IPC-TM-650 EN 2022 试验方法-- - 第520页
6.4.2 Wa veform Averaging and Number of Samples in the Measurement Zone Waveform averaging reduces the effective noise level of the measurement by M -1/2 , w here M i s the number o f acquired waveforms (typically, 8 ≤ M…
mechanical probing methods. Operators and probing equip-
ment should be tested in ability to repeat electrical probe con-
tacts.
6.3.6.1 Probes for Single-Ended Transmission Line
Measurements
The probe assembly impedance is often
chosen to be 50 Ω to match the impedance of the TDR sys-
tem. Impedance matching minimizes reflections at the inter-
face between the probe and the transmission line under test.
These reflections, which appear at and around the transition
region in the TDR pulse and can extend for some time after
this transition, are perturbations in the TDR waveform and are
undesirable because they may affect the computation of the
reference level instant, thereby increasing measurement
uncertainty. When the characteristic impedance of the trans-
mission line under test is nominally 50 Ω, these perturbations
will normally decay rapidly. If the impedance of the transmis-
sion line under test is significantly different from 50 Ω, the
magnitude of the perturbations can be large and their duration
long enough to affect the computation of the reference level
instant. The effect of these perturbations must be taken into
account when determining the appropriate waveform epoch
(see 4.1.2). The design and quality of manufacture of the
probe has a large effect on the magnitude and duration of
reflections generated between the TDR system and the trans-
mission line under test.
When probing non-50 Ω lines, it is possible to separate, in the
TDR waveform, the large signal perturbations caused by the
TDR/probe interface from those caused by the probe/
transmission line interface. To do this, a specially designed
probe is required that is impedance matched to the transmis-
sion line under test and that also has a long propagation delay
between the TDR/probe connection and the probe tip. The
long propagation delay can effectively move the large pertur-
bations at the TDR/probe interface out of the waveform
epoch.
6.3.6.2 Probes for Coupled-Signal-Line (Differential)
Transmission Line Measurements
The probe consider-
ations described in 4.3.3 apply for probes used in differential
transmission line measurements. However, the necessity to
simultaneously probe two signal lines and one or two refer-
ence plane contacts makes differential probing more difficult
than probing single signal line structures. In a PB manufactur-
ing environment, the use of two probes that were previously
used for single-ended measurements may not be possible.
This is because the operator is required to use both hands for
probing, which leaves them unable to operate the instrument.
Contact your instrument manufacturer for their probing solu-
tions and advice. Probes from one manufacturer can also be
used with another manufacturer’s TDR if the impedance val-
ues and connectors are compatible.
6.4 Adjustable Measurement Parameters
6.4.1 Sampling Interval (Point Spacing)
The temporal
resolution of the TDR unit is an issue only if it affects the dura-
tion of the transitions in the TDR waveforms (see 4.1.2) that
are used to compute t
d
. The temporal resolution of the TDR is
affected by the transition duration of the TDR step response,
the transition duration of the step response of all intervening
electrical components (connectors, cables, adapters), mea-
surement jitter, the interval between sampling instances, and
timebase errors. For typical TDR measurements, timebase
errors and sampling intervals should not be an issue (both are
or can be made to be less than 10 ps). The effect of measure-
ment jitter can be modeled by convolving the jitter distribution
with the TDR step response to yield an effective TDR step
response. The effect of jitter on the bandwidth of the TDR
measurement can be assessed from the jitter spectrum,
which can be described by:
J(,) = e
−2(πσ,)
2
,
where
J is the jitter spectrum,
f is frequency, and
σ is the rms jitter value.
If the effective jitter step response differentially impacts the
duration of the two or more waveform transitions used to
compute t
d
, then jitter must be reduced. More than likely, jit-
ter will be nearly identically distributed for each transition. But
if the jitter is so great as to affect the accuracy of computing
the transition instants, then the user must reduce the duration
of the waveform period or reduce the system jitter. Reduction
in the duration of the waveform period may introduce a bias in
the voltage values and this may affect the computed value of
t
d
. If the rms jitter value is less than 20% of the transition
duration of the TDR step response, then the jitter is small and
can be ignored. For typical TDR systems, however, rms jitter
is less than 10 ps and will not affect the t
d
measurements.
Similarly, the effect of cables, connectors, and adapters on
the measurement can be modeled by convolving their step
responses with that of the TDR unit. If the transition duration
of this new step response meets the requirements of 4.1.2,
then the performance of the cables, connectors, and adapters
is adequate.
Number
2.5.5.11
Subject
Propagation Delay of Lines on Printed Boards by TDR
Date
04/2009
Revision
IPC-TM-650
Page
15
of
16
6.4.2 Waveform Averaging and Number of Samples in
the Measurement Zone
Waveform averaging reduces the
effective noise level of the measurement by M
-1/2
, where M is
the number of acquired waveforms (typically, 8 ≤ M ≤ 256).
Consequently, averaging can reduce measurement noise.
This reduction is limited by the number of bits of the analog-
to-digital converter of the TDR system and the linearity of the
timebase. However, if the TDR system exhibits drift in the
timebase, averaging too many waveforms may result in a
reduction of t
sys
and a commensurate reduction in the
temporal/spatial resolution of the TDR.
