80GHz Notch Filter Design - people9 80GHz Notch Filter Design Figure 3 3D Notch filter HFSS...

DIGITAL PRODUCTIVITY FLAGSHIP

80GHz Notch Filter Design

Mark De Alwis

10 June 2015

ii 80GHz Notch Filter Design

iii 80GHz Notch Filter Design

Important disclaimer

CSIRO advises that the information contained in this publication comprises general statements based on

scientific research. The reader is advised and needs to be aware that such information may be incomplete

or unable to be used in any specific situation. No reliance or actions must therefore be made on that

information without seeking prior expert professional, scientific and technical advice. To the extent

permitted by law, CSIRO (including its employees and consultants) excludes all liability to any person for

any consequences, including but not limited to all losses, damages, costs, expenses and any other

compensation, arising directly or indirectly from using this publication (in part or in whole) and any

information or material contained in it.

4 80GHz Notch Filter Design

Contents

Executive summary....................................................................................................................................... 6

1 Introduction .................................................................................................................................... 7

2 Design Method Layout and Fabrication .......................................................................................... 7

2.1 Layout Options .................................................................................................................................. 7

2.2 3D Simulations .................................................................................................................................. 8

2.3 Second Iteration .............................................................................................................................. 10

2.4 Third Iteration ................................................................................................................................. 14

3 Test Results ................................................................................................................................... 16

4 Analysis and Conclusions .............................................................................................................. 23


Figures

Figure 1 Notch filter layout variants . .......................................................................................................... 7

Figure 2 3D Notch file structure in HFSS . ..................................................................................................... 8

Figure 3 3D Notch filter HFSS simulation results……………………………………………………………………………………...9

Figure 4 Schematic of 2 Section 78.5GHz notch filter version 3 from the first iteration . ........................... 9

Figure 5 S-Parameter plot of the microwave office linear simulation ............................. .........................10

Figure 6 2D GSG pad layout created in Microwave office . ........................................................................ 10

Figure 7 microwave office EM sight simulated circuit . .............................................................................. 11

Figure 8 Simulation results in Microwave office ……………………………………………………………………………………11

Figure 9 3D HFSS model of the filter structure with GSG pads at the input and output. .......................... 12

Figure 10 HFSS Solution Setup . ................................................................................................................. 12

Figure 11 HFSS 3D Simulation Results.. ...................................................................................................... 13

Figure 12 layout files .................................................................................................................................. 14

Figure 13 Layout Mask…….……………………………………………………………………………………………………………………..14

Figure 14 HFSS 3D model for filter variant V7 with four resonators……………….……………………………………….15

Figure 15 Simulated HFSS data plot for the four resonator notch filter variant V7 ................................... 15

Figure 16 Plotted test data for Filter variant V5EM ................................................................................... 16

Figure 17 Fabricated test substrates attached to a test carrier using conductive epoxy .......................... 17

Figure 18 Notch Filter V2-5 simulated ........................................................................................................ 18

Figure 19 Notch Filter V-5 measured ......................................................................................................... 18

Figure 20 Notch Filter V2-6 simulated ....................................................................................................... 19

Figure 21 Notch Filter V2-6 measured ....................................................................................................... 19

Figure 22 Notch filter V3-4 simulated ....................................................................................................... 20


Figure 24 Notch Filter V7 simulated ........................................................................................................... 21

Figure 25 Notch Filter V7 measured ........................................................................................................... 21

Figure 26 Notch Filter V7-1 simulated ........................................................................................................ 22



Executive summary

A low latency E Band (71-76GHz and 81-86GHz) radio with a high data rate has been designed. The E-

Band RF front end requires filtering of the LO signal at 78.5GHz. A set of notch filters have been designed

to be placed in front of the transmitter (TX) power amplifier. A new design methodology along with a

nonstandard substrate material with plated via holes was modelled and fabricated. The fabricated filter

test data shows agreement to the design simulation.


