IPC-TM-650 EN 2022 试验方法.pdf - 第481页

Step 1 – Hold the probe in air (see Figure 5-9) and measure the average voltage levels for each of those parts of the TDR waveform corresponding to the open step, V open , and to the reference air line, V std, 0 . Calcul…

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directly
to Step 5. The reflection coefficient, ρ
TL
for
the trans-
mission line is given by:
ρ
TL
=
V
r,1
V
i,1
Step
5 –
Calculate
the characteristic impedance, Z
C,x
,o
fthe
transmission line using:
Z
C,x
= Z
tran
1 +ρ
TL
1 −ρ
TL
The Z
C,x
, therefore,
corresponds to the average, minimum, or
maximum characteristic impedance value of the transmission
line under test.
5.2.2
In-situ Reference Method
In
this method, the
impedance of the reference air line is the basis from which the
characteristic impedance of all test transmission lines is com-
puted. The in-situ reference method requires the least number
of acquired waveforms compared to the other two single-
ended methods described herein. This method provides a
direct comparison between the reference impedance and the
impedance of the transmission line under test, the other two
methods do not, and this reduces the effects of drift in TDR
amplitude offset on the Z
0
.
However, because the reference is
always in use, it is more likely to become damaged and this
may cause measurement error.
5.2.2.1
Measurement Calibration Procedure
This
method
does not require a calibration step other than the
general system calibration described in 5.1.1.
5.2.2.2
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).
IPC-2257a-5-7
Figure 5-7 Calibration of Transfer Standard
TIME
TDR
INSTRUMENT
SPD
TRANSFER
STANDARD
V
tran,1
AIRLINE
V
std
V
r,0
PRECISION
RF CABLE
MEASUREMENT ZONE
for REFERENCE LINE
t
i,Ref
t
f,Ref
MEASUREMENT ZONE
for AIR LINE
t
f,std
t
i,std
IPC-2257a-5-8
Figure
5-8 TDR Measurement of Transmission Line Under Test
TDR
INSTRUMENT
SPD
TRANSFER
STANDARD
TIME
V
tran,3
V
C,ave
V
r,1
MEASUREMENT ZONE
for TRANSMISSION
LINE UNDER TEST
TRANSMISSION LINE UNDER TEST
PRECISION
RF CABLE
MEASUREMENT ZONE
for TRANSFER STANDARD
t
i,Ref
t
f,Ref
t
i,TL
t
f,TL
IPC-TM-650
Number
2.5.5.7
Subject
Characteristic
Impedance of Lines on Printed Boards by TDR
Date
03/04
Revision
A
Page
12 of 23
电子技术应用　　　　　　　ｗｗｗ．ＣｈｉｎａＡＥＴ．ｃｏｍ
Step
1 –
Hold
the probe in air (see Figure 5-9) and measure
the average voltage levels for each of those parts of the TDR
waveform corresponding to the open step, V
open
, and
to the
reference air line, V
std,0
. Calculate the amplitude, V
i,0
,ofthe
incident voltage step using:
V
i,0
= V
open
− V
std,0
Step
2 –
Probe
the transmission line (see Figure 5-10) and
measure the mean, minimum, and maximum voltage values,
V
C,ave
,V
C,min
, and V
C,max
, of
that part of the TDR waveform
corresponding to the measurement zone of the transmission
line.
Step
3 –
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 reference air line, V
std,1
,
and
compute the voltage difference, V
r,1
:
V
r,1
= V
std,1
− V
C,x
where V
C,x
the
subscript ‘‘X’’ refers to ‘‘ave,’’ ‘‘min,’’ or
‘‘max.’’
Step
4 –
Calculate
the reflection coefficient of the transmis-
sion line und er test relative to the reference air line. If the TDR
system already provides reflection coefficient values, go
IPC-2257a-5-9
Figure
5-9 Calibration of Incident Step Amplitude
t
1,TL
TIME
V
i,0
AIRLINE
PROBE
SPD
TDR
INSTRUMENT
V
std,0
V
open
PRECISION
RF CABLE
t
i,std
t
f,std
MEASUREMENT ZONE
for AIR LINE
IPC-2257a-5-10
Figure
5-10 TDR Measurement of Transmission Line
V
std,1
V
C,ave
V
r,1
MEASUREMENT ZONE
for TRANSMISSION LINE UNDER TEST
TRANSMISSION LINE UNDER TEST
AIRLINE
PROBE
PRECISION
RF CABLE
MEASUREMENT ZONE
for AIR LINE
t
i,std
t
f,std
TDR
INSTRUMENT
SPD
TIME
t
i,TL
t
f,TL
IPC-TM-650
Number
2.5.5.7
Subject
Characteristic
Impedance of Lines on Printed Boards by TDR
Date
03/04
Revision
A
Page
13 of 23
电子技术应用　　　　　　　ｗｗｗ．ＣｈｉｎａＡＥＴ．ｃｏｍ
directly
to Step 5. The reflection coefficient, ρ
TL
for
the trans-
mission line is given by:
ρ
TL
=
V
r,0
V
i,0
Step
5 –
Calculate
the characteristic impedance, Z
C,X
, of
the
transmission line using:
Z
C,x
= Z
std
1 +ρ
TL
1 −ρ
TL
The Z
C,x
, therefore,
corresponds to the average, minimum, or
maximum characteristic impedance value of the transmission
line under test.
5.2.3
Stored Reference Waveform Method
The
stored
reference method requires a fewer number of measurements
than the transfer standard method (see 5.2.1) because the
reference waveform is stored and subsequent waveforms
compared to it. However, it is important in this method that
the amplitude offset of the TDR unit does not drift.
5.2.3.1
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).
Step
1 –
Hold
the probe in air (see Figure 5-11) and measure
the average voltage levels for each of those parts of the TDR
waveform corresponding to the open step, V
open
, and
to the
reference standard, V
std,0
.
Calculate the amplitude, V
i,0
,o
fthe
incident voltage step using:
V
i,0
= V
open
− V
std,0
Step
2 –
Record
the voltage, V
check,0
, in
the TDR waveform
corresponding to the precision RF cable.
5.2.3.2
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).
Step
1 –
Probe
the transmission line (see Figure 5-12) and
measure the mean, minimum, and maximum voltage values,
V
C,ave
,V
C,min
, and V
C,max
, of
that part of the TDR waveform
corresponding to the measurement zone of the transmission
line.
Step
2 –
Record
the voltage, V
check,1
,
in the TDR waveform
corresponding to the precision RF cable and compute the fol-
lowing:
V
diff
=
?
V
check,0
− V
check,1
V
i,0
?
If V
diff
is
greater than 0.002, then the TDR system must be
recalibrated (see 5.2.3.1).
IPC-2257a-5-11
Figure
5-11 Calibration for Stored Reference Method
AIRLINE
PROBE
SPD
TDR
INSTRUMENT
V
std,0
V
open
PRECISION
RF CABLE
V
check,0
t
f,std
t
i,std
MEASUREMENT ZONE
for AIR LINE
t
1,TL
TIME
V
i,0
IPC-TM-650
Number
2.5.5.7
Subject
Characteristic
Impedance of Lines on Printed Boards by TDR
Date
03/04
Revision
A
Page
14 of 23
电子技术应用　　　　　　　ｗｗｗ．ＣｈｉｎａＡＥＴ．ｃｏｍ