HP 8753D - Page 428

Connection Considerations
A
dapters
T
o
minimize
the
error introduced
when you
add an
adapter to
a measurement
system, the
adapter
needs
to
have
low
SWR
or mismatch,
low loss
, and
high repeatability
.
Figure
6-98. A
dapter Considerations
In
a
reection
measurement,
the
directivity of
a system
is
a
measure
of
the
error
introduced
by
an imperfect
signal
separation
device
.
It
typically
includes
any
signal
that
is
detected
at
the
coupled
port which
has
not
been
reected
by
the
test
device
.
This
directivity
error
will
add
with
the true
reected
signal
from
the
device
,
causing
an
error
in
the
measured
data.
Overall
directivity
is the
limit to
which
a
device's
return
loss
or
reection
can
be
measured.
Therefore
,
it
is important
to have
good
directivity
to
measure
low
reection
devices
.
F
or
example
,
a
coupler
has
a
7
mm
connector
and
40 dB
directivity,
which is
equivalent to
a
reection coecient of
 =0.01 (directivity in dB =
0 20 log
 ). Suppose we want to
connect to
a device with an SMA male connector
.W
e need to adapt from 7 mm to SMA.
If we choose a precision 7 mm to SMA adapter with a SWR of 1.06, which has
 =0.03, the
overall directivity becomes
 =0.04 or 28 dB
. However
, if we use two adapters to do the same
job, the reection from each
adapter adds up to degrade the directivity to 17 dB
. The last
example shown in Figure 6-98 uses
three adapters that shows an even worse directivity of
14 dB
. It is clear that
a low SWR is desirable to avoid degrading the directivity of the system.
Application and Operation Concepts 6-161