Limits may be set up for the measurement in the following way:
- Step 1. Press the LIMIT key to display the Limit Menu.
- Step 2. If a Single Limit (single value across the entire distance range) is required, press the SINGLE LIMIT soft key. Select the ON/OFF soft key to turn the Limit On. Press the EDIT soft key to update the value of the Limit. Enter the desired numerical value using the keypad. Press the Up/Down Arrow key.
The Limit line may be turned off by pressing the ON/OFF soft key.
- Step 3. If more than one limit value across the entire distance range is needed, press the MULTIPLE LIMITS soft key. Select the SEGMENT 1 soft key. Press the ON/OFF soft key to turn on the first segment of the limit line. Press the EDIT soft key and enter the start distance using the numeric keypad and press
ENTER. A new data entry display will appear. Enter the desired value of the limit at the start distance and press ENTER. A new data entry display will appear. Enter the desired end distance of the first limit segment and press ENTER. A new data entry display will appear. Enter the desired value of the limit at the end distance and press ENTER. This process can be repeated for each new segment by selecting the NEXT SEGMENT soft key. When eachsegment is selected information is displayed at the bottom of the display area. Pressing the PREV SEGMENT and NEXT SEGMENT will advance the displayed information to the previous or next segment.
Saving a Setup
Press the SAVE SETUP key. Using the Up/Down Arrow key, select an <Empty> location. Press ENTER. The measurement setup and calibration will be saved.
Saving a Trace
Press the SAVE DISPLAY key. Enter a trace name using the soft keys and press ENTER when data entry is complete.
Optimizing Frequency Range
Selecting the appropriate frequency range is not as obvious as it may seem. For return loss measurements, the specification usually calls out the frequency range over which the data is to be taken. For Distance To Fault analysis, the resolution and maximum distance range are dependent upon the frequency sweep range, the number of frequency data points and the relative propagation velocity of the cable being tested. Therefore, the frequency range must be chosen carefully. When checking the return loss of the antenna in DTF mode, the operating frequency range of the antenna should be used.
For checking transmission lines, a large frequency span is desirable to highlight potential faults or areas of performance degradation. However there is a constraint that limits the frequency range. The maximum distance is inversely related to the frequency range
The wider the frequency range, the smaller the maximum distance that can be measured. Graphs illustrating this relationship are shown in Figure 7.
There is also a relationship between resolution and frequency range. The wider the frequency range, the smaller the resolution. Wider frequency sweeps improve the resolution of DTF measurements.
With adequate frequency sweep range, 0.6 centimeters can be resolved. Distance range can exceed 600 kilometers using narrow frequency sweeps.
The insertion loss of a cable varies with frequency - the higher the frequency the greater the loss of a cable. Most cable manufacturers specify the loss of their cables at one or more specific frequencies. If the loss is not specified for your particular frequency range, or the loss of the cable is unknown, the DTF feature can be used to find the loss.
Using a small piece of the type of cable to be tested, connect it to the instrument with the other end open (not connected to anything). Perform a DTF measurement over the frequency range of operation. A spike in return loss should be visible where the open is located (at the end of the cable). An open circuit should have 0 dB return loss (full reflection). Adjust the cable loss parameter until the open at the end of the cable measures 0 dB return loss. Use the marker function to display the value.
The relative propagation velocity of a cable is equal to 1/[SQRT (relative dielectric constant)]. The dielectric constant is determined by several factors including the dielectric type of the transmission line and the diameter thickness of that dielectric. It is specified by the manufacturer of the cable. Flexible cables may have more than ±10% variation in dielectric constant along the cable’s length due to manufacturing tolerances. Dielectric constant does not vary with frequency. If the correct relative propagation velocity is not used the distance calculation will be incorrect. If the relative propagation velocity is unknown it can be found using the DTF feature.
A known length of cable (the type being tested) can be used to determine the propagation velocity. Connect it to the instrument with the other end open (not connected to anything). Perform a DTF measurement. A spike in return loss should be visible where the open is located (at the end of the cable). An open circuit should have 0 dB return loss (full reflection). Adjust the relative propagation velocity parameter until the open at the end of the cable indicates the correct cable length.
"DTF Instrumentation Accuracy" is better than 0.1%, but the more practical concern is "Measurement Accuracy." Return loss measurement accuracy is influenced by many factors; the quality of the calibration (including the calibration components and calibration method), the accuracy of the information entered by the user, and the quality of the cables being tested. Precision calibration components allow greater measurement accuracy. For accurate calibration results, all measurement system uncertainties need to be compensated for by ensuring that the calibration components are connected to same point that will be connected to the device being tested (at the end of any extension cables or adapters being used).
Distance calculations are based on the assumption of a specific propagation velocity value for the cable or transmission line. If the propagation velocity is set incorrectly, the fault location will be identified at the wrong distance. Relative propagation velocity is calculated as 1/[SQRT(relative dielectric constant)]. The dielectric constant is determined by several factors including the dielectric type of the transmission line and the diameter thickness of that dielectric. Cable manufacturers routinely have dielectric constant variations. The variation may be ±10% or more along the cable’s length. Low cost cables generally have even greater variation in dielectric constant.
Further practical impediments to absolute distance accuracy include the various filters, diplexers, adapters, and differing cable types that are typical of most RF transmission lines. Despite the fact that the instrument itself is extremely accurate, the characteristics of the device under test confound attempts to specify absolute distance accuracy requirements for practical, in-service measurements. The net effect is that each transmission line will have its own “signature” or “finger print” on a DTF display. The ability to store DTF displays, download them to a computer and overlay traces makes analysis of these unique signatures simple. When historical data is compared to recent data, large changes in the "signature" indicate a serious problem. Small changes may indicate aging or dimensional changes due to seasonal temperature conditions.
Typical absolute measurement accuracy for tower mounted transmission lines is within one foot, slightly better than a technician’s ability to measure physical length on a tower mounted cable. Further, most service problems are either physical damage or connector problems. Physical characteristics such as connectors, adapters and bends show up clearly on the DTF display. Thus, identifying a problematic transmission line section is straightforward. As compared to return loss measurements where test accuracy is critical because small performance changes may indicate big problems. Comparison of DTF "before" and "after" plots isolates problems quickly and easily.