94 Keysight N9923-90001 User’s GuideDTF (Distance to Fault) MeasurementsDTF Measurement SettingsCable (Correction) SpecificationsBy default, the FieldFox does NOT correct DTF measurements to account forthe inherent loss of a cable. However, to make more accurate DTFmeasurements, the Cable Loss and Velocity Factor values should beconsidered.About Velocity Factor and Cable Loss— Velocity Factor is a property of the physical material of a cable. A VF of 1.0corresponds to the speed of light in a vacuum, or the fastest VF possible. Apolyethylene dielectric cable has VF = 0.66 and a cable with PTFE dielectrichas VF = 0.7.— Cable Loss is specified in dB/meter. In addition to the length of the cable,loss is also directly proportional to the frequency of the signal that passesthrough the cable.The following is an example showing how DTF cable correction works:The DUT is a 100 meter transmission cable. The Cable Loss value is 1dB/meter. This means that a signal traveling ONE WAY through the cable willlose 10 dB of power (100 m * 1dB/m). Because the FieldFox performs thismeasurement with 1 port, the test signal travels down the cable and then back,for a total loss of 20 dB.After a calibration has been performed, for the purpose of illustrating thispoint, connect an OPEN to the end of the cable – a maximum-sized fault - for100% reflection of the 300 MHz test signal.Without compensation for the loss of the cable, a –20 dB response would bevisible at 100 meters, which is the OPEN at the end of the DUT. This is from 10dB of loss through the cable in each direction.With compensation for the loss using the manufacturer’s specification, theFieldFox compensates the trace as though the signal traveling through 100meters was increased by +20 dB. Therefore the response will show 0 dB for100% reflection.How to enter Cable Loss and Velocity FactorCable Loss and Velocity factor can be entered using one of the followingmethods:— Manually enter cable loss and velocity factor for the measurement.— Select or create a cable file which contains the cable loss and velocityfactor.With a DTF measurement present:The following describes Coax Media type. However, you can create or editWaveguide Media Standards. Learn how in “Waveguide Calibrations” onpage 117.
