IPC-TM-650 EN 2022 试验方法.pdf - 第475页

sufficient robustness so that its impedance remains constant between calibration cycles. 4.3.8 Adapter, Airline-to-Probe Contact Pad A preferred method of calibrating the TDR amplitude response is to con- nect a probe co…

transmission

lines (see 5.2.1). This probe can be constructed

using a printed circuit board differential transmission line that

has a differential impedance similar to the traces being tested

or by using short identical lengths of semi-rigid cable that

connect the instrument cables to the differential probe. The

probe or semi-rigid cable should be sufficiently long to provide

an adequate duration for the measurement zone (see 4.1.2)

used during calibration and measurement.

4.3.4

Terminations

Many

instruments perform differently

under different electrical loads. If a test is performed with open

circuited lines, the calibration of the reference should also be

done using an open circuit termination.

4.3.5

ESD Protection

Static

build up on specimens prior

to test can damage the sampling heads in the TDR equip-

ment. Therefore, it is recommended that ESD protection be

used. Such protection can be supplied internally to the TDR

system or externally. If supplied externally, using a coaxial

switch for example, then the switch should be placed

between the transmission line under test and the TDR instru-

mentation. The switch, or static protection device (SPD),

should have a return and insertion loss less than 16 dB and

0.3 dB at 18 GHz. A maximum of 31 cm [12.2 in] of high

quality, high frequency cable may be used to connect the TDR

instrument to the protection switch. Samples should be

grounded to remove any residual static and/or passed

through some type of deionization device prior to testing.

Keeping the relative humidity in the test area between 45 %

and 55 % may minimize the buildup of static. Automation

software can be used to enhance the effectiveness of the

static isolation unit by switching the static isolation unit on/off

as required to minimize the amount of time that the TDR sam-

pling unit is exposed to potential ESD.

4.3.6

Calibration Artifacts (Reference or Reference

Standard)

precision coaxial air line with calibration trace-

able to a national metrology institute (such as the National

Institute of Standards and Technology), 3.5 mm or 2.92 mm

connectors at both ends, and at least 10 cm [3.94 in] long

should be used. The characteristic impedance of the air line is

based on the geometry of a coaxial transmission line using air

as the dielectric between the center conductor (signal line)

and the ground shield. The center conductor may be held in

place by glass beads located at both ends of the air line or by

the external connectors that are attached to the connectors of

the air line. The uncertainty in the nominal characteristic

impedance of the air line should be less than or equal ±

0.015 Z

ref

, where Z

ref

the characteristic impedance of the

reference air line.

4.3.7

In-Line (Transfer) Standard

The

transfer standard

is placed between the probe and the TDR unit (see Figure

4-3). This standard could be an airline, a semi-rigid coax cable

assembly with the same specifications as the flexible coax

cable assemblies, or other. The transfer standard should be of

IPC-2257a-4-3

Figure

4-3 Reference Airline and Probe Contact Pad

REFERENCE

AIRLINE

PROBE

TRANSFER

STANDARD

PROBE

CONTACT

PAD

ADAPTOR

IPC-TM-650

Number

2.5.5.7

Subject

Characteristic

Impedance of Lines on Printed Boards by TDR

Date

03/04

Revision

age6of23

电子技术应用　　　　　　　ｗｗｗ．ＣｈｉｎａＡＥＴ．ｃｏｍ

sufficient

robustness so that its impedance remains constant

between calibration cycles.

4.3.8

Adapter, Airline-to-Probe Contact Pad

preferred

method of calibrating the TDR amplitude response is to con-

nect a probe contact pad directly to the airline and to probe

the airline through the contact pad (see Figure 4-3). The con-

tact pad should have a nominal characteristic impedance of

50 Ω ± 1.0 Ω. An increase in measurement accuracy can be

achieved by using a reference impedance standard that is

closely matched to the impedance of the transmission line to

be tested.

Procedures

5.1 Measurement Preliminaries

this section, common

considerations for the calibration of the TDR measurement

system and performing the TDR measurements are provided.

5.1.3 describes a method for establishing the measurement

zone that can be applied to the measurement methods

described in 5.2 and 5.3.

5.1.1

System Calibration

the TDR instrument

manufacturer’s recommendation for the frequency of factory

calibration. TDR system ‘‘field’’ calibrations are to be per-

formed at regular intervals in addition to the less regular fac-

tory calibrations. The field calibrations are required for the fol-

lowing reasons:

a. TDR instrument specifications vary with temperature.

b. TDR instrument specifications vary with time (drift).

c. TDR instrument specifications vary due to minor ESD dam-

age.

d. TDR instrument factory calibration usually does not include

user supplied auxiliary components (e.g., cables, probes,

etc.).

TDR system field calibrations should also be performed after

a change of any system component (such as, cable, probes,

etc.). Ensure that the TDR instrument has been operating for

at least 30 minutes prior to any calibration or test measure-

ment procedure.

