IPC-TM-650 EN 2022 试验方法.pdf - 第479页
Step 1 – 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, V open , and to the transfer standard, V tran, 0 . Calcul…
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
Step
1 –
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, V
open
,
and to the
transfer standard, V
tran,0
. Calculate the amplitude, V
i,0
,ofthe
incident voltage step using:
V
i,0
= V
open
− V
tran,0
Step
2 –
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, V
std
, and
to the transfer standard,
V
tran,1
,
and compute the voltage difference, V
r,0
:
V
r,0
= V
std
− V
tran,1
The
instants, t
i,std
and t
f,std
,
shown in Figure 5-7 are anala-
gous to t
i,TL
and t
f,TL
except
that the former are for the air line
standard from which the transfer standard impedance is
determined.
Step
3 –
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, ρ
tran
for
the transfer standard is given by:
ρ
tran
=
V
r,0
V
i,0
Step
4 –
Calculate
the impedance, Z
tran
, of
the transfer stan-
dard using the following formula:
Z
tran
= Z
std
1 +ρ
tran
1 −ρ
tran
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).
Step
1 –
Repeat
the measurements of Step 1 of 5.2.1.1. Cal-
culate the amplitude, V
i,1
,
of the incident voltage step using:
V
i,1
= V
open
− V
tran,2
where V
tran,2
is
the average voltage level of that part of the
TDR waveform corresponding to the transfer standard.
Step
2 –
Probe
the transmission line (see Figure 5-8) 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 transfer standard, V
tran,3
,
and
compute the voltage difference, V
r,1
:
V
r,1
= V
tran,3
− 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 under test relative to the transfer standard. If the TDR
system already provides reflection coefficient values, go
IPC-2257a-5-6
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
Number
2.5.5.7
Subject
Characteristic
Impedance of Lines on Printed Boards by TDR
Date
03/04
Revision
A
Page
11 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,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