IPC-TM-650 EN 2022 试验方法.pdf - 第606页

4.1 In this test, characteristic impedance is measured by TDR. Commercial TDRs are readily available and consist of pulse generator and sampling oscilloscopes. Rise times of the pulses are usually less than 250 picosecon…

Scope

This

method describes the test procedures

required to measure the characteristic impedance of flat

cables.

To keep this test method as simple and straightforward as

possible, balanced and differential signal lines are not

addressed. Also, the effect of flat cable against a ground

plane is not shown, because of the difficulty in determining

what a lab standard ground plane should be.

1.2

General

Characteristic

impedance (Z

)

for high fre-

quency pulses is defined electrically as the square root of the

inductance divided by the capacitance (C). In equation form:

√

Accuracy

and consistency of impedance is required to match

the characteristics of the other electronic circuit components.

Variations and mismatches in impedance create undesirable

pulse reflections and pulse distortions. These reflections and

distortions increase attenuation and crosstalk. The character-

istic impedance of flat cables is primarily dependent upon the

dielectric properties of the insulation and the cable geometry.

It is directly proportional to conductor spacing and is inversely

proportional to conductor size and the effective dielectric con-

stant of the insulation. Therefore, consistency of impedance is

achieved by maintaining uniformity of the insulation dielectric

constant and by maintaining accurate control over conductor

dimensions and spacing of adjacent conductors.

Characteristic impedance (Z

)

is usually measured by time

domain reflectometry (TDR).

Measurement of Z

with

a TDR consists of sending a pulse

down a length of cable and then comparing the reflection

obtained to that obtained from a laboratory standard of known

impedance. Z

a cable is fully defined when three values

have been measured:

1. The average Z

for all signal lines in a length of cable when

the cable is suspended in air.

2. The maximum change in impedance (or reflection coeffi-

cient) at any point on any signal line of the cable when the

cable is suspended in air.

3. The maximum change in impedance when the cable is

clamped against a ground plane.

Measurement of the preceding values is performed by use of

the setup illustrated in Figure 1. The laboratory standard is

connected to the TDR generator output, and the cable with

unknown Z

is connected to the end of the laboratory stan-

dard. When a single-ended (unbalanced) cable is to be tested,

connection to the laboratory standard consists of (1) the cable

signal conductor to the laboratory-standard signal conductor,

and (2) the ground conductors associated with the cable sig-

nal conductor to the laboratory standard ground. The far end

of the cable may be left unterminated, or it may be terminated

with a precision resistor to verify the laboratory standard. Bal-

anced cable (which carries simultaneous positive and negative

pulses) cannot be directly tested for impedance in this man-

ner; however, a close approximation can be achieved by

selecting an axis of symmetry between two signal conductors

and then testing only one signal conductor and its associated

ground conductor.

The typical oscilloscope trace obtained when testing a cable

is illustrated in Figure 2.

Test Specimen

3.1

One

pre-production or production sample 0.9 m to 3 m

long. The number of test samples should be determined by

the manufacturer and/or user.

Equipment/Apparatus

IPC-2-5-18-1

Figure

1 TDR Test Set-up for Measuring Characteristic

Impedance

TRD

Ref Z Airline

Hang

Test Cable

in Air

RG 58 C

Cable

Connection

Device

Non-Metallic

Surface

The

Institute for Interconnecting and Packaging Electronic Circuits

2215 Sanders Road • Northbrook, IL 60062

IPC-TM-650

TEST

METHODS MANUAL

Number

2.5.18

Subject

Characteristic

Impedance Flat Cables (Unbalanced)

Date

7/84

Revision

Originating Task Group

Material

in this Test Methods Manual was voluntarily established by Technical Committees of the IPC. This material is advisory only

and its use or adaptation is 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 is for the convenience of the user and does not imply endorsement by the IPC.

age1of4

电子技术应用　　　　　　　ｗｗｗ．ＣｈｉｎａＡＥＴ．ｃｏｍ

4.1

this test, characteristic impedance is measured by

TDR. Commercial TDRs are readily available and consist of

pulse generator and sampling oscilloscopes. Rise times of the

pulses are usually less than 250 picoseconds (250 x 10-12

sec.), which gives a resolution sufficient to detect discontinui-

ties smaller than 2.5 cm in length. Since the pulse rise times

generally used now in electronic equipment are not this fast, a

TDR is adequate for testing. Also required for this test is a lab

standard air line to establish a reference impedance (Z

ref.)

and

a standard cable connection device at the air line output

(see Figure 1).

4.2

TDR, such as a Hewlett-Packard 1415A, Hewlett-

Packard 1815A, Tektronix 1 S2, or equivalent

4.3

The

standard air line used should be a General radio

874-L20 (20 cm), 874-L30 (30 cm), or equivalent for Z

50Ω.

4.4

Cable Holders

Fixture

of plexiglass or other nonmetal-

lic material. Cable hangers to suspend the cable in air. Refer

to Figure 3.

4.5

The

standard cable connection device used should

match Figure 4. It is made from a General radio cable connec-

tor type 874-C62A.

IPC-2-5-18-5

Figure

2 TDR Trace for a Typical Cable

IPC-2-5-19-1-4

Figure

3 Sample Cable Hanger

IPC-TM-650

Number

2.5.18

Subject

Characteristic

Impedance Flat Cables (Unbalanced)

Date

7/84

Revision

age2of4

电子技术应用　　　　　　　ｗｗｗ．ＣｈｉｎａＡＥＴ．ｃｏｍ

4.6

Coaxial Cable

Impedance:

50 - 2Ω RG-58A, RG-58C,

or equivalent; Termination: GR874 connectors, both ends;

Length: approximately 61 cm

4.7

Load

General

Radio type GR874 or equivalent 50Ω

load. This is an optional item, which is used to calibrate the

TDR.

Procedure

5.1

Allow

a minimum of one hour for TDR warm-up and

calibrate the instrument per manufacturer’s instructions.

5.2

Prepare

the test specimen by stripping approximately 13

mm of insulation from one end of cable. Separate the ground

and signal conductors and solder a copper buss across the

grounds (see Figure 5).

5.3

Adjust

the TDR settings as follows:

Vertical: 0.1 e/cm

Distance/time: 20 ns/cm.

Magnifier: 50 x (For equipment other than Hewlett-Packard,

use settings as close as possible to these.)

Insert the 30 cm air line into the output of the TDR. This will

serve as the 50Ω reference. Attach the coaxial cable to the air

line and terminate with the impedance probe. Vertically center

the 50Ω reference line on the TDR graticule.

5.4

Press

the probe against the conductor to be tested

insuring the ground of the probe is against the cable ground

(see Figure 5) and check the vertical placement of the 50Ω

reference; re-center if necessary.

5.5

Adjust

the distance/time magnifier to 5 or 10 and rotate

the magnifier delay dial until the total length of the cable is vis-

ible on the screen. Measure the vertical reflection coefficient

(e) in cm as illustrated in Figure 2.

5.7

Calculate

the characteristic impedance (Z

)

as follows:

= 50

(

1 + e

1 − e

)

(Ω)

Calculate

the cable measuring as shown in Figure 2.

Calculate Z

max., e = e max;

min., e = e min.

IPC-2-5-18-3

Figure

4 Cable Connection Device. Refer circled items to parts list. Made from General Radio Co. Type 874-C62A.

IPC-TM-650

Number

2.5.18

Subject

Characteristic

Impedance Flat Cables (Unbalanced)

Date

7/84

Revision

age3of4

电子技术应用　　　　　　　ｗｗｗ．ＣｈｉｎａＡＥＴ．ｃｏｍ