Coupled Line Bandpass Filter

ByAshutosh Jain

07BEC053B.Tech. E.C.E

VIT University

Design of a Narrowband Coupled Line Band Pass Filter

Guided By Dr. Srinivasa RaoAssistant Professor (Sr.)

Objective

To design, optimise, fabricate and test a narrowband Coupled line band pass filter with operating frequency around 1.38 GHz and pass band in the range of 1.35 GHz to 1.45 GHz with bandwidth as narrow as possible.

Coupled Line

When two unshielded transmission lines are placed in close proximity to each other, a fraction of the power present on the main line is coupled to the secondary line. Coupled microstrip lines are used in a number of circuit functions. The principal application areas are directional couplers, filters, and delay lines.

Cross section of a coupled microstrip line

Coupled Microstrip Line

Coupled line structure supports two quasi-TEM modes, the even mode and the odd mode. For an even mode excitation, both microstrip lines have the same voltage potentials or carry the same sign charges, resulting in a magnetic wall at the symmetry plane. In the case where an odd mode is excited, both microstrip lines have the opposite voltage potentials or carry the opposite sign charges, so that symmetric plane is an electric wall. Coupled microstrip lines are characterized by the characteristic impedances as well as the effective dielectric constants for the two modes.

Quasi-TEM modes of a pair of coupled microstrip lines: (a) even mode; (b) odd mode

Band pass Filter

A two port coupled line section having a band pass response.

Design of Coupled Line Bandpass Filter

Zo Zo

J-90o

Equivalent Circuit of a coupled line section

• The admittance inverter is quarter wavelength long of characteristic impedance, 1/J.

• Narrow Bandpass filters can be made with cascaded coupled line sections.


General structure of parallel (edge)-coupled microstrip Bandpass filter.


• Firstly the order N of the filter is determined by Insertion loss method.

• Then the low pass prototype elements g0 .. gN+1 are used to find the admittance inverters impedance.

• Design equations for a Bandpass filter with N+1 coupled line sections are

𝑍 0 𝐽 𝑛=𝜋 ∆

2√𝑔𝑛−1𝑔𝑛For n=2,3…, N

𝑍 0 𝐽 𝑁+1=√ (𝜋∆

2𝑔𝑁𝑔𝑁+1)

Here

• f2 and f1 are the pass band edge frequencies and f0 is the center frequency.


• The even and odd mode characteristic impedances for each section are found using :

0e= Z0[1 + JZ0 + (JZ0)2] = Z0[1 - JZ0 + (JZ0)2]

• From the even and odd mode impedance the width(W), length (L) and spacing(S) are found for each section.

• The first step is to find the two single line shape ratios (W/H)se and (W/H)so corresponding to the impedance Z0e/2 and Z0o/2 respectively.

()(d-1)-( )

Where d=

Design of Coupled Line Bandpass Filter• W/H and S/H for the coupled lines are found by simultaneous

solution of the following equations :

for

for Where


Schematic of the designed filter with center frequency 1.38GHz, fractional bandwidth 5% and order 5.


Frequency response of the filter with Insertion Loss at fo -0.1843 dB, Bandwidth 188 MHz


Schematic of the designed filter with center frequency 1.38GHz, fractional bandwidth 0.5% and order 7.

EM SimulationLayout of the designed filter with center frequency 1.38GHz, fractional bandwidth 5% and order 5.

EM SimulationFrequency response of the filter with Insertion Loss at fo -0.6152 dB, Bandwidth 159MHz

EM SimulationFrequency response of the filter with

EM SimulationFrequency response of the filter with Insertion Loss at f0 -1.274dB, Bandwidth 111MHz

EM SimulationFrequency response of the filter with Insertion Loss at f0 -0.95232 dB ,Bandwidth 82 MHz.

EM SimulationFrequency response of the filter with

Spurious Passbands Suppression

Basic drawback of a parallel coupled line filter is the appearance of repeated pass-bands approximately at integer multiples of the centre frequency.

Spurious Passbands Suppression

We’ve used two techniques for suppression of spurious passbands:

1. Using Open Circuit Stubs

2. Defected Ground Structures (DGS)

Open Circuit Stubs

• The proposed filter configuration is composed of two attached open-circuit stubs at the open-end of each section of the conventional coupled-line filter.

