IPC-TM-650 EN 2022 试验方法.pdf - 第468页

8.7 The temperature of the test fixture, if not in the 21°C to 23°C range. 8.8 Any conditioning prior to measurement. 8.9 The orientation of the resonator with respect to X or Y axis of the specimen. 8.10 For each resona…

7.2.1.1

For

the three point measurement described in 6.5.1,

the calculation is

1/Q

loaded

[(f

-f

)/f

]

[4]

A more exact calculation can be used that does not require

that the values of f

and

at exactly half the power level of

the maximum at resonance. This is especially suited for auto-

mated testing. The formula is

1/Q

loaded

1-(f

))

(10

∆1//10

-1

)

-0.5

[5]

((f

)

-1) (10

∆2/10

-1

)

-0.5

where:

∆1

is the positive dB difference in power level from f

and

∆2

is the positive dB difference in power level from f

7.2.1.2 For

the many point measurements of the resonance

described in 6.5.2, the non-linear regression to fit the formula

1 derives the Q

loaded

value.

7.2.2

Correcting the Resonator Loss Factor for Load-

ing

The

probe gap set for about 50 dB insertion loss at

resonance is intended to make Q

loaded

approximately

equiva-

lent to Q

unloaded

Nevertheless, corrections in the measured

total loss value, 1/Q

loaded

are

desireable. With the assumption

that the S21 parameter with straight through connection with-

out the test fixture is at 0 dB, dBr, the insertion loss or S21

parameter in dB units at the resonant peak, is related to the

power ratio by

(-dBr

/10)

where

the dBr value at resonance is taken as positive. Then

the correction is

unloaded

loaded

1-(P

)

0.5

]

unloaded

loaded

1-10

(-dBr

/20)

]

[6]

As can be seen from the following tabulation at high degrees

of insertion loss such as 50 dB errors in the straight through

connection assumption above are not as important as they

would be at lower values such as 20 or 15.

60 50 40 30 20 15 10 5

1.00

1.00 1.01 1.03 1.11 1.22 1.46 2.28

7.3 Calculation of 1/Q

The

following calculation scheme

is used to estimate the conductor loss

(5,6)

needed

for formula

1/Q

= α

C/(π f

(ε

)

0.5

)

[7]

where:

/ (377

= attenuation constant,

nepers/mm

0.00825 f

0.5

surface resistivity of copper, Ohms

377/(4 ε

0.5

(W/(B - T))))

= characteristic impedance of resonator, Ohm

377 = 120 π. = free space impedance, Ohm

(2 X log

(X+1)-(X-1)log

-1))/ π

= X+2WX

B+X

+ T / B) log

[(X

+ 1) / (X - 1)] / π

X = 1/(1-T/B)

relative permittivity

B = ground plane spacing, mm

W = resonator width, mm

T = resonator conductor thickness, mm

Proven data is not currently available for correcting this calcu-

lated value to account for increased conductor loss associ-

ated with roughness of the copper foil or surface treatments

for adhesion. When smooth rolled copper foil is used in Type

A specimens the estimate seems quite reliable in the 0.4 to 15

GHz range based on work done with neat (PTFE) poly(tet-

rafluoroethylene) sheet specimens

(3)

8.0

Report

The

report shall contain the following:

8.1

The

type of specimen: A, B, C, or D.

8.2

For

specimen type A, if not copper foil type W (wrought),

grade 5 (as rolled-wrought), bond enhancement N (none, no

stain proof), or for specimen types B, C, or D, state at least:

metal, type, grade, and bond.

8.3

The

measured length of the resonator and specimen

dielectric.

8.4

The

measured thickness of specimen cards or, if appli-

cable, of stacks.

8.5

The

center conductor width.

8.6

The

center conductor total thickness (for type C, this is

twice the cladding thickness).

IPC-TM-650

Number

2.5.5.5.1

Subject

Stripline

Test for Complex Relative Permittivity of Circuit Board

Materials to 14 GHz

Date

3/98

Revision

Page

10 of 11

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8.7

The

temperature of the test fixture, if not in the 21°C to

23°C range.

