Rohde and Schwarz ZNL-K3 - Distance to Fault Measurement Option for ZNL/ZNLE

The distance-to-fault measurements option (R&S ZNL-K3) allows the detection of cable discontinuities, which is important for base station antenna installation, for example. Users can select from a range of common cable types with predefined velocity factor and frequency dependent attenuation, or create their own cable profiles.

Faults produce peaks on the impulse response (in the time domain) that is calculated from the measured reflection S-parameter trace via inverse Fourier transformation. The distance between the reference plane and the fault can then be calculated from the propagation time, accounting for the electrical properties of the transmission line. Moreover it is possible to define the peaks to be considered as being caused by a fault.

The following example shows the reflection at the end of a faultless open cable connected to test port 2. A full one-port calibration ensures that the distance is measured from the test port position (reference plane = distance zero). The horizontal position of marker "M2" corresponds to the cable length of 1417 mm, the additional peak at position "M1" is due to a bad connection between the cable and the test port.

Adjust the frequency sweep to the length of the transmission line and the expected distance to fault:

• The maximum distance that can be measured is proportional to the number of sweep points. The larger the number of sweep points, the longer the maximum distance between the calibrated reference plane and the fault to be located.
• The maximum distance that can be measured is inversely proportional to the frequency span. The smaller the frequency span, the longer the maximum distance between the calibrated reference plane and the fault to be located. However, a smaller frequency span comes at the cost of a coarser distance resolution.

The R&S ZNL/ZNLE can automatically determine a suitable number of sweep points for a given maximum distance to fault ("Auto Number of Points"). Moreover it is possible to define which of the peaks are considered as being due to a fault, and to draw up and export a list of the detected faults.