The number of samples (data points) in the waveform epoch
will affect the accuracy and uncertainty of the computed value
of t
d
because this value is typically computed by interpolating
between adjacent datum values. Therefore, the more samples
in the waveform epoch, the smaller will be the error and stan-
dard deviation of the computed t
d
value.
6.4.3 Selection of Waveform Period
Inconsistency in
defining the waveform epoch may cause a significant but
unknown error than can exceed two sample intervals. Speci-
fying the waveform period to be consistent for both long and
short line measurements improves t
d
repeatability, and this
can improve assessment of design and fabrication quality and
vendor capability. This waveform epoch should be long
enough to accurately determine if the waveform has settled on
both sides of the waveform transition but should be short
enough, given the number of samples in the waveform, to
accurately compute the transition instant. The waveform
epoch is defined in 5.1.3.
Number
2.5.5.11
Subject
Propagation Delay of Lines on Printed Boards by TDR
Date
04/2009
Revision
IPC-TM-650
Page
16
of
16
1 Scope
This document describes five methods for deter-
mining the amount of signal propagation loss caused by
material characteristics of conductors and accompanying
structures on printed boards. These losses result in frequency
dependent attenuation, α, as described in IPC-2141. Four of
these methods to assess this loss are time domain based,
and one is frequency domain (FD) based. These methods are:
• Method A: Effective Bandwidth (EBW) method
• Method B: Root Impulse Energy (RIE) method
• Method C: Short Pulse Propagation (SPP) method
• Method D: Single-Ended TDR to Differential Insertion Loss
(SET2DIL) method
• Method E: Frequency Domain (FD) method
Method A and B and one aspect of E reduce the attenuation
to a single number. Method C and D, and another aspect of
E, report the frequency attenuation versus frequency.
Table 1-1 provides an overview of the five methods described
in this document for determining the amount of signal propa-
gation loss on printed board conductors.
1.1 EBW (Method A Description)
In this method a TDR
step with a specified rise time is injected into an unterminated
conductor, and the conductor’s loss is determined from the
degradation of the maximum slew rate of the step rise time at
the open end of the interconnect. The maximum slope
method described here does not use a 10 - 90% rise time
measurements method. Instead, it uses the maximum slew
rate to extrapolate an effective bandwidth parameter. The via
loss, skin effect loss, and dielectric loss all influence the rise
time of the TDR step as it appears at the end of the intercon-
nect. This method is not intended to measure absolute loss or
each component of loss. Rather, it determines a relative total
loss factor called EBW that can be used to discern loss varia-
tions in transmission lines from panel to panel or lot to lot.
1.1.1 EBW Measurement System Caveats
This method
is not intended for rigorous analysis of the signal attenuation
of printed board interconnects but for a simple production
test. Therefore, there are several recognized limitations in the
measurement methodology:
a) This procedure does not deliver absolute values of loss in
dB but instead delivers a parameter called EBW which is a
qualitative measure of transmission line loss α.
b) There is no attempt to separate the various loss compo-
nents (i.e., skin effect, dielectric, via loss, etc.).
Instrument TDR TDR/VNA TDT TDR/TDT VNA/TDT
Stimulus
Selected for
appropriate
spectral content
250 ps or specified 11-35 ps 11-35 ps
300 KHz to 10 GHz
or as specified
Coupon >5 cm
1.25 cm and 20.32
cm or as specified
3.0 cm and
10.0 cm
4″
(8″ effective length)
20.32 cm
or as specified
SW Scope Algorithm
Algorithm and IPC
web site pointer
Algorithm and IPC
web site for software
Algorithm Algorithm
Probe
Matched impedance
probe
Matched impedance
probe
Matched impedance
probe, RF connector
High Frequency
hand-held probe
Matched impedance
probe, RF connector
Test Quantity
Maximum slope
in MV/sec
Averaged loss (dB)
Tanδ, ε
r
, α, β, and Z
0
vs. frequency
SDD21 vs. frequency Loss fit and slope
Applicability
Printed board
fabrication testing
Printed board
fabrication testing
Printed board
material qualification,
printed board
model generation
Printed board
fabrication
qualification
and testing
Printed board
fabrication testing,
printed board design
guide specification
3000 Lakeside Drive, Suite 309S
Bannockburn, IL 60015-1249
IPC-TM-650
TEST METHODS MANUAL
Number
2.5.5.12
Subject
Test Methods to Determine the Amount of Signal
Loss on Printed Boards
Date
07/12
Revision
A
Originating Task Group
Propagation Loss Test Methods Task Group (D-24b)
Association
Connecting
Electronics
Industries
Table
1-1
Methods
Overview
EBW
RIE
SPP
SET2DIL
FD
Material
/n
this
Test
Methods
Manual
was
voluntarily
established
by
Technical
Committees
of
I
PC.
This
material
/s
advisory
only
and
"s
use
or
adaptation
，
s
entirely
voluntary.
IPC
disclaims
all
liability
of
any
kind
as
to
the
use,
application,
or
adaptation
of
this
material.
Users
are
also
wholly
responsible
for
protecting
themselves
against
all
claims
or
liabilities
for
patent
infringement.
Equipment
referenced
/s
for
the
convenience
of
the
user
and
does
not
imply
endorsement
by
IPC.
Page
1
of
24