IPC-TM-650 EN 2022 试验方法.pdf - 第477页
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/…
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
电子技术应用 ｗｗｗ．ＣｈｉｎａＡＥＴ．ｃｏｍ
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
IPC-2257a-5-4
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
IPC-2257a-5-5
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
Number
2.5.5.7
Subject
Characteristic
Impedance of Lines on Printed Boards by TDR
Date
03/04
Revision
A
Page
10 of 23
电子技术应用 ｗｗｗ．ＣｈｉｎａＡＥＴ．ｃｏｍ