Characterization of PCB

Insertion Loss with a New

Calibration Method

Jia Gongxian

Zhu Wenxue Huawei Technologies

Long Faming

Cheng Ning

Keysight Technologies

Mike Resso

Agenda

• Background - The Importance of characterizing PCB

insertion loss

• Traditional methods for characterizing PCB insertion loss

• New Characterization method – 1xAFR

• Comparisons and validations of different methods – With simulation

– With measurements (different layers, reference impedance, stub or backdrill)

• Considerations on the gating range of the 1xAFR method

• Conclusion

Determination of PCB unit length insertion loss is critical for:

• Material electrical parameters extraction

• PCB electrical performance estimation

• Evaluation of the passive channel design

• PCB material selection

• Evaluation of the PCB fabrication quality, etc

Traditional Characterization Methods

Direct Loss Subtraction TRL Calibration Automatic Fixture Removal

With 2xThru

Direct Loss Subtraction

Two differential transmission

lines with different lengths on

the PCB

Subtracting the insertion loss of the two

differential lines, resulting in the insertion

loss of PCB trace with length of ΔL.

Limitations:

• The launches of the two lines must be

consistent

• The launches must have good match

Any mismatch in the launches will become residual error after

subtraction and will be transferred to the insertion loss measurement of

the PCB trace, causing ripples on the insertion loss characterization

Removing launch effects with TRL Calibration

TRL Calibration Error Model

A typical TRL calibration kit on PCB

Concept:

• TRL cal kit is fabricated with the same

characteristics of the launches

• Total 10 measurements are made to calculate the 8

error terms

• Error correction is applied to remove the launch

parts, resulting only the PCB trace S-parameters

Limitations:

• It needs much experience in TRL cal kit design,

fabrication and verification

• The phase difference between Thru and lines must be

within (20 ~ 160) + N * 180 degrees in the frequency

range used

• The launches of the cal standards must be consistent

• The Thru and lines should have small impedance

variations

• Cal kit model needs to be characterized for calibration

Automatic Fixture Removal with 2xThru

Two differential transmission

lines with different lengths on

the PCB

• The launches are fully extracted from a shorter

2xThru, then de-embedded from the longer

differential line, resulting in insertion loss of PCB

trace with length of ΔL.

• Based on time domain gating and signal flow

diagram calculations.

• Same accuracy with TRL calibration, more accurate

than Direct Subtraction.

Limitations:

• The launches of the 2xThru must be consistent with

that of the longer line.

• The 2xThru and longer line need to be symmetrical

top to bottom

• The return loss and insertion loss of the 2xThru

cannot cross each other in the measurement

frequency range, often at least 5 dB separation is

required

Separation between RL and IL of the 2xThru

Proposed New PCB Loss Characterization

Method using 1xAFR Technique

PCB Loss Characterization with 1xAFR

Two differential Open

standards with different

lengths on the PCB

• Extracting the 4-port S-parameters of the two Opens

with 1xAFR, which is also based on Time Domain

Gating and Signal Flow Diagram calculations

• De-embed the 4-port S-parameters of the shorter

Open from the longer Open, resulting in the S-

parameters of the PCB trace with length of ΔL.

• Same accuracy with TRL calibration and 2xThru AFR,

but save more PCB area and measurement time.

Limitations:

• The launches of the two Opens must be consistent.

• The impedance variations of the PCB trace must be

as small as possible, especially when getting close to

the Open response

• The gating range needs to be selected carefully to

achieve good accuracy (will be discussed later)

Comparisons and Validations with

ADS Simulation

Simulation Procedure

• Create structures of PCB trace

without launch, launches,

differential lines, differential Opens

with ADS

• Simulate the S-parameters of all

these structures separately

• Characterize the PCB trace loss

with the above methods and

compare to the actual data

• 100 Ohm and 90 Ohm Z0 cases

are studied independently

Structure Line length (mil)

PCB trace length without

launches

11000

Launch 150

2xThrough AFR Longer line

(Direct Subtraction longer line)

11300

2xThrough AFR Shorter line

(Direct Subtraction shorter line)

1300

1xAFR Longer Open 11150

1xAFR Shorter Open 1150

Dielectric Constant: 3.85

Simulation Schematic of the Differential Open

• The S-parameters of the

Differential Open standard can

be simulated with the schematic

on the left

• S-parameters of other structures

can be simulated similarly

Overcome the Gating Effects

The intended measurement

bandwidth is 20 GHz, but

AFR may have error at high

frequency due to gating

effects.