1 Introduction

The LO signal is a CW signal entering a sub harmonic mixer. The mixer doubles the LO signal frequency to

78.5GHz prior to performing an up conversion for the transmitter. Additional filtering is required at the

front end since the LO signal is in band to the TX power amplifier. The most suitable filter type for this task

is a notch filter. Initial design considerations required a high precision sputtered gold etch back process for

fabrication of the filters. Traditionally the substrate material of choice used for this process is ceramic

Alumina. However, the dielectric constant of Alumina is too high (9.7 @ 10GHz) to realize the thin film

circuit required. Therefore, Quartz was selected for its lower dielectric constant of 3.826. A substrate

thickness of 4mil was chosen to minimize the step from the GaAs Amplifier chip and the waveguide feed

substrate.

Three iterations were required to perfect the design method. In the initial run the substrates were

modelled and fabricated with no ground signal ground (GSG) probable input and output pads. This required

the test substrates to be wire bonded to 50 Ohm RF input and output lines with GSG pads. The results were

not satisfactory since the additional circuitry added unwanted parasitics which distorted the results.

The next iteration added input and output GSG pads into the simulated models and resulting fabricated

substrates. However, the full 3-D simulation carried out in HFSS had an error which uniformly shifted the

notch frequency of all the fabricated variants up in frequency by 2GHz to 3GHz. A third iteration was

carried out correcting the error in HFSS to yield good results.

2. Design Method Layout and Fabrication

2.1 Layout Options Notch filters are formed by introducing coupled RF lines adjacent to the main transmission line carrying the

RF signal. The number of coupled lines determines the depth of the notch. The notch frequency is

determined by the length of the coupled section of RF line. The coupled line needs to be terminated at one

end with an open stub. The length of the open stub and the length of the coupled line segment are

between λ/4 and λ/8 where λ= wave length of the RF frequency of the notch. The open stub tends to be

closer to λ/4 while the coupled line length tends to be closer to λ/8. If more than one coupled line segment

is used the distance between coupled line segments is also critical to the simulation. The width of the main

RF transmission line is maintained at 50 Ohm impedance width. The width of the coupled line segments are

usually held to one uniform value determined by simulation and optimization. Applying simulation and

optimization methods described next four layout variants shown in figure 1 were designed. The radial open

stub (in V3EM & V3LN) was used to obtain a slightly wider notch. Two coupled line segments were chosen

due to the available space and the target specification of -15dB rejection.

Figure 1 Notch filter layout variants


2.2 3D Simulations An accurate 3D simulation is required for a successful filter design in the E-Band. The Microwave office

circuit simulator’s EM simulation tool EM Sight was used only for an initial estimation of the filter

performance. Once the approximate results are obtained a GDS file of the microwave office layout is

imported into HFSS. This file is for a two dimensional layout which is then converted into a three

dimensional structure to accurately represent the filter. The filter enclosure is modelled to represent the

actual enclosure in the E-Band radio TX housing. The 3D structure is given two wave ports with three modes

as the input and output excitation ports. The S-Parameter plot is then evaluated. If the required notch

location has not been met the microwave office simulation is used to move the notch location up or down

in frequency so as to arrive at a precise notch at 78.5GHz in the HFSS simulation. This is an iterative

procedure that is repeated until the desired result is obtained. Shown below in figure 2 is the three

dimensional structure of one of the filter variants modelled in HFSS.

Figure 2 3D Notch file structure in HFSS from the first iteration

Shown next in figure 3 is the HFSS simulation of the 3D structure shown in Figure 2. The microwave office

circuit used to create the 3D HFSS structure shown in figure 4. The linear S-Parameter plot of this circuit in

microwave office is also shown in figure 5.


Figure 3 3D Notch filter HFSS simulation results

Figure 4. – Schematic of 2 Section 78.5GHz notch filter version 3 from the first iteration

67.50 70.00 72.50 75.00 77.50 80.00 82.50 85.00 87.50 90.00Freq [GHz]

