ABB ETL600 Instruction Manual page 210

ABB Switzerland Ltd
It is defined by the following equation:
Ar
Z0
Z
A low value signifies a poor match, which results in a reduced transfer
of power from the PLC transmitter onto the transmission line. The main
disadvantage, however is the intermodulation phenomenon and
associated cross-talk it causes.
The level of return loss should be measured in the equipment room at
the coaxial cable input. The instruments used and the test set-up can
be seen from Fig. 7-1 and Fig. 7-2. A typical return loss characteristic
is shown in Fig. 7-3. In the case of short lines (line attenuation
< 15 dB), the remote end must be terminated at rated impedance.
The measurement of the return loss over the total bandwidth of the
coupling filter and the PLC line traps is recommended. Wherever
possible, the measurements should be carried out twice under the
following conditions:
HV transmission line grounded behind the line trap
HV transmission line open behind the line trap
If the minimum return loss is in the range 6 to 12 dB, the value of the
input impedance should also be measured. In cases where the
magnitude of the system impedance is too low or too high, an
improvement can be achieved by changing the rated impedance of the
RF hybrid (75 or 125 Ohms). Where this is not possible, the output
power must be reduced in accordance with the following relationship to
avoid non-linear distortion:
A (dB) = 1/4 [12 - Ar min ]
Minimum values of return loss < 6 dB indicate either a defective
coupling or PLC line trap, or an unacceptable property of the
transmission line.
In such instances, the coupling equipment at both ends of the line
must be checked using a dummy load as follows:
1. Carefully ground the LV end of the coupling capacitor.
2. Interrupt the connection between the coupling filter and the
coupling capacitor and terminate with a dummy load as shown in
Fig. 7-5 or Fig. 7-6.
3. Measure the return loss of the coupling filter within the rated
frequency band.
The instruments and test set-up are shown in Fig. 7-1 and a typical
return loss characteristic for a bandpass coupling filter in Fig. 7-4.
7-4
+
Z 0 Z
=
20 log
−
Z 0 Z
rated impedance
actual input impedance
November 2005
1KHW001489-EN ETL600
Commissioning