Distance-To-Fault (Dtf) - Anritsu MS2024B Manual

5-9 Distance-To-Fault (DTF)

Field Measurements
5-9 Distance-To-Fault (DTF)
The DTF measurements display Return Loss or VSWR values versus distance. These
measurements are standard on the MS20xxB VNA Master and the S412E LMR Master. If
frequency measurements fail or indicate a problem in the system, then the DTF
measurement can be used to identify and pinpoint the exact location of the problem. The DTF
measurement shows the return loss value of all of the individual components, including
connector pairs and cable components.
For more information about distance measurements, refer to Section 9-6 "Distance
Information" on page 9-6.
When DTF Return Loss or DTF VSWR is chosen from the Graph Type Selector list box, the
Freq/Dist function hard key displays the "Freq/Dist Menu (Distance-Based)" on page 7-4, not
the Frequency menu.
Windowing is a frequency filter that is applied to the frequency-domain data when it is
converted to distance-domain data. When DTF graph types are selected, the Windowing soft
key is displayed in the "DTF Setup Menu" on page 7-6. For more details about windowing,
refer to Section 9-4 "Windowing" on page 9-4 and to Appendix B.
When measuring cable distance, DTF measurements can be made with an open or a short
connected at the end of the cable. The peak indicating the end of the cable should be between
0 dB and 5 dB.
An open or short should not be used when DTF is used for troubleshooting because the open
or short will reflect everything, and the true value of a connector might be misinterpreted. A
good connector could look like a failing connector.
A 50 ohm load is the best termination for troubleshooting DTF problems because it will be
50 ohm over the entire frequency range. The antenna can also be used as a terminating
device, but the impedance of the antenna will change over different frequencies because the
antenna is designed to have only 15 dB or better return loss in the passband of the antenna.
DTF measurement is a frequency domain measurement, and the data is transformed to the
time domain using mathematics. The distance information is obtained by analyzing how
much the phase is changing when the system is swept in the frequency domain.
Frequency selective devices such as Tower Mounted Amplifiers (TMA), duplexers, filters, and
quarter wave lightning arrestors will change the phase information (distance information) if
they are not swept over the correct frequencies. Care needs to be taken when setting up the
frequency range whenever a TMA is present in the path.
Because of the nature of the measurement, maximum distance range and fault resolution are
dependent on the frequency range and number of data points. The instrument will take care
of all the math, but knowing whether the cable is longer than DMax is important. The only
way to be able to improve the horizontal range is to reduce the frequency span or to increase
the number of data points. Similarly, the fault resolution is inversely proportional to the
frequency range, and the only way to improve the fault resolution is to widen the frequency
span.
5-14 PN: 10580-00289 Rev. K Vector Network Analyzer MG