Agilent Technologies Agilent 4396B - Manual (Page 98)

Re
ection Measurement
When making a re
ection measurement, the analyzer monitors the
signal going to the DUT and uses it as the reference . It compares the
re
ected signal from the DUT to the reference signal. The ratio of the
incident and re
ected signals is the re
ection coecient of the DUT
or , when expressed in decibels , the return loss .
Re
ection measurements require the connection of a directional
device , such as a directional coupler , to separate the power re
ected
from the DUT . This separation is necessary so that it can be measured
independently of the incident power (see the following gure).
Figure 5-11. Re
ection Measurement
Multi-P ort T est Devices
When the device has more than one port, connect high-quality terminations (loads) to
all unused DUT ports to terminate them into their characteristic impedance (usually
50 or 75
). If this is not done , re
ections o the unused ports will cause
measurement errors .
The S-parameter test set automatically switches the termination at the unused port
for each S-parameter measurement. When using a transmission/re
ection test set,
terminate the unused input port of the analyzer with a high quality load.
The signal re
ected from the DUT is measured as a ratio with the
incident signal. It can be expressed as a re
ection coecient, a return
loss , or as SWR. These measurements are mathematically de ned as:
5-12 Network Measurement Examples