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Y1

HFSSDesign1XY Plot 1 ANSOFT

m1

m2

Based on Linear Microw ave Office simulation

Curve Info

dB(S(1:1,1:1))Setup1 : Sw eep

dB(S(1:1,2:1))Setup1 : Sw eep

Name X Y

m1 78.6000 -14.8669

m2 78.4000 -13.7359

W1

W2

1

2

3

4

M2CLINID=TL2W1=8.5 milW2=w2 milS=x2 milL=L2 milAcc=4

W1

W2

1

2

3

4


MBEND90XID=MS1W=w1 milM=0.7


MLEFXID=MO1W=w2 milL=0.2 mil


MLINID=TL1W=8.5 milL=25 mil

MLINID=TL3W=8.5 milL=L3 mil

MLINID=TL4W=8.5 milL=25 mil

MRSTUB2WID=TL5W=w1 milRo=y1 milTheta=90 Deg

MRSTUB2WID=TL6W=w2 milRo=y2 milTheta=90 Deg

MSUBEr=3.826H=4 milT=0.08 milRho=1Tand=0.000015ErNom=3.83Name=SUB1

PORTP=1Z=50 Ohm

PORTP=2Z=50 Ohm

x2=1.2

y2=15.65

L2=13.6

L1=13.4

w1=7.2

y1=15.85

w2=7.2

L3=25


Figure 5. – S-Parameter plot of the microwave office linear simulation.

The inaccuracy of the linear circuit simulator can be seen in the required frequency shift of the notch

location which in this case has to be at 82.4GHz in order to arrive at the desired notch frequency of

78.5GHz in the 3D simulation. Tuning the 3D structure in HFSS is not recommended for this type of circuit

due to the simulation time taken and the complexity of varying the physical layout in the 3D environment.

In order that the layout remains accurate and free of errors tuning of the notch location is always carried

out in microwave office where the circuit remains a two dimensional structure. When the HFSS simulation

confirms that the notch frequency is at 78.5GHz the corresponding 2D GDS file is imported into AUTOCAD

and used to prepare the mask drawing for fabrication.

2.3 Second Iteration In this iteration a ground-signal-ground-pad (GSG) was introduced to both 2D AWR simulation and the 3D

HFSS simulation. The 2D layout of the GSG pad modelled in microwave office is shown figure 6. The EM

sight simulator was used to simulate this 2D structure and extract a set of S-Parameter files. The two port S-

Parameter file was then used at the input and output of the 2D filter layout in Microwave office.

Figure 6 2D GSG pad layout created in Microwave office

68 70 72 74 76 78 80 82 84 86 88 90Frequency (GHz) (GHz)

78.5GHz Notch Filter 2 Section V3

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-5

0

5

S21

& R

etur

n Lo

ss (

dB)

78.45 GHz 81 GHz76 GHz

82.4 GHz-40.5 dB

DB(|S(1,1)|)Notch Filter 2 section V3



21


Figure 7 microwave office EM sight simulated circuit

Figure 8 simulation results in Microwave office

Figure 7 shows the microwave office simulation circuit with the EM sight simulator invoked. The S-

parameter files for the GSG pad is not shown but attached to the input and output of the circuit shown in

figure 7. Figure 8 shows is EM sight simulation result. The GDS file of the circuit shown in figure 7 is

imported into HFSS and given a 3D structural definition of solid gold with a metal thickness of 0.16mil. The

quartz substrate and air box surrounded by an Aluminium enclosure are added. GSG pads are created in 3D

and also added. However, HFSS gives an error if the input and output wave ports are defined at the

boundary of the GSG pads. Therefore and additional length of RF line has to be added to the layout. In this

STACKUPName=SUB2

EXTRACTID=EX1EM_Doc="Notch_2sec_EM"Name="Notch_Filter_2sec_V2"Simulator=EMSightX_Cell_Size=0.2 milY_Cell_Size=0.2 milPortType=DefaultSTACKUP="SUB2"Extension=10 milOverride_Options=YesHierarchy=OffSweepVar_Names=""

W1

W2

1

2

3

4


W1

W2

1

2

3

4






MLINID=TL1W=8.5 milL=20.2 mil

MLINID=TL3W=8.5 milL=L3 mil

MLINID=TL4W=8.5 milL=20.2 mil

MRSTUB2W$ID=TL5W=w1 milRo=y1 milTheta=90 Deg

MSUBEr=3.826H=4 milT=0.08 milRho=1Tand=0.000015ErNom=3.83Name=SUB1

MRSTUB2W$ID=TL6W=w1 milRo=y1 milTheta=90 Deg

PORTP=1Z=50 Ohm

PORTP=2Z=50 Ohm

x2=1.0

L3=24

w1=6.8

L1=12.6

y1=15.8

68 70 72 74 76 78 80 82 84 86 88 90Frequency (GHz)

