HP 8753D - Page 418

Attenuation
at Mixer
Ports
Mismatch
between the
instruments
,
cables
,
and
mixer
introduces
errors
in
the
measurement
that
you cannot
remove
with
a
frequency
response
calibration.
Y
ou
can
reduce
the
mismatch
by
using high
quality
attenuators
as
close
to
the
mixer
under
test
as
possible
.
When
characterizing linear
devices
,
you
can
use
vector
accuracy
enhancement
(measurement
calibration)
to
mathematically
remove
all
systematic
errors
from
the
measurement,
including
source
and
load
mismatches
.
This
is
not
possible
when
the
device
you
are
characterizing is
a
mixer
operating
over
multiple
frequency
ranges:
therefore
,
source
and
load
mismatches
are
not
corrected
for
and
will
add
to
overall
measurement
uncertainty
.
T
o
reduce
the
measurement
errors
associated
with
the
interaction
between
mixer
port
matches
and
system
port
matches
,
you
can
place
attenuators
at
all
of
the mixer's
ports.
Figure 6-88
shows
a
plot
of
swept
conversion
loss
where
no
attenuation
at
mixer
ports was
used. The
ripple
versus
frequency
is
due
to
source
and
load
mismatches
.
Figure
6-88.
Conversion
Loss
versus
Output
Frequency
Without
Attenuators
at
Mixer
P
orts
In
contrast,
Figure
6-90 made
use of
appropriate attenuation
at all
mixer
ports
.
Y
ou
should
give
extra
care
to
the
selection
of the
attenuator located
at the
mixer's
IF
port
to
avoid
overdriving
the
receiver
.
F
or
best results
, choose
the value
of
this
attenuator
so
that
the
power
incident
on
the
analyzer's
R
channel
port
is less
than 0
10 dBm
and greater
than
0
35
dBm.
Application and Operation Concepts 6-151