IPC-TM-650 EN 2022 试验方法--.pdf - 第478页

Figure 5-4 Determination of instant in the TDR wav eform corresponding to th e be ginning of the r eference line. A R,0 is the amplitude of the signal refle cted from the open end o f rf cable. SPD TDR INSTRUMENT PRECISIO…

100%1 / 824
Step 1 –
Step 2 –
Step 3 –
Step 4 –
Step 5 –
Figure 5-3 Determination of Measurement Zone
T
rt
,TL
TIME
t
1,TL
t
2,TL
SPD
TRANSFER
STANDARD
TDR
INSTRUMENT
TRANSMISSION LINE UNDER TEST
t
i,TL
t
f,TL
MEASUREMENT ZONE
for TRANSMISSION LINE UNDER TEST
PRECISION
RF CABLE
IPC-TM-650
Page 9 of 23
Number
2.5.5.7
Subject
Characteristic
Impedance
of
Lines
on
Printed
Boards
by
TDR
Date
03/04
Revision
A
5.1.
3.2
Procedure
for
the
Reference
Line
Remove
the
transfer
standard
(or
air
line
reference)
and
hold
the
precision
rf
cable
in
the
air
and
locate
the
instant,
L
Ref，
on
the
TDR
waveform
where
the
rf
cable/open
discon¬
tinuity
occurs
(see
Figure
5-4).
^1Ref
is
the
instant
in
the
TDR
waveform
when
the
reflection
from
the
open
circuit
has
reached
50
%
of
its
amplitude
(see
Figure
5-4),
unless
other¬
wise
specified
by
the
user.
Connect
the
rf
cable
to
the
reference
line
and
locate
the
instant,
t2
Ref,
on
the
TDR
waveform
where
the
reference
line/open
discontinuity
occurs
(see
Figure
5-5).
^‘Ref
is
the
instant
in
the
TDR
waveform
when
the
reflection
from
the
open
circuit
has
reached
50
%
of
its
amplitude
(see
Figure
5-5),
unless
otherwise
specified
by
the
user.
Compute
the
round
trip
propagation
time
of
the
transmission
line
using:
Trt,Ref
=
bkef
-
,Ref
Determine
the
initial
instant,
tiRef,
of
measurement
zone
using:
L,Ref
—
^1
,Ref
+
X/%7^,Aef
where
xi%
is
the
lower
limit
of
the
measurement
zone
and
is
30
%
unless
otherwise
specified
by
the
user.
Determine
final
instant,
tf
TL,
of
measurement
zone
using:
tf,Ref
=
t[Ref
+
XfT^rt.Ref
where
x
侠
is
the
upper
limit
of
the
measurement
zone
and
is
70
%
unless
otherwise
specified
by
the
user.
5.2
Single-Ended
TDR
Measurement
Procedures
This
section
contains
three
methods
for
measuring
the
character¬
istic
impedance
of
single-ended
transmission
lines.
The
fol¬
lowing
calibration
and
measurement
steps
should
be
used
when
the
device(s)
under
test
are
unbalanced
(single-ended)
transmission
lines.
This
process
can
be
followed
manually
but,
to
improve
measurement
repeatability
and
reduce
measure¬
ment
time,
an
automated
measurement
system
is
recom¬
mended.
Additionally,
the
use
of
a
fixture
based
or
robotic
probing
system
greatly
improves
the
accuracy
and
repeatabil¬
ity
over
hand
probe
techniques
and
further
reduces
the
mea¬
surement
time.
5.2.1
Transfer
Standard
Method
In
this
method,
the
impedance
of
the
reference
air
line
is
transferred
to
a
second¬
ary
transmission
line.
The
computed
impedance
of
the
sec¬
ondary
or
transfer
line
then
becomes
the
basis
from
which
the
characteristic
impedance
of
all
subsequent
test
transmission
lines
is
computed.
The
transfer
method
provides
a
direct
com¬
parison
of
the
impedance
of
the
transfer
standard
to
that
of
the
transmission
line
under
test.
Although
this
does
require
two
additional
measurements,
as
compared
to
the
in-situ
method
(see
5.2.2),
it
does
reduce
the
risk
of
damage
to
the
reference
impedance
standard
due
to
frequent
use
and
han¬
dling.
The
effects
of
drift
in
TDR
amplitude
offset
are
mini¬
mized
with
this
method.
5.2.1.
1
Measurement
Calibration
Procedure
The
instru¬
ment
setting
must
be
the
same
for
Steps
1
and
2.
This
pro¬
cedure
will
determine
the
characteristic
impedance
of
the
transfer
standard
from
which
characteristic
impedance
of
the
transmission
line
under
test
will
be
determined
(see
5.2.1
.2).
