IPC-TM-650 EN 2022 试验方法.pdf - 第478页

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 …

5.1.3.2

Procedure for the Reference Line

Step

1 –

Remove

the transfer standard (or air line reference)

and hold the precision rf cable in the air and locate the instant,

1,Ref

on the TDR waveform where the rf cable/open discon-

tinuity occurs (see Figure 5-4). t

1,Ref

the instant in the TDR

waveform when the reflection from the open circuit has

reached 50 % of its amplitude (see Figure 5-4), unless other-

wise specified by the user.

Step

2 –

Connect

the rf cable to the reference line and locate

the instant, t

2,Ref

on the TDR waveform where the reference

line/open discontinuity occurs (see Figure 5-5). t

2,Ref

the

instant in the TDR waveform when the reflection from the

open circuit has reached 50 % of its amplitude (see Figure

5-5), unless otherwise specified by the user.

Step

3 –

Compute

the round trip propagation time of the

transmission line using:

rt,Ref

= t

2,Ref

− t

1,Ref

Step

4 –

Determine

the initial instant, t

i,Ref

of measurement

zone using:

i,Ref

= t

1,Ref

+ x

rt,Ref

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

using:

ƒ,Ref

= t

1,Ref

+χ

ƒT

rt,Ref

where x

the upper limit of the measurement zone and is

70 % unless otherwise specified by the user.

5.2

Single-Ended TDR Measurement Procedures

This

section

contains three methods for measuring the character-

istic impedance of single-ended transmission lines. The fol-

lowing calibration and measurement steps should be used

when the device(s) under test are unbalanced (single-ended)

transmission lines. This process can be followed manually but,

to improve measurement repeatability and reduce measure-

ment time, an automated measurement system is recom-

mended. Additionally, the use of a fixture based or robotic

probing system greatly improves the accuracy and repeatabil-

ity over hand probe techniques and further reduces the mea-

surement time.

5.2.1

Transfer Standard Method

this method, the

impedance of the reference air line is transferred to a second-

ary transmission line. The computed impedance of the sec-

ondary or transfer line then becomes the basis from which the

characteristic impedance of all subsequent test transmission

lines is computed. The transfer method provides a direct com-

parison of the impedance of the transfer standard to that of

the transmission line under test. Although this does require

two additional measurements, as compared to the in-situ

method (see 5.2.2), it does reduce the risk of damage to the

reference impedance standard due to frequent use and han-

dling. The effects of drift in TDR amplitude offset are mini-

mized with this method.

5.2.1.1

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 5.2.1.2).

IPC-2257a-5-3

Figure

5-3 Determination of Measurement Zone

,TL

TIME

1,TL

2,TL

SPD

TRANSFER

STANDARD

TDR

INSTRUMENT

TRANSMISSION LINE UNDER TEST

i,TL

f,TL

MEASUREMENT ZONE

for TRANSMISSION LINE UNDER TEST

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

age9of23

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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

the

amplitude of the signal reﬂected from the open end of rf cable.

SPD

TDR

INSTRUMENT

PRECISION

RF CABLE

1,Ref

TIME

0.5

R.0

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).

R,0

the amplitude of the signal reﬂected from the open end of the reference line.

SPD

TDR

INSTRUMENT

PRECISION

RF CABLE

1,Ref

TIME

0.5

R,0

REFERENCE LINE

2,Ref

IPC-TM-650

Number

2.5.5.7

Subject

Characteristic

Impedance of Lines on Printed Boards by TDR

Date

03/04

Revision

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:

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,

tran,1

and compute the voltage difference, 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:

tran

r,0

i,0

Step

4 –

Calculate

the impedance, Z

tran

, of

the transfer stan-

dard using the following formula:

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:

i,1

= V

open

− V

tran,2

where V

tran,2

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,

C,ave

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

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

tran,0

open

i,0

MEASUREMENT ZONE

for TRANSMISSION LINE UNDER TEST

PRECISION

RF CABLE

i,TL

f,TL

MEASUREMENT ZONE

for REFERENCE LINE

i,Ref

f,Ref

IPC-TM-650

Number

2.5.5.7

Subject

Characteristic

Impedance of Lines on Printed Boards by TDR

Date

03/04

Revision

Page

11 of 23

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