8.8 Any

conditioning prior to measurement.

8.9

The

orientation of the resonator with respect to X or Y

axis of the specimen.

8.10

For

each resonance, show 8.10.1 through 8.10.9.

8.10.1

The

node number n.

8.10.2

The

calculated effective stripline permittivity.

8.10.3

The

calculated effective dielectric loss tangent.

8.10.4

The

resonant frequency, f

at maximum transmis-

sion.

8.10.5

The

insertion loss at resonance, dB

at maximum

transmission.

8.10.6

The

loaded

(optional).

8.10.7

The

calculated Q

unloaded

(optional).

8.10.8

the three point method of 6.3, 6.4, or 6.5.1 is used,

report the frequency and dB value of the two points either side

of the peak (optional).

8.10.9

the non-linear regression (NLR) method of 6.5.2 is

used, report the number of data points used, NLR uncertainty

values (for f

loaded

)

and the standard deviation of the fit

in dB units (optional).

9.0 Notes

9.1

Permittivity

The

dielectric of a stripline circuit affects

the electrical response of all the circuits printed on it. Velocity

of propagation, wavelength, and characteristic impedance all

vary with permittivity. If the permittivity varies from the design

value, the performance of such circuits is degraded.

Throughout this document, the term ‘‘permittivity’’ refers to

relative permittivity of the dielectric material, a dimensionless

ratio of the absolute permittivity of the material to that of a

vacuum.

9.2

Loss Tangent

The

attenuation and Q (figure of merit) of

stripline circuits are a function of combined copper and dielec-

tric loss. An excessively high loss tangent leads to loss in sig-

nal strength and to degraded performance of frequency selec-

tive circuits such as filters.

9.3

Dielectrics Clad with Thick Metal on One Side

This

method can be used for measurements of dielectric sub-

strates with thin foil on one side and thick cladding such as

aluminum sheet on the other by using the Type C specimen

configuration. In some cases, with very thick metal cladding it

may be necessary to use a modified part 5.1.2 (Figure 4) with

a reduced thickness dimension.

9.4

Anisotropic Materials

For

anisotropic materials, test

methods in which the electric field is not imposed on the

dielectric in a stripline configuration can give misleading values

of effective stripline permittivity and loss tangent. This test

method measures an effective stripline permittivity when the

specimen configuration is close to that of the application.

10.0

References

1. Electrical

Performance of Microwave Boards, IEEE Trans.

Components, Packaging & Manufacturing Technology,

Part B, vol. 18, no. 1, Traut, G. R, Feb. 1995.

2. The Complex Permittivity of RF Circuit Board Materials by

Resonances of a Stripline Section in the 0.2 to 15 GHZ

Range, Traut, G. Robert, Preprints of the Measurement

Science Conference 1997 January 23 & 24, Pasadena

Convention Center, Pasadena, CA

3. Complex Permittivity Over a Wide Frequency Range by

Adjustable Air Gap Probing a Stripline Resonator, Traut,

G. Robert, Proceedings of the Technical Conference, IPC

Printed Circuits Expo, March 9-13, 1997, San Jose Con-

vention Center, San Jose, CA.

4. The NIST 60-Millimeter Diameter Cylindrical Cavity Reso-

nator: Performance Evaluation for Permittivity Measure-

ments, Vanzura, E. J., Geyer, R. G. and Janezic, M.D.,

NIST Technical Note 1354, August 1993, National Insti-

tute of Standards and Technology, Boulder, CO 80303-

3328.

5. Characteristic Impedance of the Shielded-Strip Transmis-

sion Line, Cohn, S. B., IRE Trans MTT, (July 1954): pp.

52- 57.

6. Problems in Strip Transmission Lines, Cohn, S. B., IRE

Transactions MTT 3 (March 1955): pp. 119-126.

