HP 8753D - Page 398

T
o increase
the time
domain measurement
range
,
rst
increase
the
number
of
points
,
but
remember
that as
the
number
of
points
increases
,
the
sweep
speed
decreases
.
Decreasing the
frequency
span also
increases
range
,
but
reduces
resolution.
Resolution
Two
dierent resolution
terms are
used
in
the
time
domain:
response
resolution
range
resolution
Response
resolution.
Time
domain
response
resolution
is
dened
as
the
ability
to
resolve
two
closely-spaced
responses
,
or
a
measure
of
how
close
two
responses
can
be to
each other
and
still
be
distinguished
from
each
other
.
F
or
responses
of
equal
amplitude,
the response
resolution
is
equal
to
the
50%
(
0
6
dB)
impulse
width.
It
is inversely
proportional to
the measurement
frequency
span,
and
is
also
a
function
of
the
window
used
in
the transform.
The approximate
formulas
for
calculating
the
50%
impulse
width
are
given
in
T
able
6-11.
For
example,
using the
formula
for
the
bandpass
mode
with
a
normal
windowing
function
for
a
50 MHz
to 13.05
GHz
measurement
(13.0
GHz
span):
50%
cal
cul
ated
impul
se
w
idth
=
0
:
98
13
:
0
( GH
z )
2
2
=0
: 151
nanoseconds
E
l
ectrical
leng
th
(
in
air
)
=
(0
:
151
2
10
0
9
s
)
2 (30
2 10
9
cm=s
)
=
4
:
53
centimeter
s
With
this
measurement,
two
equal
responses
can
be
distinguished
when they
are
separated
by
at
least
4.53
centimeters
.
In
a
measurement
with
a20
GHz
span,
two
equal
responses
can
be
distinguished
when
they are
separated
by
at
least
2.94
cm.
Using
the
low
pass
mode
(the
low
pass
frequencies
are slightly
dierent)
with
a
minimum
windowing
function,
you
can
distinguish
two
equal
responses that
are about
1.38
centimeters
or
more
apart.
F
or
reection
measurements,
which measure
the
two-way
time
to
the
response
,
divide
the
response
resolution
by
2. Using
the example
above
,
you
can
distinguish
two
faults
of
equal
magnitude
provided
they
are
0.69
centimeters
(electrical
length)
or
more
apart.
Note
Remember
,
to
determine
the
physical
length,
the
relative
velocity
factor
of
the
transmission
medium
under
test must
be entered
into the
electrical length
equation.
For example
, a cable with a teon
dielectric (0.7 relative velocity factor), measured under the
conditions stated above
, has a fault location measurement response
resolution of
0.45 centimeters
. This is the maximum fault location response resolution.
Factors such as
reduced frequency span, greater frequency domain data windowing, and a large
discontinuity
shadowing the response of a smaller discontinuity
, all act to degrade the eective response
resolution.
Figure 6-72 illustrates the eects of response resolution. The solid line shows the actual
reection measurement of two approximately equal discontinuities (the input and output of an
SMA barrel). The dashed line shows the approximate eect of each discontinuity,iftheycould
be measured separately.
Application and Operation Concepts 6-131