IPC-TM-650 EN 2022 试验方法--.pdf - 第535页

this case, the screening has to be done for odd-mode, with TDR pulse polarity of + - , and even-mode, + +. It is also rec- ommended to perform TDR for +0 and 0 + sing le mod e to see how close to each other t he t wo lin…

Figure 1 Resistive Matching Network for Unbalanced Cables

For Z System Z Cable:

Z System

For Z System Z Cable:

Z System

Z Cable

R =

Z (Z - Z )

R = Z

(Z - Z )

R =

Z (Z - Z )

R = Z

(Z - Z )

Z Cable

IPC-TM-650

Number

Subject Date

Revision

Page 2 of 4

PC-2-5-30-1

2.5.30

Balanced

and

Unbalanced

Cable

Attenuation

Measurements

12/87

this case, the screening has to be done for odd-mode, with

TDR pulse polarity of + -, and even-mode, + +. It is also rec-

ommended to perform TDR for +0 and 0+ single mode to see

how close to each other the two lines’ characteristics are.

5.3.2 Measuring Frequency Relative Permittivity with

SPP

The capacitance be measured at 1 MHz with an

LCR meter for several lengths of lines. Such measurements

are generally made at a low enough frequency such as 1 MHz

so that the reactance associated with the lead inductance is

negligible. In a subsequent step line resistance measurements

using a 4 wire Kelvin method are also made. The measure-

ments determine the resistance per unit length and the

capacitance per unit length. By taking the difference between

results at two lengths and dividing by the difference in lengths,

the effect of parasitic end load is eliminated. The LCR meter

be also used to measure the capacitance between the

layers of the large circular disc designated for dielectric per-

mittivity determination.

Relative permittivity, ε

, is calculated with Equation 5-6 using

the known area, A, of the test specimen disc, the distance

between the layers h, and the capacitance, C, as measured

with the LCR at 1 MHz. The value for ‘‘h’’ may be determined

by cross-sectioning analysis.

[5-6]

5.3.3 Measuring Low Frequency Copper Resistivity, ρ,

with SPP

The resistivity (ρ) per unit length of the signal line

conductor is determined with Equation 5-7. R

is the resis-

tance measured using a 4 wire Kelvin method for the long line

of length l

. R

is the resistance measured using a 4 wire Kel-

vin method for the short line of length l

ρ =

− R

− l

[5-7]

A is the cross-section area (equal to the conductor width mul-

tiplied by the conductor thickness).

5.3.4 SPP Low Frequency Permittivity

It should be

noted that the two ground planes that are above and below

the signal of interest are always shorted together, in the trans-

mission line region and in the parallel plate disc area. The disc

that is used should have a diameter that is 100x the height, h

to, the nearest ground in order to be able to calculate ε

directly from (1) without any fringe capacitance consideration.

The typical diameter of the disc is 12.7 mm [0.5 in]. It is use-

ful to have a dummy structure that is nearby the disc that has

only the via connection between the surface pad and the disc

and the small lateral line extension. Typical configuration was

shown in 3.3.4.2. The capacitance of this parasitic structure is

subtracted from the total disc C so that the end effects are not

included in the result for ε

Finally, the dielectric loss, tanδ, is also measured for the large

disc using the same LCR meter in the range of 10 KHz to

1 MHz.

The line capacitance per unit length, together with the cross

sectional dimensions can also be used for determining the

dielectric constant at 1 MHz. The procedure is to calculate the

capacitance with a 2D field solver for an assumed dielectric

constant. Iteration is used on this assumed value until the

agreement is obtained between measured and calculated C.

The implicit assumption here is that the lines are uniform and

that the cross section is well known along the length. Both

these assumptions have limitations and this is why the extrac-

tion based on line C is not as accurate. On the other hand, the

composition of glass fiber and dielectric resin might differ in

the disc area from the line area which could introduce errors

in the extracted ε

at 1 MHz.

5.3.5 SPP TDT Measurement

TDT measurements are

also made with several lines, but especially with the 2 cm

[0.787 in] and 8 cm [3.15 in] lines of interest. In addition, it is

useful to measure a very short line of ‘‘zero length,’’ (e.g., 0.25

- 0.45 mm [0.0098 - 0.0177 in]) in order to obtain the band-

width of the time-domain set-up and for use as a reference for

delay extraction. The TDT measurement monitors the propa-

gation delay at 50% of the signal swing, the propagated rise

time between 10% and 90% levels. By taking the difference in

delays for the two lines and dividing by the difference in

lengths, one obtains the line propagation delay per unit length,

τ, without the effect of probes, pads, and via discontinuities.

The assumption is that these features are of similar character-

istics for the two lines. The propagated risetime through the

‘‘zero length’’ line indicates the bandwidth for the setup based

on the simplified formula for the upper 3 dB frequency given

in Equation 5-8.

, =

0.35

[5-8]

The correlation of propagation delay and rise time shape with

simulation can provide a very useful validation of the broad-

band model that is being created using this method.

Examples are given in Figure 5-6.

Number

2.5.5.12

Subject

Test Methods to Determine the Amount of Signal Loss on

Printed Boards

Date

07/12

Revision

IPC-TM-650

shall

Page

IPC-25512-5-6

0.4

-0.01

0.2

0.3

0.1

0 2 4 6 8 10

Measurement

Simulation

0.4

-0.01

0.2

0.3

0.1

0 2 4 6 8 10

Measurement

Simulation

Measurement

Simulation

l = 5 cm l = 8 cm

l = 5 cm

l = 20 cm

Time (nsec)

Voltage (V)

Time (nsec)

Voltage (V)

0.04

-0.01

0.02

0.03

0.01

0 0.2 0.4 0.6 0.8 1

Time (nsec)

Measurement

Simulation

Voltage (V)

0.04

-0.01

0.02

0.03

0.01

0 0.2 0.4 0.6 0.8 1

Time (nsec)

Number

2.5.5.12

Subject

Test Methods to Determine the Amount of Signal Loss on

Printed Boards

Date

07/12

Revision

IPC-TM-650

Figure

5-6

Typical

TDT

Measurements

and

Validation

Page