HP 8753D - Page 389

Time
domain
low
pass
This
mode
is
used
to
simulate
a
traditional
time
domain
reectometry (TDR)
measurement. It
provides
information
to
determine
the
type
of
discontinuity
(resistive,
capacitive,
or inductive)
that
is
present.
Low
pass
provides
the
best
resolution for
agiven
bandwidth in
the frequency
domain.
It
may
be
used
to
give
either
the step
or impulse
response of
the test
device.
The
low
pass
mode
is
less
general-purpose
than
the
bandpass
mode because
it places
strict
limitations
on
the
measurement
frequency
range
.
The
low pass
mode requires
that the
frequency
domain
data
points
are
harmonically
related
from
dc to
the stop
frequency.
That
is
,
stop
=
n
x
start,
where
n
=
number of
points.
For
example
,
with
a
start
frequency
of
30
kHz
and
101
points
,
the
stop
frequency
would
be 3.03
MHz. Since
the analyzer
frequency
range
starts
at
30
kHz,
the
dc
frequency
response
is extrapolated
from the
lower frequency
data. The
requirement
to
pass
dc
is
the
same
limitation
that exists
for traditional
TDR.
Setting
frequency
range
for
time domain
low pass
Before
a
low
pass
measurement is
made,
the measurement
frequency range
must
meet
the
(stop
=
n
x
start)
requirement
described
above
.
The
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SET
FREQ
LOW
PASS
softkey
performs
this
function
automatically: the
stop
frequency
is
set
close
to
the
entered
stop
frequency
,
and
the start
frequency is
set equal
to
stop/n.
If the
low end
of the
measurement frequency
range
is
critical,
it
is
best
to
calculate
approximate
values
for
the
start
and
stop
frequencies
before
pressing
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SET
FREQ
LOW
PASS
and
calibrating.
This
avoids
distortion
of
the
measurement results
.T
o
see
an
example
,
select
the
preset
values
of
201
points
and a
300 kHz
to
3
GHz
frequency
range
.
Now
press
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SET
FREQ
LOW
PASS
and
observe
the
change in
frequency
values
.
The
stop
frequency
changes
to
2.999
GHz,
and
the
start
frequency changes
to
14.925
MHz.
This
would
cause
a
distortion
of
measurement
results
for
frequencies
from
300
kHz
to 14.925
MHz.
Note
If
the start
and
stop
frequencies
do
not
conform
to
the
low
pass
requirement
before
alow
pass mode
(step
or
impulse)
is
selected
and
transform
is
turned
on,
the analyzer
resets the
start
and
stop
frequencies
.
If
error
correction
is
on
when
the frequency
range is
changed,
this
turns
it
o.
Table
6-10. Minimum
Frequency
Ranges
for
Time
Domain
Low
P
ass
Number of
P
oints
Minimum Frequency
Range
3 30
kHz
to
0.09
MHz
11 30
kHz
to
0.33 MHz
26 30 kHz to 0.78 MHz
51 30 kHz to 1.53 MHz
101 30 kHz to 3.03 MHz
201 30 kHz to 6.03 MHz
401 30 kHz to 12.03 MHz
801 30 kHz
to 24.03 MHz
1601 30 kHz to 48.03 MHz
6-122 Application and Operation Concepts