Agilent Technologies 4395A - Page 285

Reection
Measurement
(NA)
Reection Measurement
(NA)
When
making
a
reection
measurement, the
4395A 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 coecient
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 Figure
10-7).
Figure
10-7.
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 coecient, a return loss
, or as SWR. These measurements are
mathematically dened as:
Examples of Applications 10-7