Use proper ESD control methods to avoid damage to the TDR

instrument in all calibration and test measurement proce-

dures. ESD control components can include static dissipative

mats, deionizer systems, and operator gowning.

5.1.2

Premeasurement Checks

The

test measurement

should be performed after the completion of the calibration

process. Ensure that plane of the signal line of a microstrip (or

embedded microstrip) structure is at least a distance equal to

six times the width of the microstrip signal line from any mate-

rial (such as the testing table) that can affect the dielectric

environment of the microstrip line. If the tests are being con-

ducted with hand probe(s), care must be taken to ensure that

the hands and/or arms of the operator do not contact any

surface of the board over the transmission line being tested.

Probes should be applied to the test points with sufficient

force to ensure proper electrical contact between the trace

and the probe assembly. Consistent application (that is, force,

angle of placement, etc.) of the probes onto the test points is

important to ensure repeatable measurement results.

Before recording any measurement results, ensure that the

TDR waveform is stable (that is, not drifting in amplitude or

time) otherwise measurement error will occur. To improve the

accuracy of the impedance measurement, it is important to

optimize the vertical gain setting (typically in V/div or ρ/div) and

horizontal axis setting (typically in time/div) of the TDR unit so

as to maximize the duration of the measurement zone within

the TDR waveform and to increase amplitude resolution. Per-

form this TDR adjustment prior to acquiring any TDR wave-

forms from which Z

will

be computed. Ensure that the tem-

perature and humidity of the test environment is within TDR

instrument specifications and is stable.

5.1.3 Establishing the Measurement Zone

The

value of

the measurement zone is critical to the accuracy and repeat-

ability of the TDR measurement process. Measurement zone

differences are a large factor in correlation problems between

measurements. The measurement zone should be set repeat-

ability for each transmission line independent of the type of

dielectric material surrounding the transmission line or its

structure (surface microstrip, embedded microstrip, stripline,

differential pair, etc.). The following process can be incorpo-

rated into the test measurement process. There are two mea-

surement zones, one for the transmission line under test (see

5.1.3.1) and one for the reference (see 5.1.3.2), which may be

either a transfer standard or a coaxial air line.

5.1.3.1

Procedure for the Transmission Line Under Test

Step

1 –

Hold

the probe in the air and locate the instant, t

1,TL

the TDR waveform where the probe/open discontinuity

occurs (see Figure 5-1). t

1,TL

the instant in the TDR wave-

form when the reflection from the open circuit has reached

50 % of its amplitude (see Figure 5-1), unless otherwise speci-

fied by the user.

IPC-TM-650

Number

2.5.5.7

Subject

Characteristic

Impedance of Lines on Printed Boards by TDR

Date

03/04

Revision

age7of23

电子技术应用　　　　　　　ｗｗｗ．ＣｈｉｎａＡＥＴ．ｃｏｍ

Step

2 –

Place

the probe in contact with the transmission line

under test and locate the instant, t

2,TL

on the TDR waveform

where the transmission line/open discontinuity occurs (see

Figure 5-2). t

2,TL

the instant in the TDR waveform when the

reflection from the open circuit has reached 50 % of its ampli-

tude (see Figure 5-2), unless otherwise specified by the user.

Step

3 –

Compute

the round trip propagation time of the

transmission line using:

rt,TL

= t

2,TL

− t

1,TL

Step

4 –

Determine

the initial instant, t

i,TL

of measurement

zone (see Figure 5-3) using:

i,TL

= t

1,TL

+ x

rt,TL

where x

the lower limit of the measurement zone and is

30 % unless otherwise specified by the user.

Step

5 –

Determine

final instant, t

f,TL

, of

measurement zone

(see Figure 5-3) using:

ƒ,TL

= t

1,TL

+ x

ƒ%

rt,TL

where x

the upper limit of the measurement zone and is

70 % unless otherwise specified by the user.

IPC-2257a-5-1

Figure

5-1 Determination of instant in the TDR waveform corresponding to the beginning of the transmission line. A

R,1

the

amplitude of the signal reﬂected from the open end of the probe. SPD is the static protection device (see 4.3.5).

PROBE

SPD

TDR

INSTRUMENT

PRECISION

RF CABLE

TRANSFER

STANDARD

TIME

0.5A

R,1

1,TL

R,1

IPC-2257a-5-2

Figure

5-2 Determination of instant in TDR waveform corresponding to the end of the transmission line.

R,2

the

amplitude of the signal reﬂected from the open end of the transmission line under test.

SPD

TRANSFER

STANDARD

TDR

INSTRUMENT

TRANSMISSION LINE UNDER TEST

0.5

TIME

1,TL

2,TL

PRECISION

RF CABLE

IPC-TM-650

Number

2.5.5.7

Subject

Characteristic

Impedance of Lines on Printed Boards by TDR

Date

03/04

Revision

age8of23

电子技术应用　　　　　　　ｗｗｗ．ＣｈｉｎａＡＥＴ．ｃｏｍ