IPC-TM-650 EN 2022 试验方法.pdf - 第490页

where: BW - 3dB is the 3 dB attenuation bandwidth and t d is the 10 % to 90 % transition duration of the TDR step response. Note that this relationship may not accurately represent the intended operational frequencies of…

set.

Three times the standard deviation around each side of

the mode is the repeatability.

The ability to resolve a measurement value is fundamental to

the accuracy of any measurement process. The TDR instru-

ment should have sufficient measurement resolution to facili-

tate the accuracy requirements of the measurement method

described herein. The total risetime of the TDR system (includ-

ing cables, probes, etc.) and step aberrations define the

impedance resolution (see 4.1.2).

6.1.5

General Cautionary Statement

TDR

test systems

and associated accessories are precision high frequency

devices. Most TDRs include hardware to protect the static-

sensitive sampling heads. However, operators and mainte-

nance staff should take proper ESD precautions (see manu-

facturer’s recommendations). High frequency cables, because

they typically use solid center conductors, are not as flexible

as typical coaxial cable. Consequently, care should be taken

not to excessively bend and flex the high frequency cables.

The probes used in TDR systems typically use spring-loaded

contacting mechanisms and these should be checked peri-

odically to ensure proper operation. Statistical process control

methods and control charts can provide useful information

regarding the condition of the TDR system and its associated

accessories.

6.1.6

TDR Measured Values

The

units of the values out-

put by the TDR system may be in voltage, reflection coefficient

(commonly called ‘‘rho’’ for the Greek character, ρ, represent-

ing it), and impedance.

6.1.6.1

Impedance

the TDR system provides impedance

values directly, no further computation is required to obtain

the characteristic impedance of the transmission line under

test.

6.1.6.2

Reflection coefficient, ρ

the TDR unit provides

its output in terms of ρ, then the characteristic impedance of

the transmission line under test must be computed from ρ.

6.1.6.3

Voltage

the TDR unit provides its output in term

of voltages, these voltages must first be used to compute the

amplitude of the incident and reflected pulses. Note, all volt-

ages values measured in the test procedures are that of static

voltage levels. These voltage levels are used to compute pulse

amplitudes. The pulse amplitudes, in terms of voltage, are

then used to compute the reflection coefficient of the trans-

mission line under test relative to the TDR, as shown in the

test methods, and these reflection coefficients are then used

to determine the characteristic impedance of the transmission

line under test.

6.2

Calibration

6.2.1 System Verification

The

use of test reference speci-

mens corresponding to different impedance values, for

example 28 Ω,50Ω, and 100 Ω for single-ended transmis-

sion lines and 100 Ω for differential transmission lines, should

be measured according to the user-defined sampling plan

and compared to impedance control limits to ensure the sys-

tem is functioning correctly.

6.2.2

Calibration Artifacts

Air

line standards should be

checked for mechanical tolerances or replaced at regular

intervals. They should be handled with care. Worn out stan-

dards can cause a significant but unknown error than can

exceed 2 Ω. The air line should be compared to another air

line periodically to verify the air line in use has not been dam-

aged. The airline should also be calibrated and documented

periodically (not less than once every two years) by a qualified

certification laboratory and kept in an environment safe from

mechanical shocks, dust and dirt. Dust and dirt degrade the

fine threads of the connection and damage the electrical mat-

ing surfaces. Also, some TDR equipment manufacturers have

requirements for the minimum length of the airlines they rec-

ommend for calibration and standardization. Check with the

manufacturer regarding their calibration requirements. For dif-

ferential impedance of 100 Ω, each channel can be checked

with a 50 Ω airline.

Ideally, the effects of material properties of the air line should

be included in the calculation of the air line impedance

because some of the corresponding transmission line proper-

ties, such as conductor resistance, will be frequency depen-

dent. Also, beadless air lines should be used because their

geometries can be readily measured whereas the geometries

of beaded air lines are more difficult to measure.

6.3

Measurement System

6.3.1 Bandwidth/Risetime Resolution

The

frequency

components of the TDR step are approximately related to the

bandwidth by:

−3dB

≈

0.35

IPC-TM-650

Number

2.5.5.7

Subject

Characteristic

Impedance of Lines on Printed Boards by TDR

Date

03/04

Revision

Page

21 of 23

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where:

-3dB

the 3 dB attenuation bandwidth and t

the 10 %

to 90 % transition duration of the TDR step response.

Note that this relationship may not accurately represent the

intended operational frequencies of the transmission line

being tested. The bandwidth and risetime characteristics must

be adequate to ensure that the TDR can provide a measure-

ment zone long enough to accurately determine Z

for

transmission line of a given length. This constant-valued

region corresponds to the round-trip propagation time of the

TDR step on the transmission line being tested. If the TDR

transition duration is too long relative to this propagation time,

the constant-valued region (also called the measurement

zone) will be very short thereby increasing measurement error

and uncertainty. Risetime considerations, however, are not

the best method for determining TDR resolution. It is better to

consider the temporal/spatial resolution of the TDR (see 4.1.2)

than bandwidth/risetime resolution when determining the per-

formance of the TDR measurement system.

