MBR5 User Manual
MBR5 Rev 007
20
Figure 8: Measuring BR – Excluding Connector
Backreflection Accuracy and Range
The absolute accuracy of backreflection measurements made with the meter is
dependent on the level of backreflection to be measured.
The backreflection measurement range is restricted by the BR0 level. The meter
can measure backreflection levels 15 dB below BR0 to a maximum of -‐80 dB (-‐60
dB for multimode). For example, if BR0 is -‐40 dB, the minimum backreflection
of the DUT that can be measured is -‐55 dB.
An asterisk (*) is displayed near the range limit (last 5 dB of range if using BR0,
below -‐60 dB otherwise) to indicate that the setup and measurement
procedures described in the previous two sections must be followed carefully to
ensure accurate results.
The small amount of reflections that BR0 represent can be polarization sensitive,
and multiple reflections can cause interference effects that can make the
reflections very sensitive to temperature.
NOTE: To ensure accurate backreflection measurements below -‐65 dB (-‐25 dB
for multimode), perform the BR0 measurement at least once per shift.
The measured backreflection is also affected by losses that can occur between
the meter and the DUT. To compensate for these losses, measure the total
amount of loss between the output of the measurement hybrid jumper and the
input of the DUT (see “LOSS AND POWER MEASUREMENT” section on page 21),
double this value and add it to the backreflection value displayed on the meter.
In the following example, the total loss between the meter and the DUT is 2.0
dB, and the displayed backreflection is -‐29 dB:
BR = -‐29 dB + (2 x 2.0 dB) = -‐25 dB
Absolute accuracy specifications are dependent upon the accuracy of the MBR5
meter calibration, so a calibration verification needs to be performed
FC/APC
BR = -58.0dB
BR
B 0
R
POWER
DARK
1490
1550
1310
1625
O
I
(LOCAL)
Termination
Point
for BR
TOTAL
Termination
Point
for BR
0
Measurement
Jumper
DUT
