Contract R&D | Consultancy

Communications Sensors Information Systems

Specialist Products

Kate Moore

Using HFSS successfully at 77GHz

ROKE Presentation Title

2

Using HFSS successfully at 77GHz

• Data sheets give you the dielectric constant and

the loss tangent

Or do they?

• The materials properties are rarely measured at

high frequencies so the user must measure it

themselves.

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Material Properties

3

Design

a resonant structure…

Ø outer

Ø inner

Tra

ck w

idth

Gap1 Gap2

Manufacture

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Material Properties

4

Measure

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Material Properties

5

Comparewith HFSS predictions: vary εr and tanδ until the measured

results overlay the predictions

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S21 (

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Frequency (GHz)

Resonant Rings: HFSS comparison 2

Ring 4

HFSS: Er=3.11 tand=0.009 straight sides

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Track shape and profile

6

Using HFSS successfully at 77GHz

ROKE Presentation Title

Track shaping and cross section

• The tracks are not perfect shapes

• At these frequencies it makes a difference

7

ROKE Presentation Title

Track shaping and cross section

• HFSS allows you to change the profile of

the tracks easily

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ROKE Presentation Title

Track shaping and cross section

• This shaping makes a difference

9

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Bond Wires

10

Using HFSS successfully at 77GHz

ROKE Presentation Title

Bond Wires

11

HFSS models of bondwires

Real bondwires

ROKE Presentation Title

Bond Wires

12

• HFSS predictions for an identical circuit with different bondwires

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Ansoft LLC LO Match1Return Loss plot loss for different bondwire ANSOFT

Curve Info

Bondw ire 3

Bondw ire 2

Bondw ire 1

ROKE Presentation Title

Ports, complex and otherwise

13

Using HFSS successfully at 77GHz

ROKE Presentation Title

Matching to ports

14

• Bondwire

Typical port

measurement of

a 77GHz device

Matching to the port needs to

include

• Port impedance

• Matching network (stub)

ROKE Presentation Title

Complex ports – a bit quirky

• Positive S parameters!

• Positive S-parameters are possible if using

complex ports. The fields in the model are all

correct but the return loss looks scary. It is due

to the maths involved in normalising the ports.

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SAS IP, Inc. Rx port Small airbox stub matchSmith Plot 1 ANSOFT

Curve Info

S(LumpPort1,LumpPort1)Setup1 : Sw eep1

S(Tx,Tx)Setup1 : Sw eep1

What are you matching to?

Port impedance from datasheet = 29 -75j

Tune the stub to give a good match.

What is the impedance of the circuit with the stub?

29-75j

Which will not give a good match in real life. You must

manually set the port to 29 + 75j

ROKE Presentation Title

Matching to ports

To get around the positive s

parameters:

Divide the port into two parts:

1. A resistive port corresponding to the

real part of the port impedance. i.e.

29

2.A reactive section in series

corresponding to the port reactance

i.e. 0.0273pF

ROKE Presentation Title

Lumped ports vs waveports

Lumped ports

easy and small

Can fit nicely on

the end of a bond

wire.

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Lumped ports

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Lumped Ports_Fixed_WidthSmith Chart 1 ANSOFT

Curve Info

S(LumpPort1,LumpPort1)Setup1 : Sw eep1

Simple port at the end

of a track. The

dielectric is 0.1mm

thick

A simple port should be

fine. Shouldn’t it?

ROKE Presentation Title

Lumped ports

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Lumped Ports_Fixed_WidthSmith Chart 1 ANSOFT

Curve Info

S(LumpPort1,LumpPort1)Setup1 : Sw eep1

S(LumpPort1,LumpPort1)_1Imported

The port may need to be

thinner. Comparing the

input impedance shows a

significant difference.

Which is correct?

Lumped ports are not invisible

they have dimensions and

therefore parasitic L and C

ROKE Presentation Title

Lumped ports

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Lumped Ports_Fixed_Width_reduced_heightSmith Chart 1 ANSOFT

Curve Info

Lumped port - Full height

Lumped port - reduced height

Lumped port narrow

L can be reduced by

moving the ground plane

up towards the track,

shortening the port.

Or use a wave port

ROKE Presentation Title

Tolerance analysis

21

Optimetrics

ROKE Presentation Title

Optimetrics

• PCB manufacturing accuracy is critical. Optimetrics can be used effectively

with parametric sweeps show the effects of over and under etching.

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Return Loss variation with etching ANSOFT

Curve Info

Ideal w ith C

Under 10

Under 10 C

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Under 20 C

ROKE Presentation Title

Conclusions

• HFSS can be used at every step when designing at

77GHz.

• Initial designs

• Validation of measured material properties

• Further iteration with optimised material values

• Analysis of manufacturing tolerances

• However, you must be careful because effects which can

happily be ignored at lower frequencies are critical at mm

waves75.0 76.0 77.0 78.0 79.0 80.0 81.0 82.0 83.0 84.0 85.0

Freq [GHz]

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Re

turn

Lo

ss (

dB

)

Return Loss variation with etching ANSOFT

Curve Info

Ideal w ith C

Under 10

Under 10 C

Under 20

Under 20 C

Contract R&D | Consultancy

Communications Sensors Information Systems

Specialist Products

Kate Moore

Using HFSS successfully at 77GHz