables this will need to be established by experimentation, the method for this is detailed in the TX instruction manual.
VELOCITY of PROPERGATION (VoP)
The velocity of propagation of a cable is the speed at which a signal is transmitted through a cable, this is normally defined in terms of a % of the speed of light in a vacum (186,000 miles per second) or as the speed in feet or meters per micro second this speed is represented by the number 1 or 100% this would be the fastest speed which can be achieved all other speeds would be slower. A cable with a VoP of 75% would transmit a signal at 75% of the speed of light. A VoP of 50% would transmit at 50% of the speed of light etc. The VoP value is determined by the dielectric material and its thickness (insulation) which covers and separates the conductors. In a twisted pair cable the VoP value is also determined by the spacing of the conductor’s i.e. the twist rate. In a coaxial cable the material separating the centre conductor from the screen and the outer sheath is the material which determines the VoP. This is because the signal runs over the conductor i.e. between the conductor and the insulation; an initial VoP value for the cable may be established by identifying the dielectric material such as PVC, PTFE etc. The VoP value is not significantly influenced by the conductor material which may be steel, aluminium, copper, etc.
When two cables are placed in close proximity to each other a cable impedance is formed, the impedance value is determined by the spacing of the conductors and as with the VoP the insulation (dielectric) material used in the manufacture of the cable. If the cable is manufactured with constant spacing between the conductors as in the case of coaxial cable (the screen of the cable being taken as a conductor) then the impedance value for the cable will be constant along the cable length, if the conductor spacing is not constant the impedance value will vary along the cable length. The TDR could therefore be described as a comparator, comparing the impedance value of the cable under test with that set by the operator in the TDR, significant changes in impedance being identified as either positive or negative returns on the LCD, the amount of energy reflected back from the miss match will determine the amplitude of the return pulse the open of the cable being either a break in the cable or the end of the cable run will be displayed as a large positive return, a shorted end, where the two conductors under test are joined or touching will be displayed as negative return, smaller returns displayed along the waveform indicate impedance mis- matches Positive returns indicate Examples of High Impedance Faults ,that is where the impedance value of the mismatch is greater than the impedance value of the cable being tested, negative returns indicate examples of low impedance faults, that is where the impedance value of the miss match is less than the impedance value of the cable being tested.
Examples of High Impedance Faults (Positive Pulse)
Open /cable end
Tap with high cable impedance
Examples of Low Impedance Faults (Negative Pulse)
Tap With Low cable impedance
This is the distance between the rise and the fall of the launch pulse, it is as implied in the title the distance for which faults may not be seen usually an attachment lead is added which is longer than the width of the launch pulse. This takes the point at which the attachment lead is connected to the test cable beyond the width of the launch pulse thereby allowing the operator to identify faults at the start point of the cable under test.
The width of the launch pulse is pre set within the TDR and is not user adjustable and will automatically be adjusted for the length range selected. For the measurement of short cables i.e. up to 7 meters a very short pulse width is require, the pulse width is extended as the range increases. A typical launch pulse width for low end TDR’s is 3 meters wide therefore a 3meter long attachment lead would be required to view faults within the launch pulse, The TX6000 however which uses a very short pulse has a dead zone of 0.6 meters (VoP 67%)
TYPES OF TIME DOMAIN REFLECTOMETERS
TDR’s fall into 3 groups
This type gives a digital readout only displaying the length of the cable and with instruments such as the FaultCastor will show length to fault whether the cable end is open or short
The pulse TDR, as described in the preceding pages. A pulse will be sent down the cable under test, the cable voltage will be sampled at sub second intervals for a predetermined time period dependant on the range selected and the change in voltage when plotted against time is displayed on the LCD as a wave form
The cable under test will be fed with a continuous 5 volt DC signal generated by the TDR circuit. The amplitude of this voltage (the Edge) will be maintained at the same level until a fault is identified. At the point where the fault occurs the voltage will rapidly drop further impedance miss matches occurring along the cable will be identified by the reflected energy which will again push down or pull up the generated voltage dependent upon weather the fault is open or short. The resultant trace is displayed as a square wave.