IPC-TM-650 EN 2022 试验方法.pdf - 第485页

Step 2 – Probe the reference airline using suitable adapter and obtain a TDR waveform similar to that shown in Figure 5-16 for each channel. Measure the average voltage levels for the high and low states for the two diff…

100%1 / 824
5.3.2
Determination of Measurement Zone
The
deter-
mination of the measurement zone is done similarly to that for
single-ended transmission line test methods as discussed in
5.1.3. However, for this differential test method, the TDR
waveform will appear similar to that shown in Figure 5-14.
The instants shown in Figure 5-14, t
i,TS
,t
f,TS
,t
i,TL
, and t
f,TL
,
are
the initial and final instants of the measurements zones for
the transfer standard and transmission line under test. The
t
i,TS
,t
f,TS
are
the same as t
i,Ref
and t
f,Ref
described
for single-
ended transmission lines in 5.1.3.
5.3.3
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 5.3.4).
Step
1 –
Hold
the probe in air and measure the average volt-
age levels corresponding to the high and low states of the two
differential TDR waveforms, which are labeled V
TS,Ch1,1
,
V
TS,Ch2,1
, V
open,Ch1
,
and V
open,Ch2
in
Figure 5-15. There are a
total of four states, two for each of the differential waveforms.
Calculate the amplitude, V
inc,Ch1
,
of the incident voltage step
for Channel 1 (Ch1) using:
V
inc,Ch1
= V
TS,Ch1,1
− V
open,Ch1
and
the amplitude, V
inc,Ch2
,
of the incident voltage step for
Channel 2 (Ch2) using:
V
inc,Ch2
= V
TS,Ch2,1
− V
open,Ch2
where:
V
TS,Ch1,1
is
the average voltage level of that part of the Ch1
TDR waveform corresponding to the transfer standard,
V
TS,Ch2,1
is
the average voltage level of that part of the Ch2
TDR waveform corresponding to the transfer standard,
V
open,Ch1
is
the average voltage level of that part of the Ch1
TDR waveform corresponding to the high state of the differen-
tial signal applied to Ch1, and
V
open,Ch2
is
the average voltage level of that part of the Ch2
TDR waveform corresponding to the high state of the differen-
tial signal applied to Ch2.
IPC-2257a-5-13
Figure
5-13 Differential TDR Waveform
-0.4
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
0.4
Time
Signal (V)
Ch1
Ch2
T
ransmission Line
Measurement Zone
t
i
t
f
IPC-TM-650
Number
2.5.5.7
Subject
Characteristic
Impedance of Lines on Printed Boards by TDR
Date
03/04
Revision
A
Page
16 of 23
电子技术应用　　　　　　　ｗｗｗ．ＣｈｉｎａＡＥＴ．ｃｏｍ
Step
2 –
Probe
the reference airline using suitable adapter
and obtain a TDR waveform similar to that shown in Figure
5-16 for each channel. Measure the average voltage levels for
the high and low states for the two differential TDR wave-
forms, which are labeled V
TS,Ch1,2
, V
TS,Ch2,2
, V
std,Ch1
,
and
V
std,Ch2
in
Figure 5-16. There are a total of four states, two for
each of the differential waveforms. Calculate the voltage differ-
ence, V
r,Ch1
,
for Channel 1 (Ch1) using:
V
r,Ch1
= V
TS,Ch1,2
− V
std,Ch1
and
the voltage difference, V
r,Ch2
,
for Channel 2 (Ch2) using:
V
r,Ch2
= V
Ts,Ch2,2
− V
std,Ch2
where:
V
TS,Ch1,2
is
the average voltage level of that part of the Ch1
TDR waveform corresponding to the transfer standard (not
the same value as used in Step 1),
V
TS,Ch2,2
is
the average voltage level of that part of the Ch2
TDR waveform corresponding to the transfer standard (not
the same value as used in Step 1),
V
std,Ch1
is
the average voltage level of that part of the Ch1
TDR waveform corresponding to the reference standard (the
airline), and
V
lo,Ch2
is
the average voltage level of that part of the Ch2 TDR
waveform corresponding to the reference standard (the air-
line).
This calibration step can be performed using either one refer-
ence airline or two. Because the reference airline contains only
one signal conductor, if one airline is used, then calibration of
the two channels must be performed sequentially (in which
case, Figure 5-16 is a composite of two TDR waveforms, one
for each differential TDR channel). If two airlines are used, then
the calibration of the two channels can be performed simulta-
neously.
Step
3 –
Calculate
the characteristic impedance, Z
TS,Ch1
,o
f
the transfer standard for Channel 1 (Ch1) using:
Z
TS,Ch1
=
(
V
inc,Ch1
− V
r,Ch1
V
inc,Ch1
+ V
r,Ch1
)
Z
std
IPC-2257a-5-14
Figure
5-14 Measurement Zones for Differential TDR
-0.4
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
0.4
Time
Signal (V)
Ch1
Ch2
TDR/Probe Interf
ace
Probe/T
ransmission Line Interface
T
ransfer Standard
Measurement Zone
T
ransmission Line
Measurement Zone
t
i,TS
t
i,TL
t
f
,TL
t
f
,TS
IPC-TM-650
Number
2.5.5.7
Subject
Characteristic
Impedance of Lines on Printed Boards by TDR
Date
03/04
Revision
A
Page
17 of 23
电子技术应用　　　　　　　ｗｗｗ．ＣｈｉｎａＡＥＴ．ｃｏｍ
IPC-2257a-5-15
Figure
5-15 Measuring Amplitude for Incident Step
-0.4
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
0.4
Signal (V)
Time
t
i,TS
t
f
,TS
V
open,Ch1
V
open,Ch
2
Ch1
Ch2
V
TS
,Ch2,1
V
TS
,Ch1,1
-0.5
-0.6
t
i,TL
t
f
,TL
0.5
0.6
IPC-2257a-5-16
Figure
5-16 Calibration of Transfer Standard
-0.4
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
0.4
Signal (V)
Time
V
TS
,Ch2,2
V
TS
,Ch1,2
t
i,TS
t
i,std
t
f
,std
t
f
,TS
Ch1
Ch2
V
std,Ch1
V
std,Ch2
IPC-TM-650
Number
2.5.5.7
Subject
Characteristic
Impedance of Lines on Printed Boards by TDR
Date
03/04
Revision
A
Page
18 of 23
电子技术应用　　　　　　　ｗｗｗ．ＣｈｉｎａＡＥＴ．ｃｏｍ