78.5GHz Notch Filter V5 2 Section EM

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-5

0

S21

& R

etur

n Lo

ss (

dB)

d3

d2d1

78.5 GHz

80.4 GHz-33.6 dB

DB(|S(1,1)|)Notch_Filter_2sec_V4

DB(|S(2,1)|)Notch_Filter_2sec_V4

DB(|S(2,2)|)Notch_2sec_EM


case a 20 mil length RF line of 50 ohm line width was added at the input and output of the circuit. Figure 9

shows the complete 3D layout of this structure. The size of this enclosure is 146.4mil x 94mil x 54.16mil.

This includes the ground plane metallization thickness which is also defined as 0.16mil. This value is derived

from the design rules document published by the fabrication house Applied Thin-film Products (ATP) based

in the United States. 2mil thick Aluminium covers are added to the air box on the top and the left and right

sides.

Figure 9 3D HFSS model of the filter structure with GSG pads at the input and output.

In the solution setup window under analysis shown in figure 10, an error occurred due to a

recommendation made by HFSS technical support. The solution frequency was specified at 90GHz which is

the highest frequency the simulation is set up to sweep instead of 78.5GHz which is the point of interest

and where the solution frequency should be specified in order to run the 3D simulator accurately.

Figure 10 HFSS Solution Setup


Figure 11 HFSS 3D Simulation Results

3D simulation results are shown for S11 (red and blue) and S21 (brown and green) in figure 11. The plots

shown here are two different set up files. Setup 1 has the solution frequency specified at 90GHz while

setup 2 has the solution frequency set up at 78.5GHz. The shift up in frequency corresponds to the actual

measured data. This measured data plot is shown in figure 15 under test results.

67.50 70.00 72.50 75.00 77.50 80.00 82.50 85.00 87.50 90.00Freq [GHz]

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1HFSSDesign1XY Plot 1 ANSOFT

m1

m2

m3m4

m5

m6

m7

Version 5 w ith Solution Freq at 78.5GHz and 90GHz

Curve Info

dB(S(1,1))Setup1 : Sw eep


dB(S(1,1))_1Setup2 : Sw eep

dB(S(2,1))_1Setup2 : Sw eep

Name X Y

m1 78.5000 -16.7737

m2 78.1000 -17.6657

m3 75.0000 -1.4546

m4 71.0000 -0.6556

m5 82.0000 -1.7869

m6 86.0000 -0.6612

m7 81.2000 -13.2029


2.4 Third Iteration The third iteration follows the same procedure described above. Five layout options were designed which

included two designs with four resonators. Shown in figure 12 are the AUTOCAD layout files followed by

the layout mask shown in figure 13.

Figure 12 layout files

Figure 13 Layout Mask

Front SideDimensions are in Inches

Material - Fused Silica, Quartz

Thickness 4milGold Thickness is 160micro Inches front and back

Back side is solid gold

Use Etch Back Process

For Au Thickness of 160micro Inches all critical gapsare biased 0.0004" narrower while all critical line widthsare biased 0.0004" wider as indicated below.


The HFSS 3D drawing for the four resonator notch filter V7 is shown in Figure 14 followed by the results of

the 3D EM simulation for this filter. All filter variants were modelled and simulated similarly but they are

not shown.

Figure 14 HFSS 3D model for filter variant V7 with four resonators

Figure 15 Simulated HFSS data plot for the four resonator notch filter variant V7

The conductivity of gold was modified upon a recommendation from 4.1x 10^7 Siemens/m to 1.5 x 10^7 to

account for the changes in conductivity at E band frequencies. The die electric loss tangent of the quartz

substrate was raised from 0.000015 to 0.0015 also to account for increased loss at E Band frequencies.

These modifications improve the accuracy of the simulation. They are applied to all five filter variants.

Using four resonators creates a wider notch.

