IPC-TM-650 EN 2022 试验方法-- - 第607页

Figure 4 Cable Connectio n Device. Refer circled items t o parts list. Made from Ge ner al Radio Co. T ype 874-C62A. IPC-TM-650 Number Subject Date Revision Page 3 of 4 2.5.18 Characteristic Impedance Flat Cables (Unbala…

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Figure 2 TDR Trace for a Typical Cable
Figure 3 Sample Cable Hanger
IPC-TM-650
Number
Subject Date
Revision
Page 2 of 4
2.5.18
Characteristic
Impedance
Flat
Cables
(Unbalanced)
7/84
B
I
PC-2-5-1
8-5
4.1
In
this
test,
characteristic
impedance
is
measured
by
TDR.
Commercial
TDRs
are
readily
available
and
consist
of
pulse
generator
and
sampling
oscilloscopes.
Rise
times
of
the
pulses
are
usually
less
than
250
picoseconds
(250
x
10-12
sec.),
which
gives
a
resolution
sufficient
to
detect
discontinui¬
ties
smaller
than
2.5
cm
in
length.
Since
the
pulse
rise
times
generally
used
now
in
electronic
equipment
are
not
this
fast,
a
TDR
is
adequate
for
testing.
Also
required
for
this
test
is
a
lab
standard
air
line
to
establish
a
reference
impedance
(Zo
ref.)
and
a
standard
cable
connection
device
at
the
air
line
output
(see
Figure
1).
4.2
A
TDR,
such
as
a
Hewlett-Packard
1415A,
Hewlett-
Packard
1815A,
Tektronix
1
S2,
or
equivalent
4.3
The
standard
air
line
used
should
be
a
General
radio
874-L20
(20
cm),
874-L30
(30
cm),
or
equivalent
for
Zo
二
50Q.
4.4
Cable
Holders
Fixture
of
plexiglass
or
other
nonmetal-
lic
material.
Cable
hangers
to
suspend
the
cable
in
air.
Refer
to
Figure
3.
4.5
The
standard
cable
connection
device
used
should
match
Figure
4.
It
is
made
from
a
General
radio
cable
connec¬
tor
type
874-C62A.
Figure 4 Cable Connection Device. Refer circled items to parts list. Made from General Radio Co. Type 874-C62A.
IPC-TM-650
Number
Subject Date
Revision
Page 3 of 4
2.5.18
Characteristic
Impedance
Flat
Cables
(Unbalanced)
7/84
B
4.6
Coaxial
Cable
Impedance:
50
-
2c
RG-58A,
RG-58C,
or
equivalent;
Termination:
GR874
connectors,
both
ends;
Length:
approximately
61
cm
4.7
Load
General
Radio
type
GR874
or
equivalent
50Q
load.
This
is
an
optional
item,
which
is
used
to
calibrate
the
TDR.
5
Procedure
5.1
Allow
a
minimum
of
one
hour
for
TDR
warm-up
and
calibrate
the
instrument
per
manufacturer's
instructions.
5.2
Prepare
the
test
specimen
by
stripping
approximately
1
3
mm
of
insulation
from
one
end
of
cable.
Separate
the
ground
and
signal
conductors
and
solder
a
copper
buss
across
the
grounds
(see
Figure
5).
5.3
Adjust
the
TDR
settings
as
follows:
Vertical:
0.1
e/cm
Distance/time:
20
ns/cm.
Magnifier:
50
x
(For
equipment
other
than
Hewlett-Packard,
use
settings
as
close
as
possible
to
these.)
Insert
the
30
cm
air
line
into
the
output
of
the
TDR.
This
will
serve
as
the
50Q
reference.
Attach
the
coaxial
cable
to
the
air
line
and
terminate
with
the
impedance
probe.
Vertically
center
the
50Q
reference
line
on
the
TDR
graticule.
5.4
Press
the
probe
against
the
conductor
to
be
tested
insuring
the
ground
of
the
probe
is
against
the
cable
ground
(see
Figure
5)
and
check
the
vertical
placement
of
the
50Q
reference;
re-center
if
necessary.
5.5
Adjust
the
distance/time
magnifier
to
5
or
10
and
rotate
the
magnifier
delay
dial
until
the
total
length
of
the
cable
is
vis¬
ible
on
the
screen.
Measure
the
vertical
reflection
coefficient
(e)
in
cm
as
illustrated
in
Figure
2.
5.7
Calculate
the
characteristic
impedance
(Zo)
as
follows:
1
+
Q
Zo
=
5O
宙
(
Q)
i
y
Calculate
Zo
of
the
cable
measuring
as
shown
in
Figure
2.
Calculate
Zo
max.,
e
=
e
max;
Zo
min.,
e
=
e
min.
Figure 5 Connection of Impedance Probe to Sample
under Test
IPC-TM-650
Number
Subject Date
Revision
Page 4 of 4
7/84
2.5.18
B
Characteristic
Impedance
Flat
Cables
(Unbalanced)
6
Notes
6.1
The
TDR
employs
a
pulse
rise
time
less
than
250
pico¬
seconds.
A
pulse
of
this
rise
time
is
extremely
rich
in
harmon¬
ics
extending
well
into
the
GHz
region
of
the
frequency
spec¬
trum.
The
impedance
probe
illustrated
in
Figure
1
is
designed
to
minimize
the
effects
of
impedance
mismatch
at
the
con¬
nection;
therefore,
it
is
suggested
that
a
probe
of
this
type
be
used
for
the
impedance
measurement.
The
importance
of
a
good
connection
between
the
cable
under
test
and
the
TDR
can
not
be
overemphasized.
Cables
longer
than
3
m
in
length
may
be
tested,
but
care
must
be
exercised
so
as
not
to
confuse
the
effect
of
increased
wire
resistance
with
an
apparent
increase
in
impedance
as
the
magnifier
delay
dial
is
rotated
to
observe
the
longer
cable
length
(function
of
attenuation,
which
includes
wire
size).
6.2
Under
no
circumstances
should
the
cable
be
tested
while
in
a
coiled
form
due
to
the
effect
of
increased
induc¬
tance.
6.3
Keep
cable
a
minimum
of
1
5
cm
away
from
any
dielec¬
tric
or
ground
plane
including
metal,
wood,
etc.
(except
in
step
5.5).
6.4
Measurement
of
Zo
of
unknown
cable
length
should
be
made
as
close
as
possible
to
the
cable
connection
device
(after
overshoot
and
undershoot).
6.5
The
reference
Zo
cable
may
be
positioned
after
the
RG58C
cable
and
before
the
cable
connection
device.
There¬
fore,
the
reference
Zo
is
adjacent
to
the
test
cable
on
the
TDR
trace.