6.3.2

Temporal/Spatial Resolution

The

TDR unit may

not be the only limiting factor for temporal resolution. The

probe connecting the TDR unit to the test specimen may also

limit resolution and this needs to be considered. Because of

the nature of TDR, it is easy to include the effects of the TDR

unit and all of the probe devices collectively, by defining t

sys

the

fall time of the TDR step that has reflected from a short

circuit placed at the end of the probe and returned to the TDR

head. The mean value of Z

less susceptible to TDR reso-

lution than are the minimum and maximum values.

6.3.3

Amplitude Scale

a coarse vertical scale is used,

quantization error can be significant. Many instruments

change accuracy when their scales are changed, and this can

result in significant but unknown errors that can exceed 3 Ω.

6.3.4

Baseline and Amplitude Drift

The

ability of the TDR

instrument to maintain a constant baseline voltage and con-

stant amplitude step pulse are critical to the repeatability of

the TDR measurement process. TDR step generators and

sampling units are sensitive to time and temperature drifts.

Drift should be minimized and have a value that corresponds

to less than one-tenth the desired impedance uncertainty.

6.3.5

Electrostatic Discharge Damage

ESD

damage to

TDR instrumentation is often not easily detected and may

unknowingly affect measurement accuracy. Therefore, system

calibration should be performed regularly to check for this (see

5.1.1). All cables should have a termination attached to one

end when not in use and while they are being connected to

the TDR instrumentation. The use of a static protection switch

helps eliminate ESD damage to the TDR. Operators should

have anti-static awareness training and should perform all

measurements in anti-static work areas while wearing anti-

static wrist straps.

6.3.6

Probes

Hand-held

(manual) probing solutions are

more sensitive to operator technique than are automated

methods and, consequently, the measurements made using

manual probing methods will exhibit higher variability than

automated methods. Operators should be trained and tested

in their ability to repeatably probe and obtain a consistent

measurement.

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 affect the length of the measure-

ment zone and may increase measurement uncertainty. When

the characteristic impedance of the transmission line under

test is nominally 50 Ω, these perturbations will normally decay

rapidly. If the impedance of the transmission line under test is

significantly different from 50 Ω, the magnitude of the pertur-

bations can be large and their duration long enough to affect

the measurement zone. The effect of these perturbations

must be taken into account when determining the measure-

ment zone (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

transmission 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 measurement

zone.

IPC-TM-650

Number

2.5.5.7

Subject

Characteristic

Impedance of Lines on Printed Boards by TDR

Date

03/04

Revision

Page

22 of 23

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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 PCB manufac-

turing environment, the use of two probes that were previ-

ously used for single-ended measurements may not be pos-

sible. 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 solutions and advice. Probes from one manufacturer

can also be used with another manufacturer’s TDR if the

impedance values 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 impacts the

duration of the constant-valued regions in the TDR waveform

(see 4.1.2) that are used for computing Z

The temporal reso-

lution 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), measurement 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 measurement jitter can be modeled by con-

volving 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(ƒ)=e

−2(πσƒ)

where:

J is

the jitter spectrum,

f is frequency, and

σ is the rms jitter value

If the effective step response impacts the duration of the mea-

surement zones, then jitter must be reduced. If the jitter has

an observable effect, then the user must reduce the duration

of the measurement zone (by increasing the lower limit and

decreasing the upper limit, (see 5.1.3) from which Z

com-

puted or reduce the system jitter. Reduction in the duration of

the measurement zone may introduce a bias in the voltage or

reflection coefficient values and this affect the computed value

of Z

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 Z

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.

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. However, if the TDR-

system exhibits drift in the timebase, averaging too many

waveforms may result in a reduction of t

sys

and

a commensu-

rate reduction in the temporal/spatial resolution of the TDR.

The number of samples (data points) in the measurement

zone will affect the standard deviation of the computed value

of Z

because

this value is the result of averaging all the

samples in the measurement zone. Therefore, the more

samples in the measurement zone, the smaller will be the

standard deviation of the computed Z

value.

6.4.3

Selection of Constant-Valued Region (Measure-

ment Zone)

Inconsistency

in defining where the constant-

valued region is located in the TDR waveform may cause a

significant but unknown error than can exceed 5.0 Ω. Speci-

fying the measurement zone improves measurement repeat-

ability of the same or similar samples, and this can improve

assessment of design and fabrication quality and vendor

capability. This measurement zone should be far enough

away from the launch and the open end of the transmission

line under test to minimize the effects of these discontinuities.

The measurement zone is to be given as the separation

between two positions on the transmission line, and these

positions are to be given as a percentage of the transmission

line length referenced from the TDR/transmission line inter-

face. The measurement zone is defined in 5.1.3.

6.5

Acknowledgments

The

majority of the figures used

herein were provided by Mr. Bryan C. Parker of the Introbot-

ics Corporation, Albuquerque, NM.

IPC-TM-650

Number

2.5.5.7

Subject

Characteristic

Impedance of Lines on Printed Boards by TDR

Date

03/04

Revision

Page

23 of 23

电子技术应用　　　　　　　ｗｗｗ．ＣｈｉｎａＡＥＴ．ｃｏｍ