67.50 70.00 72.50 75.00 77.50 80.00 82.50 85.00 87.50 90.00Freq [GHz]

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m1 m2

m3 m4

Version 7 with solution frequency at 78.5GHz with the Conductivity of Gold modified to = 1.5x10 7̂ s/mDie-electric loss tangent = 0.0015

Curve Info





Name X Y

m1 78.1000 -17.7149

m2 78.5000 -17.9296

m3 76.0000 -2.3253

m4 81.0000 -2.6255


3. Test Results

Figure 16 Plotted test data for Filter variant V5EM

Figure 15 shows the plotted test data for the notch filter from the second iteration discussed above in

section 3.3. The 3D HFSS simulation of this filter was shown in figure 11. In this simulation the setup 2

simulation data plotted out corresponds to the actual test data. Maker m7 in figure 11 is at 81.2GHz. This

reveals the close agreement of the simulation data to the test data once the error in the solution frequency

of the HFSS simulation was eliminated.

70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90Frequency (GHz)

Notch Filter V5EM

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-5

0

81.2 GHz78.5 GHz76 GHz

76 GHz-0.6578 dB

71 GHz-0.5853 dB

81.2 GHz-14.29 dB

DB(|S(1,1)|)V5EM 1.$FPRJ

DB(|S(2,1)|)V5EM 1

DB(|S(2,2)|)V5EM 1.$FPRJ


Figure 17 Fabricated test substrates attached to a test carrier using conductive epoxy

The substrates in figure 16 were tested in the GHz lab using 200 micron pitch GSG probes on the high

frequency (110GHz) probe station connected to an Agilent network analyser. The via holes show

conductive epoxy has come up the holes and has been scrapped off. However, this does not cause a

problem since the grounding is thru the pad and the active RF transmission line remains undisturbed. S-

Parameters were extracted on a total of ten substrates (two of each type).

Presented next are plots of the 3D HFSS simulations of all five filter types paired with plots of the measured

test data. The roughness of the measured data plots for S11 and S22 are caused by the network analyser.


Figure 18 Notch Filter V2-5 simulated

Figure 19 Notch Filter V-5 measured

67.50 70.00 72.50 75.00 77.50 80.00 82.50 85.00 87.50 90.00Freq [GHz]

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m1

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m3 m4

Version 2 with solution frequency at 78.5 full set of S-Paramterswith the Conductivity of Gold modified to = 1.5x10^7 s/m

Curve Info





Name X Y

m1 78.2000 -16.4894

m2 78.5000 -14.9757

m3 75.0000 -0.9754

m4 82.0000 -1.0165

70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90Frequency (GHz)

Notch Filter V2-5

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-5

0 d3

d2d1

71 GHz 86 GHz81 GHz78.5 GHz76 GHz

86 GHz-0.8869 dB

81 GHz-2.13 dB

71 GHz-0.4484 dB 76 GHz

-1.266 dB

78.5 GHz-17.37 dB

DB(|S(1,1)|)V2_5_1.$FPRJ

DB(|S(2,2)|)V2_5_1.$FPRJ

DB(|S(2,1)|)V2_5_1



Figure 21 Notch Filter V2-6 measured

In the simulation of this filter the notch location was offset 400 MHz lower to 78.1GHz. This was done in

anticipation of a slight drift of the frequency due to variations in the fabrication process. However, no such

variations can be seen from the measured data.

67.50 70.00 72.50 75.00 77.50 80.00 82.50 85.00 87.50 90.00Freq [GHz]

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m1

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m3 m4

Version 2 w ith solution f rw equency at 78.5 full set of S-Paramtersw ith the Conductivity of Gold modif ied to = 1.5x10^7 s/m

Curve Info





Name X Y

m1 78.1000 -14.7809

m2 78.5000 -12.2902

m3 75.0000 -0.9520

m4 82.0000 -0.8388

70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90Frequency (GHz)

Notch Filter V2-6

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-5

0


78.1 GHz-16.14 dB

86 GHz-1.041 dB

81 GHz-1.694 dB

71 GHz-0.478 dB

76 GHz-1.342 dB

78.5 GHz-14.96 dB

DB(|S(1,1)|)V2_6_1.$FPRJ

DB(|S(2,1)|)V2_6_1

DB(|S(2,2)|)V2_6_1.$FPRJ


Figure 22 Notch filter V3-4 simulated


Again the simulated notch frequency was given an offset of 400MHz to 78.1GHz in anticipation of a slight

frequency drift lower and in this case a drift of 100MHz is observed but not as large as 400MHz.

