IPC-TM-650 EN 2022 试验方法.pdf - 第476页
Step 2 – Place the probe in contact with the transmission line under test and locate the instant, t 2, TL , on the TDR waveform where the transmission line/open discontinuity occurs (see Figure 5-2). t 2, TL is the insta…
sufficient
robustness so that its impedance remains constant
between calibration cycles.
4.3.8
Adapter, Airline-to-Probe Contact Pad
A
preferred
method of calibrating the TDR amplitude response is to con-
nect a probe contact pad directly to the airline and to probe
the airline through the contact pad (see Figure 4-3). The con-
tact pad should have a nominal characteristic impedance of
50 Ω ± 1.0 Ω. An increase in measurement accuracy can be
achieved by using a reference impedance standard that is
closely matched to the impedance of the transmission line to
be tested.
5
Procedures
5.1 Measurement Preliminaries
In
this section, common
considerations for the calibration of the TDR measurement
system and performing the TDR measurements are provided.
5.1.3 describes a method for establishing the measurement
zone that can be applied to the measurement methods
described in 5.2 and 5.3.
5.1.1
System Calibration
Follow
the TDR instrument
manufacturer’s recommendation for the frequency of factory
calibration. TDR system ‘‘field’’ calibrations are to be per-
formed at regular intervals in addition to the less regular fac-
tory calibrations. The field calibrations are required for the fol-
lowing reasons:
a. TDR instrument specifications vary with temperature.
b. TDR instrument specifications vary with time (drift).
c. TDR instrument specifications vary due to minor ESD dam-
age.
d. TDR instrument factory calibration usually does not include
user supplied auxiliary components (e.g., cables, probes,
etc.).
TDR system field calibrations should also be performed after
a change of any system component (such as, cable, probes,
etc.). Ensure that the TDR instrument has been operating for
at least 30 minutes prior to any calibration or test measure-
ment procedure.
Use proper ESD control methods to avoid damage to the TDR
instrument in all calibration and test measurement proce-
dures. ESD control components can include static dissipative
mats, deionizer systems, and operator gowning.
5.1.2
Premeasurement Checks
The
test measurement
should be performed after the completion of the calibration
process. Ensure that plane of the signal line of a microstrip (or
embedded microstrip) structure is at least a distance equal to
six times the width of the microstrip signal line from any mate-
rial (such as the testing table) that can affect the dielectric
environment of the microstrip line. If the tests are being con-
ducted with hand probe(s), care must be taken to ensure that
the hands and/or arms of the operator do not contact any
surface of the board over the transmission line being tested.
Probes should be applied to the test points with sufficient
force to ensure proper electrical contact between the trace
and the probe assembly. Consistent application (that is, force,
angle of placement, etc.) of the probes onto the test points is
important to ensure repeatable measurement results.
Before recording any measurement results, ensure that the
TDR waveform is stable (that is, not drifting in amplitude or
time) otherwise measurement error will occur. To improve the
accuracy of the impedance measurement, it is important to
optimize the vertical gain setting (typically in V/div or ρ/div) and
horizontal axis setting (typically in time/div) of the TDR unit so
as to maximize the duration of the measurement zone within
the TDR waveform and to increase amplitude resolution. Per-
form this TDR adjustment prior to acquiring any TDR wave-
forms from which Z
0
will
be computed. Ensure that the tem-
perature and humidity of the test environment is within TDR
instrument specifications and is stable.
5.1.3 Establishing the Measurement Zone
The
value of
the measurement zone is critical to the accuracy and repeat-
ability of the TDR measurement process. Measurement zone
differences are a large factor in correlation problems between
measurements. The measurement zone should be set repeat-
ability for each transmission line independent of the type of
dielectric material surrounding the transmission line or its
structure (surface microstrip, embedded microstrip, stripline,
differential pair, etc.). The following process can be incorpo-
rated into the test measurement process. There are two mea-
surement zones, one for the transmission line under test (see
5.1.3.1) and one for the reference (see 5.1.3.2), which may be
either a transfer standard or a coaxial air line.
