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3.7 Return Loss / Fault Severity
A unique feature of Model 1205CXA is AUTOMATIC
dBRL calculation. This eliminates the need to visually
and/or manually calculate the RETURN LOSS at a par-
ticular point on the waveform.
The RETURN LOSS (dBRL) reading is calculated us-
ing the signal
amplitudes
and
waveform data samples
taken just to the left and right of each cursor. It is best
to position the cursors along the leading edge of both
pulses.
Return Loss is a way of measuring impedance changes
in a cable. The algorithm for determining return loss
is:
dBRL=20 Log
10
V
O
/V
R
Where V
O
is the amplitude of the transmitted
pulse and V
R
is the amplitude of the reflected
pulse.
A small value dBRL number means that most of the
pulse energy is reflected by the cable fault. An open
or short would reflect all the energy so its return loss
is zero.
Remember, the larger the dBRL reading, the smaller
the problem and vice versa.
The Model 1205CXA can be set up to display dBRL in
two possible modes, Total dBRL and Fault dBRL.
Total dBRL displays the dBRL of the fault, plus the
attenuation of the cable or cable loss.
Fault dBRL displays the dBRL of the fault and
factors out the attenuation of the cable or cable
loss.
In the Fault dBRL mode, the attenuation (loss) of the
cable has been subtracted out of the display reading.
Therefore, a complete open or short will read 0 dBRL,
regardless of the length of cable.
NOTE: The only way to get an accurate measure-
ment in the Fault dBRL mode is to use the Setup
menu to enter the type of cable under test.
To select Fault dBRL mode, press the Menu key and
select CABLE TYPE. Select Fault dBRL. After Fault
dBRL is selected, you will be prompted to select the
type of cable under test. This is necessary to ac-
curately cancel out attenuation effects on the dBRL
reading.