67.50 70.00 72.50 75.00 77.50 80.00 82.50 85.00 87.50 90.00Freq [GHz]

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m1

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m3 m4

Conductivity of Gold Modified to 1.5x10 7̂ s/m

Version 3.4 with solution frequency at 78.5GHz

Curve Info





Name X Y

m1 78.1000 -14.1395

m2 78.5000 -11.2408

m3 75.0000 -1.0737

m4 82.0000 -0.8142

70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90Frequency (GHz)

Notch Filter V3-4

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-5

0


78.2 GHz-15.65 dB

78.5 GHz-14.13 dB

76 GHz-1.399 dB

71 GHz-0.4969 dB

86 GHz-1.043 dB

81 GHz-1.601 dB

DB(|S(1,1)|)V3_4_1.$FPRJ

DB(|S(2,1)|)V3_4_1

DB(|S(2,2)|)V3_4_1.$FPRJ


Figure 24 Notch Filter V7 simulated

Figure 25 Notch Filter V7 measured

Notch filter V7 and V7-1 were included to provide a wider notch and observe the accuracy of the

simulation. Filter V7 has the notch centred at 78.5the desired frequency and shows good agreement. Filter

V7-1 is offset 400MHz to the low side as in some of the other variants.

67.50 70.00 72.50 75.00 77.50 80.00 82.50 85.00 87.50 90.00Freq [GHz]

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m1m2

m3 m4

Version 7 with solution frequency at 78.5GHz with the Conductivity of Gold modified to = 1.5x10 7̂ s/mDie-electric loss tangent = 0.0015

Curve Info





Name X Y

m1 78.1000 -17.7149

m2 78.5000 -17.9296

m3 76.0000 -2.3253

m4 81.0000 -2.6255

70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90Frequency (GHz)

Notch Filter V7

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-5

0

d2d1


86 GHz-1.108 dB

81 GHz-1.928 dB

71 GHz-0.6312 dB

76 GHz-2.098 dB

78.5 GHz-17.46 dB

DB(|S(1,1)|)V7_2.$FPRJ

DB(|S(2,1)|)V7_2

DB(|S(2,2)|)V7_2.$FPRJ




67.50 70.00 72.50 75.00 77.50 80.00 82.50 85.00 87.50 90.00Freq [GHz]

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m1

m2

m3

m4

Version 7.1 with solution frwequency at 78.5 GHz with the Conductivity of Gold modified to = 1.5x10^7 s/mDie-electric loss tangent = 0.0015

Curve Info





Name X Y

m1 78.1000 -17.4503

m2 78.5000 -15.9270

m3 76.0000 -3.4449

m4 81.0000 -1.7544

70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90Frequency (GHz)

Notch Filter V7-1

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0

d2d1


86 GHz-1.169 dB

81 GHz-1.754 dB

71 GHz-0.5976 dB

76 GHz-2.257 dB

78.5 GHz-16.59 dB

DB(|S(1,1)| )V7_1_2.$FPRJ

DB(|S(2,2)| )V7_1_2.$FPRJ

DB(|S(2,1)| )V7_1_2


4. Analysis and Conclusions An accurate method has been developed to design filters in the E-Band frequency range. This method has

demonstrated good agreement between simulated and actual performance. The use of a three dimensional

EM simulation tool HFSS has been combined with a linear circuit simulator, Microwave office to achieve the

design objectives. This approach can be duplicated in the design of MMIC chips in the E band frequency

range as well as in the design of other filter types. Three dimensional simulation of ground signal ground

pads are crucial in the design of MMICs since they are vital layout features that enable accurate on wafer

testing.

A minimal drift of approximately 100MHz is observed between simulated and measured data which is

significantly lower than the anticipated 400MHz. The value for the anticipated drift of 400MHz was arrived

at from measurement of the two previous iterations where other factors such as simulation error and bond

wire parasitics in the first iteration contributed to the frequency drift.


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FOR FURTHER INFORMATION

Digital Productivity Flagship

Mark De Alwis

t +61 2 9372 4169

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Digital Productivity Flagship

Hajime Suzuki

t +61 2 9372 4121

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w www.csiro.au

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