Chapter 3
Structured Cable Field Calibration and Testing
3-28
Troubleshooting Return Loss Problems
Problem: Excessive Return Loss (Value of 10 dB or less)
Probable Causes
Open, shorted, or damaged cable.
Installed cable, cable segments, or patch cord have improper
characteristics.
Damaged or worn cable or connectors.
Poor punch-down.
Factory splice in cable.
Other Tests Affected
Test
Possible
Result
Attenuation
May be high.
Capacitance and
Could be affected if the impedance
Average Impedance mismatch is caused by cable damage.
DC Loop Resistance
May be high if due to a poor punch-down.
Impedance Test
Average impedance is derived from electrical delay and capacitance measurements.
The results of this test are expressed in ohms. Average impedance testing can help
identify physical damage to the cable, connector defects, or cable segments with
incorrect characteristic impedance.
This test uses capacitive measurements; therefore, it is necessary to specify the
correct cable type in order to accurately perform the test.
Note: If a CAT 3 cable is selected (specified as the cable type where PVC is used in
the cable insulation) but a CAT 5 cable (where Teflon
®
is used as the cable insulation)
is actually used, the average impedance will be calculated incorrectly. To avoid this
problem, be sure to specify the correct cable type.
Figure 3-18: Impedance Test Screen
Impedance Errors
Impedance errors cause signal reflection and strength reduction. Average impedance
of each pair should be equal to the LAN system impedance of 100, 120, or 150
Ω
.
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