Figure 5-4 Determination of instant in the TDR waveform corresponding to the beginning of the reference line. A
R,0
is the
amplitude of the signal reflected from the open end of rf cable.
SPD
TDR
INSTRUMENT
PRECISION
RF CABLE
t
1,Ref
TIME
0.5
A
R.0
A
R,0
Figure 5-5 Determination of instant in TDR waveform corresponding to the end of the reference line (transfer standard or
air line reference).
A
R,0
is the amplitude of the signal reflected from the open end of the reference line.
SPD
TDR
INSTRUMENT
PRECISION
RF CABLE
t
1,Ref
TIME
0.5
A
R,0
A
R,0
REFERENCE LINE
t
2,Ref
IPC-TM-650
Page 10 of 23
Number
2.5.5.7
Subject
Characteristic
Impedance
of
Lines
on
Printed
Boards
by
TDR
Date
03/04
Revision
A
Step 1 –
Step 2 –
Step 3 –
Step 4 –
Step 1 –
Step 2 –
Step 3 –
Step 4 –
Figure 5-6 Measurement of Incident Step Amplitude
TIME
TDR
INSTRUMENT
SPD
TRANSFER
STANDARD
V
tran,0
V
open
V
i,0
MEASUREMENT ZONE
for TRANSMISSION LINE UNDER TEST
PRECISION
RF CABLE
t
i,TL
t
f,TL
MEASUREMENT ZONE
for REFERENCE LINE
t
i,Ref
t
f,Ref
IPC-TM-650
Page 11 of 23
Number
2.5.5.7
Subject
Characteristic
Impedance
of
Lines
on
Printed
Boards
by
TDR
Date
03/04
Revision
A
Hold
the
probe
in
air
(see
Figure
5-6)
and
measure
the
average
voltage
levels
for
each
of
those
parts
of
the
TDR
waveform
corresponding
to
the
open
step,
Vopen,
and
to
the
transfer
standard,
Vtran
0.
Calculate
the
amplitude,
Vi0,
of
the
incident
voltage
step
using:
二
Vopen
—
%rac,0
Probe
the
reference
airline
(see
Figure
5-7)
through
the
appropriate
interface
(see
4.3.8)
and
measure
the
average
voltage
levels
for
each
of
those
parts
of
the
TDR
waveform
corresponding
to
the
airline,
Vstd,
and
to
the
transfer
standard,
%c,i,
and
compute
the
voltage
difference,
Vr0:
%0
=
Vstd
-
Vtran,1
The
instants,
ti>std
and
tfstd,
shown
in
Figure
5-7
are
anala-
gous
to
ti
TL
and
tf
TL
except
that
the
former
are
for
the
air
line
standard
from
which
the
transfer
standard
impedance
is
determined.
Calculate
the
reflection
coefficient
of
the
transfer
standard
relative
to
the
air
line.
If
the
TDR
system
already
pro¬
vides
reflection
coefficient
values,
go
directly
to
Step
4.
The
reflection
coefficient,
ptran
for
the
transfer
standard
is
given
by:
—必
p
痴
F
Calculate
the
impedance,
Ztran,
of
the
transfer
stan¬
dard
using
the
following
formula:
7
_7
1
+
Ptran
/
二
^std
~
1
-
Ptran
5.2.
1.2
Measurement
Process
The
instrument
setting
must
be
the
same
for
Steps
1
and
2.
This
procedure
will
determine
the
characteristic
impedance
of
the
transmission
line
under
test.
This
process
should
be
done
after
the
mea¬
sure
zone
has
been
defined
(see
5.1
.3).
Repeat
the
measurements
of
Step
1
of
5.2.1
.1.
Cal¬
culate
the
amplitude,
忆，
of
the
incident
voltage
step
using:
匕
1
二
Vopen
—
匕
ran,
2
where
Vtran
2
is
the
average
voltage
level
of
that
part
of
the
TDR
waveform
corresponding
to
the
transfer
standard.
Probe
the
transmission
line
(see
Figure
5-8)
and
measure
the
mean,
minimum,
and
maximum
voltage
values,
心
a”，
％
min>
and
J
of
that
part
of
the
TDR
waveform
corresponding
to
the
measurement
zone
of
the
transmission
line.
Without
moving
the
probe
from
the
position
used
in
Step
2,
measure
the
average
voltage
level
for
that
part
of
the
TDR
waveform
corresponding
to
the
transfer
standard,
Vtran3,
and
compute
the
voltage
difference,
%:
=
%ra〃,3
-
Vgx
where
\/c(x
the
subscript
“X”
refers
to
"ave,”
"min,''
or
''max.''
Calculate
the
reflection
coefficient
of
the
transmis¬
sion
line
under
test
relative
to
the
transfer
standard.
If
the
TDR
system
already
provides
reflection
coefficient
values,
go
IPC-2257a-5-6