IPC-TM-650

Number

2.5.5.5.1

Subject

Stripline

Test for Complex Relative Permittivity of Circuit Board

Materials to 14 GHz

Date

3/98

Revision

Page

11 of 11

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Scope

This

document describes time domain reflectom-

etry (TDR) methods for measuring and calculating the charac-

teristic impedance, Z

of a transmission line on a printed cir-

cuit board (PCB). In TDR, a signal, usually a step pulse, is

injected onto a transmission line and the Z

the transmis-

sion line is determined from the amplitude of the pulse

reflected at the TDR/transmission line interface. The incident

step and the time delayed reflected step are superimposed at

the point of measurement to produce a voltage versus time

waveform. This waveform is the TDR waveform and contains

information on the Z

the transmission line connected to the

TDR unit.

Note: The signals used in the TDR system are actually rect-

angular pulses but, because the duration of the TDR wave-

form is much less than pulse duration, the TDR pulse appears

to be a step.

1.1

Applicability

The

observed voltage or reflection coeffi-

cient change in the TDR waveform is related to the difference

between Z

the transmission line and the impedance of the

TDR. If the impedance of the TDR unit is known via proper

calibration, then the Z

the transmission line attached to the

TDR unit may be determined. Thus, the TDR method is use-

ful for measuring Z

and

changes in Z

a transmission line.

These impedance values thus determined can be used to

verify transmission line design (engineering development),

measure production repeatability, and qualify manufacturers

via transfer or artifact standards.

Engineering development requires detailed information on the

electrical performance of prototype units to assure the trans-

mission line design yields the expected performance charac-

teristics. Detailed laboratory analysis of the effect of variations

in design features expected in actual manufacture can be

done to assure the proposed design can be manufactured at

a useful quality level.

1.2

Measurement System Limitations

Measurements

often

vary greatly, depending on equipment used and how

the tests were performed. Following a specified method helps

assure accurate and consistent results. Both single-ended

and differential line measurements have limitations in com-

mon, including the following:

a. The Z

measured

units are derived and not directly mea-

sured.

b. The value of characteristic impedance obtained from TDR

measurements is traceable to a national metrology insti-

tute, such as the National Institute of Standards and Tech-

nology (NIST), through coaxial air line standards. The char-

acteristic impedance of these transmission line standards

is calculated from their measured dimensional and material

parameters.

c. A variety of methods for TDR measurements each have

different accuracies and repeatabilities.

d. If the nominal impedance of the line(s) being measured is

significantly different from the nominal impedance of the

measurement system (typically 50 Ω), the accuracy and

repeatability of the measured numerical valued will be

degraded. The greater the difference between the nominal

impedance of the line being measured and 50 Ω, the less

reliable the numerical value of the measured impedance

will be.

e. Measurement variation (repeatability, reproducibility) may

only be a small component of the total uncertainty in the

value of the characteristic impedance. For example, if the

uncertainty in the characteristic impedance of the reference

air line is ± 0.5 Ω (for a 95 % confidence interval), then the

uncertainty in the measured characteristic impedance of

the test line can be no better than ± 0.5 Ω even if measure-

ment variation is much less.

f. The particular TDR methods described herein are not

suited for measuring the characteristic impedance as a

function of position along the transmission line (impedance

profiling) because signal reflections within the transmission

line under test and between the TDR unit and transmission

line under test may adversely affect measurement results.

g. The requirements for the length of the transmission line

under test given in Section 3 of this test method as well the

IPC-2141 must be met.

Further measurement considerations and notes are provided

in Section 6.

1.3

Sample Limitations

The

type of test sample used may

also impact Z

values

(see IPC-2141). The sample-based limi-

tations include:

2215

Sanders Road

Northbrook, IL 60062-6135

IPC-TM-650

TEST

METHODS MANUAL

Number

2.5.5.7

Subject

Characteristic

Impedance of Lines on Printed

Boards by TDR

Date

03/04

Revision

Originating Task Group

TDR Test Method Task Group (D-24a)

Material

in this Test Methods Manual was voluntarily established by Technical Committees of 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 IPC.

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