5.1.3.1
Procedure for the Transmission Line Under Test
Step
1 –
Hold
the probe in the air and locate the instant, t
1,TL
,
on
the TDR waveform where the probe/open discontinuity
occurs (see Figure 5-1). t
1,TL
is
the instant in the TDR wave-
form when the reflection from the open circuit has reached
50 % of its amplitude (see Figure 5-1), unless otherwise speci-
fied by the user.
IPC-TM-650
Number
2.5.5.7
Subject
Characteristic
Impedance of Lines on Printed Boards by TDR
Date
03/04
Revision
A
P
age7of23
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Step
2 –
Place
the probe in contact with the transmission line
under test and locate the instant, t
2,TL
,
on the TDR waveform
where the transmission line/open discontinuity occurs (see
Figure 5-2). t
2,TL
is
the instant in the TDR waveform when the
reflection from the open circuit has reached 50 % of its ampli-
tude (see Figure 5-2), unless otherwise specified by the user.
Step
3 –
Compute
the round trip propagation time of the
transmission line using:
T
rt,TL
= t
2,TL
− t
1,TL
Step
4 –
Determine
the initial instant, t
i,TL
,
of measurement
zone (see Figure 5-3) using:
t
i,TL
= t
1,TL
+ x
i%
T
rt,TL
where x
i%
is
the lower limit of the measurement zone and is
30 % unless otherwise specified by the user.
Step
5 –
Determine
final instant, t
f,TL
, of
measurement zone
(see Figure 5-3) using:
t
ƒ,TL
= t
1,TL
+ x
ƒ%
T
rt,TL
where x
f%
is
the upper limit of the measurement zone and is
70 % unless otherwise specified by the user.
IPC-2257a-5-1
Figure
5-1 Determination of instant in the TDR waveform corresponding to the beginning of the transmission line. A
R,1
is
the
amplitude of the signal reflected from the open end of the probe. SPD is the static protection device (see 4.3.5).
PROBE
SPD
TDR
INSTRUMENT
PRECISION
RF CABLE
TRANSFER
STANDARD
TIME
0.5A
R,1
t
1,TL
A
R,1
IPC-2257a-5-2
Figure
5-2 Determination of instant in TDR waveform corresponding to the end of the transmission line.
A
R,2
is
the
amplitude of the signal reflected from the open end of the transmission line under test.
SPD
TRANSFER
STANDARD
TDR
INSTRUMENT
TRANSMISSION LINE UNDER TEST
0.5
A
R2
TIME
t
1,TL
t
2,TL
A
R2
PRECISION
RF CABLE
IPC-TM-650
Number
2.5.5.7
Subject
Characteristic
Impedance of Lines on Printed Boards by TDR
Date
03/04
Revision
A
P
age8of23
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5.1.3.2
Procedure for the Reference Line
Step
1 –
Remove
the transfer standard (or air line reference)
and hold the precision rf cable in the air and locate the instant,
t
1,Ref
,
on the TDR waveform where the rf cable/open discon-
tinuity occurs (see Figure 5-4). t
1,Ref
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.
Step
2 –
Connect
the rf cable to the reference line and locate
the instant, t
2,Ref
,
on the TDR waveform where the reference
line/open discontinuity occurs (see Figure 5-5). t
2,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.
Step
3 –
Compute
the round trip propagation time of the
transmission line using:
T
rt,Ref
= t
2,Ref
− t
1,Ref
Step
4 –
Determine
the initial instant, t
i,Ref
,
of measurement
zone using:
t
i,Ref
= t
1,Ref
+ x
i%
T
rt,Ref
where x
i%
is
the lower limit of the measurement zone and is 30
% unless otherwise specified by the user.
Step
5 –
Determine
final instant, t
f,TL
,
of measurement zone
using:
t
ƒ,Ref
= t
1,Ref
+χ
ƒT
T
rt,Ref
where x
f%
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).
IPC-2257a-5-3
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
Number
2.5.5.7
Subject
Characteristic
Impedance of Lines on Printed Boards by TDR
Date
03/04
Revision
A
P
age9of23
电子技术应用 ｗｗｗ．ＣｈｉｎａＡＥＴ．ｃｏｍ