Design of Combline Bandpass Filters

Interdigital BPF included

Microwave & Millimeter-wave Lab.

Comline Bandpass Filters

2

Input Output

Typical Combline Bandpass Filter

Microwave & Millimeter-wave Lab.

Equivalent circuit

3

J12 J23 J34

GA

b1 b3Yin

a

J01

b2

GB

Input coupling

Output coupling

Microwave & Millimeter-wave Lab.

Coupled section

equivalent circuit

4

Y2 , l

Y1 , l

Y3 , l

Zoe, Zoo

Microwave & Millimeter-wave Lab.

Slope parameter

5

Use the following resonant condition:

Microwave & Millimeter-wave Lab.

Coupling coefficient(1)

J-inverter:

Coupling coefficient:

6

Microwave & Millimeter-wave Lab.

Coupling coefficient(2)

Coupling coefficient inversely proportional to the frequency

7

Microwave & Millimeter-wave Lab.

Combline Bandapss Filter

8

Microwave & Millimeter-wave Lab.

Equivalent circuit for 1st and last stage

Interdigital type feeding

9

Microwave & Millimeter-wave Lab.

Coupling stage

Combline type

10

Microwave & Millimeter-wave Lab.

Equivalent circuit(1)

Equivalent circuit

11

Microwave & Millimeter-wave Lab.

Design equations

12

Microwave & Millimeter-wave Lab. 13

Microwave & Millimeter-wave Lab. 14

Microwave & Millimeter-wave Lab.

Equivalent circuit(2)

Reduced circuit

15

Microwave & Millimeter-wave Lab.

Equivalent circuits

16

Microwave & Millimeter-wave Lab.

Interdigital BPF-Introduction

Attractive features Very compact Required tolerances in manufacturing are

relatively relaxed because of the large spacing between resonator elements

The spurious response appears at there times the center frequency of the passband

The rates of cutoff and the strength of the stop bands are enhanced by multiple poles attenuation at dc and at even multiples of the passband center

17

Microwave & Millimeter-wave Lab.

Interdigital Bandpass Filter

Input feeding – shorted

30% bandwidth or less

18

Microwave & Millimeter-wave Lab.

Equivalent circuit

19

Y2 , lY1 , l Y3 , l

YA

1:N

Y12 , l Y23 , l Y34 , l

……

……

Microwave & Millimeter-wave Lab.

Design procedures

20

Microwave & Millimeter-wave Lab.

Design equations

21

Microwave & Millimeter-wave Lab.

Designed Filter

22

Microwave & Millimeter-wave Lab.

Input feeding –Open

30% bandwidth or more

23

Microwave & Millimeter-wave Lab.

Equivalent circuit

24

J12 J23 J34

YA

b1 b3

J01

b2

YB

Microwave & Millimeter-wave Lab.

Design equations(1)

25

Microwave & Millimeter-wave Lab.

Design equations(2)

26

Microwave & Millimeter-wave Lab.

From Filter Specifications

27

Microwave & Millimeter-wave Lab.

Design Example

Open ended feeding At 2 GHz 10% bandwidth, Order 3

28

Microwave & Millimeter-wave Lab.

Layout of microstrip interdigital BPF

29

SPEC

Order : 3FBW : 10%Center frequency : 2GHzResonator width : 모두 같게 설계

Resonator lengthCase 1.

Case 2.

Substrate

Dielectric : 6.15H=1.27mmTanD : 0.0013

90o

(1 )2 2

FBW

Microwave & Millimeter-wave Lab. 30

Step 1 - 에 따른 resonator length 를 구한다 .

Step 2

-slope parameter 를 정해

even mode addmittance 값을 구할 수 있다 .(resonator width)

Step 3-J inverter 값과 even mode addmittance 값을 통해 odd mode addmittance 값을 구한다 .(resonator 들 간의 간격 s)

Layout of microstrip interdigital BPF

0

1

2

3

4

1

0.6292

0.9702

0.6292

1

g

g

g

g

g

01

12

23

34

0.1137

0.5208

0.5208

0.1137

J

J

J

J

Microwave & Millimeter-wave Lab.

Simulated Results(1)

31

(1 )2 2

FBW 90o

Center frequency

: 1.952GHz

1dB BW :210MHz(11%)

Microwave & Millimeter-wave Lab.

Simulated Results(2)

32

(1 )2 2

FBW

Center frequency

: 2.040GHz

1dB BW

:210MHz(11%)