
Making
A
ccurate
Measurements
of
Electrically
Long
Devices
A
device
with
a
long
electrical
delay
,
such
as
a
long
length
of
cable
or
a
SA
W
lter
,
presents
some
unusual
measurement
problems
to
a
network
analyzer
operating
in
swept
frequency
mode
.
Often
the
measured
response
is
dependent
on
the
analyzer's
sweep
time
,
and
incorrect
data
may
be
obtained.
At
faster
sweep
rates
,
the
magnitude
of
the
response
may
seem
to
drop
and
look
distorted,
while
at
slower
sweep
rates
it
looks
correct.
The
results
may
indicate
that
a
cable
has
more
loss
than
it
truly
does
,
or
that
a
lter
has
some
unusual
ripple
in
the
passband
which
isn't
really
there
.
This
section
describes
the
cause
of
this
behavior
,
and
how
to
accurately
measure
these
electrically
long
devices
.
The
Cause
of
Measurement
Problems
When
using
a
vector
network
analyzer
to
measure
a
device
that
has
a
long
electrical
delay
(1T),
the
device's
time
delay
causes
a
frequency
shift
between
its
input
and
output
signals
.
The
frequency
shift,
1F
,
equals
the
product
of
the
sweep
rate
and
the
time
delay:
1F=
dF/dt
*
1T
Since
frequency
is
changing
with
time
as
the
analyzer
sweeps
,
the
time
delay
of
the
DUT
causes
a
frequency
oset
between
its
input
and
output.
In
the
analyzer
receiver
,
the
test
and
reference
input
signals
will
dier
in
frequency
by
1F
.
Because
the
test
signal
frequency
is
slightly
dierent
than
the
receiver
frequency
,
the
analyzer
will
err
in
measuring
its
magnitude
or
phase
.
The
faster
the
analyzer's
sweep
rate
,
the
larger
1F
becomes
,
and
the
larger
the
error
in
the
test
channel.
The
HP
8753D
network
analyzers
do
not
sweep
at
a
constant
rate
.
The
frequency
range
is
covered
in
several
bands
,
and
the
sweep
rate
may
be
dierent
in
each
band.
So
if
an
operator
sets
up
a
broadband
sweep
with
the
minimum
sweep
time
,
the
error
in
measuring
a
long
device
will
be
dierent
in
each
band,
and
the
data
will
be
discontinuous
at
each
band
edge
.
This
can
produce
confusing
results
which
make
it
dicult
to
determine
the
true
response
of
the
device
.
T
o
Improve
Measurement
Results
T
o
reduce
the
error
in
these
measurements
,
the
frequency
shift,
1F
,
must
be
reduced.
1F
can
be
reduced
by
using
the
following
three
methods:
decreasing
the
sweep
rate
decreasing
the
time
delay
(1T)
Decreasing
the
Sweep
Rate
The
sweep
rate
can
be
decreased
by
increasing
the
analyzer's
sweep
time
.
T
o
increase
the
analyzer's
sweep
time
,
press
4
MENU
5
NNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNN
SWEEP
TIME
[MANUAL]
and
use
the
front
panel
knob
,
the
step
4
*
5
4
+
5
keys
,
or
the
front
panel
keypad
enter
in
the
appropriate
sweep
time
.
Selection
of
the
appropriate
sweep
time
depends
on
the
device
being
measured;
the
longer
the
electrical
delay
of
the
device
under
test,
the
slower
the
sweep
rate
must
be
.
A
good
way
to
tell
when
the
sweep
rate
is
slow
enough
is
to
put
the
vector
network
analyzer
into
a
list
frequency
mode
of
sweeping,
and
compare
the
data.
In
this
mode
,
the
vector
network
analyzer
does
not
sweep
the
frequency
,
but
steps
to
each
listed
frequency
point,
stops
,
makes
a
measurement,
then
goes
on
to
the
next
point.
Because
errors
do
not
occur
in
the
list
frequency
mode
,
it
can
be
used
to
check
the
data.
The
disadvantage
of
the
list
frequency
mode
is
that
it
is
slower
than
sweeping.
5-48
Optimizing
Measurement
Results
Содержание HP 8753D
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