Simulate/measure to higher

frequencies to overcome the

gating effects, in this case

study, the simulation is up to

25 GHz.

Direct Subtraction Result (100 Ohm Z0)

The maximum difference

between actual data and

Direct Loss Subtraction is

0.2216 dB at 19.99 GHz (ΔL

= 11 inches)

Ripples at high frequency

due to residual mismatch

error

2xThru AFR Result (100 Ohm Z0)

The maximum difference

between actual data and

2xThrough AFR is 0.0012 dB

at 810 MHz (ΔL = 11 inches)

1xAFR AFR Result (100 Ohm Z0)

The maximum difference

between actual data and

1xAFR is 0.0848 dB at 10

MHz (ΔL = 11 inches)

Direct Subtraction Result (90 Ohm Z0)

The maximum difference

between actual data and

Direct Loss Subtraction is

0.193 dB at 17.55 GHz (ΔL =

11 inches)

Ripples at high frequency

due to residual mismatch

error

2xThru AFR Result (90 Ohm Z0)

The maximum difference

between actual data and

2xThrough AFR is 0.0561 dB

at 19.98 GHz (ΔL = 11

inches)

1xAFR AFR Result (90 Ohm Z0)

The maximum difference

between actual data and

1xAFR is 0.08 dB at 10 MHz

(ΔL = 11 inches)

Summary of Simulation Results

• The 2xThru AFR and 1xAFR

are both very close to the

actual data, either with 100

Ohm or 90 Ohm Z0

• The Direct Subtraction

shows more error at high

frequency due to residual

mismatch

Z0 Characterization

Method

Error (dB/inch)

100 Ohm Direct Subtraction 0.022

2xThru AFR 0.0001

1x AFR 0.0008

90 Ohm Direct Subtraction 0.02

2xThru AFR 0.0056

1x AFR 0.008

Comparisons and Validations with

Measurement

Comparison outlines

For 100 Ohm and 90 Ohm Z0, compare the

performance of above methods in following

cases:

Comparisons

Layer 4

Stub

Backdrill

Layer 6

Cross-

comparis

on of

1xAFR

8 layer PCB

Measurement Structures

Dielectric Constant: 3.9 at 5 GHz

Structure Line length (mil)

PCB trace length without

launches

10000

TRL Open 500

TRL thru 1000

TRL Line1 3480

TRL Line2 1496

TRL Line3 1099

TRL DUT 11000

2xAFR Longer line (Direct

Subtraction longer line)

12327

2xAFR Shorter line (Direct

Subtraction shorter line)

2327

1xAFR Longer Open 11400

1xAFR Shorter Open 1400

8-layer PCB layout and fabricated PCB

(coaxial connectors are not mounted yet)

Measurement results after TRL

calibration is treated as reference for

comparisons

100 Ohm, Layer 4, stub

Error compared to TRL:

Char

method

Error

(dB/inch)

Direct

Subtraction

0.055

2xThru AFR 0.024

1xAFR 0.023

Even some deviation in TRL here

The big deviation in TRL

should be due to cal kit

fabrication error

100 Ohm, Layer 4, backdrill

Error compared to TRL:

Char

method

Error

(dB/inch)

Direct

Subtraction

0.06

2xThru AFR 0.016

1xAFR 0.02

100 Ohm, Layer 6

Error compared to TRL:

Char

method

Error

(dB/inch)

Direct

Subtraction

0.044

2xThru AFR 0.027

1xAFR 0.026

Cross comparisons

Very repeatable results with

1xAFR on different layers,

either Stub or backdrill

90 Ohm, Layer 4, stub

Error compared to TRL:

Char

method

Error

(dB/inch)

Direct

Subtraction

0.068

2xThru AFR 0.029

1xAFR 0.039

90 Ohm, Layer 4, backdrill

Error compared to TRL:

Char

method

Error

(dB/inch)

Direct

Subtraction

0.057

2xThru AFR 0.034

1xAFR 0.037

TRL has big deviation in this range

90 Ohm, Layer 6

Error compared to TRL:

Char

method

Error

(dB/inch)

Direct

Subtraction

0.039

2xThru AFR 0.026

1xAFR 0.021

Cross comparisons

Difference < 0.015 dB/inch

with 1xAFR on different

layers, either Stub or

backdrill

Measurement Comparison Summary

• 2xThru AFR and 1xAFR are both very close to TRL calibration result

• Direct Loss Subtraction result shows ripples at high frequency due to

mismatch effects

• TRL calibration result may have big deviation, too, due to TRL cal kit

fabrication quality

• The new 1xAFR works for both 100 Ohm and 90 Ohm Z0

• The 1xAFR results are very repeatable on different PCB layers, either

with Stub or backdrill

Considerations on the Gating Range of

1xAFR Method

If the gating range is too narrow

• In the 1xAFR method,

bandpass time domain

gating is used for the

extraction of the fixture

insertion loss from the Open

standard.

• If the gating range is too

close to the Open response,

part of the Open response

may be gated off and causes

the extracted insertion loss

to have some ripples

If the gating range is too wide

• If the gating range is too

wide, although the complete

Open response will be

maintained, some mismatch

effects caused by the

impedance variations of the

PCB trace will also be

included after gating and

introduce some ripples to the

extracted insertion loss

Optimum gating range

• The optimum gating range

needs to be adjusted as a

compromise of the two

aspects above

Extracted insertion loss with different gating range

In real applications

• Optimize the fabrication

quality to make sure the

impedance variations are as

small as possible

• Select the gating range

carefully to optimize the

extracted insertion loss if

there are still some

impedance variations

Conclusion

Direct Loss

Subtraction

TRL

calibration

2xThrough

AFR

1xAFR

Complexity Easy Complicated Easy Easy

Accuracy Low High High High

Cost Low High Low Lowest

Measurement

Bandwidth

Low High High High

Thank You!

Q & A