HP 8753D - Page 255

Making A
ccurate Measurements
of Electrically
Long Devices
A
device
with
a
long electrical
delay,
such as
a long
length of
cable or
aSA
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
oset
between
its
input
and
output.
In
the
analyzer
receiver
,
the
test and
reference
input
signals
will
dier
in
frequency
by
1F
.
Because
the
test
signal
frequency
is
slightly
dierent
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
dierent
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
dierent
in
each
band,
and
the
data
will
be
discontinuous
at
each band
edge
.
This
can
produce
confusing
results
which
make
it
dicult
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
NNNN
NNNNNNNNNNNNNNNNNNNNNNNNN
NNNNNNNNNNNNNNNNNNNNNNNNN
NNNNN
SWEEP TIME [MANUAL]
and use the front panel knob
, the
step 4
*
54
+
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,itcan
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