A Background to Return Loss Measurement with the HP 81534A Return Loss Measurement
Agilent 8163A Lightwave Multimeter, Agilent 8164A Lightwave Measurement System, & Agilent 8166A Lightwave
Multichannel System, Fourth Edition
149
For “Measuring the Reflections from the DUT” on page 147, the value of the
reflection factor of the DUT is called R
DUT
. This gives the following equation:
If we substitute equation 3 into equations 2 and 4, this gives us the following two
equations:
If we subtract P
T
from equations 5 and 6, this gives us the following equations:
If we divide equation 8 by equation 7, this gives us the following equations:
Thus we can use the equation below to calculate return loss:
The return loss of the reference reflection is given by:
Calculating the Front Panel Delta
The Front Panel Delta is the change in loss variation that is caused by replacing
the reference cable, as used in “Measuring the Reflection Reference using a
Reference Cable” on page 140, with the measurement cable, as used in
“Measuring the Return Loss” on page 142. This is caused by differences in
reflections from the front panel connector and also differences in the backscatter
level of the fibers.
To measure the front panel delta you must measure the power transmitted through
the reference cable, “Measuring the Power Transmitted Through the Reflection
Reference” on page 146, and the power transmitted through the measurement
cable, see “Measuring the Power Transmitted Through the Measurement Cable”
on page 146. The system may be represented by the general diagram shown in
Figure 107.
P
DUT
k
1
k
2
R
DUT
P
src
sP
src
+=
(4)
P
Ref
k
1
k
2
R
Ref
P
src
P
T
+=
(5)
P
DUT
k
1
k
2
R
DUT
P
src
P
T
+=
(6)
P
Ref
P
T
– k
1
k
2
R
Ref
P
src
=
(7)
P
DUT
P
T
– k
1
k
2
R
DUT
P
src
=
(8)
R
DUT
P
DUT
P
T
–
P
Ref
P
T
–
--------------------------
R
Ref
=
(9)
RL
DUT
10 R
DUT
log– 10
P
DUT
P
T
–
P
Ref
P
T
–
--------------------------
10 Rlog–
Ref
log–==
(10)
RL
Ref
10 R
Ref
log–=
(11)
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