IPC-TM-650 EN 2022 试验方法.pdf - 第609页

1 Scope This method describes the test procedures required to measure propagation delay in flat cables. Propa- gation delay is defined as the time required for a pulse to traverse a unit length of cable. Excessive propag…

6 Notes

6.1

The

TDR employs a pulse rise time less than 250 pico-

seconds. A pulse of this rise time is extremely rich in harmon-

ics extending well into the GHz region of the frequency spec-

trum. The impedance probe illustrated in Figure 1 is designed

to minimize the effects of impedance mismatch at the con-

nection; therefore, it is suggested that a probe of this type be

used for the impedance measurement. The importance of a

good connection between the cable under test and the TDR

can not be overemphasized.

Cables longer than3minlength may be tested, but care

must be exercised so as not to confuse the effect of increased

wire resistance with an apparent increase in impedance as the

magnifier delay dial is rotated to observe the longer cable

length (function of attenuation, which includes wire size).

6.2

Under

no circumstances should the cable be tested

while in a coiled form due to the effect of increased induc-

tance.

6.3

Keep

cable a minimum of 15 cm away from any dielec-

tric or ground plane including metal, wood, etc. (except in

step 5.5).

6.4

Measurement

of Z

unknown cable length should be

made as close as possible to the cable connection device

(after overshoot and undershoot).

6.5

The

reference Z

cable

may be positioned after the

RG58C cable and before the cable connection device. There-

fore, the reference Z

adjacent to the test cable on the TDR

trace.

IPC-2-5-18-4

Figure

5 Connection of Impedance Probe to Sample

under Test

IPC-TM-650

Number

2.5.18

Subject

Characteristic

Impedance Flat Cables (Unbalanced)

Date

7/84

Revision

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Scope

This

method describes the test procedures

required to measure propagation delay in flat cables. Propa-

gation delay is defined as the time required for a pulse to

traverse a unit length of cable. Excessive propagation delay

will result in the malfunction of critical circuits due to the late

arrival of pulses. Propagation delay is directly proportional to

the effective dielectric constant of the insulation.

Applicable Documents

None

Test Specimen

3.1

One

pre-production or production sample1mto3m

long. The number of test samples should be determined by

the manufacturer and/or user.

Apparatus

4.1

In this test, propagation delay is measured using time

domain reflectometry (TDR). Commercial TDRs are readily

available and consist of a pulse generator and sampling oscil-

loscopes. The TDR to be used should be a Hewlett-Packard

1415A, Hewlett-Packard 1815A, Tektronix 1 S2 or equal.

4.2 Two

standard cable connection devices to terminate

each end of the test cable, which should match Figure 1. It is

made from a General Radio cable connector type 874-C62A.

4.3

509 load, type GR874 or equivalent, to terminate the

output of the TDR

IPC-2-5-18-3

Figure

1 Cable Connection Device

The

Institute for Interconnecting and Packaging Electronic Circuits

2215 Sanders Road • Northbrook, IL 60062

IPC-TM-650

TEST

METHODS MANUAL

Number

2.5.19

Subject

Propagation

Delay of Flat Cables Using Time

Domain Reflectometer

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.

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4.4

Fixture

of plexiglass or other nonmetallic material. Cable

hangers to suspend the cable in air (see Figure 2)

Procedure

5.1

Allow

a minimum of one hour for TDR warmup and cali-

brate the instrument per manufacturer’s instructions.

5.2

Prepare

the test specimen by stripping approximately 13

mm of insulation from both ends of the cable. Separate the

ground and signal conductors and solder a copper buss

across the grounds of each end (see Figure 3). Solder a stan-

dard cable connection device to each end of the cable (see

Figure 4).

5.3

Adjust

the TDR settings as follows: Vertical-0.2 p/cm;

Distanceltime-20 ns/cm; Magnifier-10X. (For equipment other

than Hewlett-Packard, use settings as close as possible to

these.)

5.4

Terminate

TDR output using the 509 load.

5.5

Adjust

the magnitude delay dial so the 50Ω termination

is visible and positioned to the left on the screen. Adjust the

vertical position so the pulse trace leading edge crosses the

horizontal graticule center line at 10% of pulse height (see

Figure 5). Mark the position of the leading edge of the pulse

on the horizontal graticule (mentally or by camera). If a cam-

era is used, don’t advance the film; a second exposure will be

made in 5.8.

5.6 Remove

the U section of coaxial GR connectors con-

necting the step out and signal in. Position each L connector

(made from the U connector) in the ‘‘STEP OUT’’ and ‘‘SIG-

NAL IN’’ connectors.

5.7

Connect

the test specimen, one end to the ‘‘Step Out’’

and the other end to ‘‘Signal In’’ (see Figure 6).

5.8

The

trace on the TDR screen will have moved to the

right from its original position in 5.5. Mark the position of the

leading edge of the pulse on the horizontal graticule (again at

10% of pulse height). At this time, a second exposure on the

same film used in 5.5 can be made. This will result in both

traces on one film. The distance between this mark and the

mark in 5.5 is the measured propagation delay (TD). Multiply

the measured T

20 (distance/time set at 20 ns/cm), then

IPC-2-5-19-1-4

Figure

2 Sample Hanging Device

IPC-2-5-19-2

Figure

3 Cable Preparation

IPC-2-5-19-3

Figure

4 Cable Connection

IPC-TM-650

Number

2.5.19

Subject

Propagation

Delay of Flat Cables Using Time Domain

Reflectometer

Date

7/84

Revision

age2of3

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