Parallel Coupled Line Filter with Open-Circuit Stubs

Open Circuit Stubs

• If the short circuit is replaced by an open circuit stub with suitable length, then the spurious response can be suppressed.

• The initial lengths of upper stubs are set equal to a quarter wavelength (at double of centre frequency). The initial lengths of lower stubs are set equal to a quarter wavelength (at triple of centre frequency).

Open Circuit StubsSchematic of the designed filter with center frequency 1.38GHz, fractional bandwidth 1% and order 3 with open-circuit stubs.

Open Circuit StubsFrequency response of the filter with Insertion Loss at fo -1.061dB, Bandwidth 84 MHz

Open Circuit StubsSchematic of the designed filter with center frequency 1.38GHz, fractional bandwidth 1% and order 3 with open-circuit stubs and bends.

Open Circuit StubsFrequency response of the filter with Insertion Loss at fo -0.8269 dB, Bandwidth 46 MHz

Open Circuit StubsLayout of the designed filter with center frequency 1.38GHz, fractional bandwidth 1% and order 3 with bends for suppression of first two harmonics.

Open Circuit StubsFrequency response of the filter with Insertion Loss at fo -2.529 dB, Bandwidth 72 MHz

DGS

• Here the ground plane metal of a microstrip circuit is intentionally modified to enhance performance.

• The name for this technique simply means that a “defect” has been placed in the ground plane, which is typically considered to be an approximation of an infinite, perfectly-conducting current sink.

• The basic element of DGS is a resonant gap or slot in the ground metal, placed directly under a transmission line and aligned for efficient coupling to the line.

DGS

• The equivalent circuit for a DGS is a parallel-tuned circuit in series with the transmission line to which it is coupled.

• DGS structure for microstrip parallel coupled-line filters is proposed to suppress the second, third and fourth harmonics simultaneously

• The design procedure is very simple and doesn’t need a recalculation of the coupled line dimensions.

DGS

• Configuration of the proposed DGS on the ground plane of the microstrip line is shown

Schematic top view of a slanted dumb-bell shaped DGS unit.

(w= 3.09 mm, a=b=5 mm, g=0.5 mm)

DGS

• The configuration of the proposed parallel coupled-line filter with the five slanted dumb-bell shaped DGS sections is shown below.

• Except the length of the resonators, other design parameters of the conventional parallel coupled-line filter remain intact.

DGS Design I

• The lattice dimension a×b of the 1st and 5th DGS sections and the 2nd, 3rd and 4th DGS sections are similar and are assigned to be 5.5 mm × 5.5 mm and 6 mm × 6 mm respectively.

• The etched gap width is chosen to be 0.5 mm for both types of the DGS units. The length of the narrow gap is the same as the line-width of the corresponding open transmission line resonators.

• As the electrical length of the filter increases due to addition of DGS elements; we’ve reduced the length of each coupled line section by 5 mm.

DGS Design I

EM structure of filter configuration using DGS

Design I

DGS Design I

Frequency response of filter configuration using DGS design IInsertion Loss at f0 -2.012 dB

DGS Design II

• There is only a slight modification in this configuration, here the length of the narrow gap is increased and is equal to twice the width of open transmission resonator plus the gap.

• The lattice dimensions of the DGS sections and the etched gap width is the same as in Design I.

• Here the electrical length of the filter has further increased and we’ve reduced the length of each coupled line section by 8 mm.

DGS Design II

EM structure of filter configuration using DGS

Design II

DGS Design II

Frequency response of filter configuration using DGS design IIInsertion Loss at f0 -2.012 dB

Fabrication

Top view of the fabricated filter

Fabrication

Bottom view of the fabricated filter

Measurement Results

The fabricated filter is analyzed using vector network analyzer and measured S21 and S11 are shown below.

Conclusions

• It’s observed that in higher order filters the gap ratios obtained were high and this led to decoupling of resonators. Hence their EM Simulation results were unsatisfactory.

• The filter configuration using open stubs is able to suppress the first two spurious passbands but the filter configuration is dimensionally big and the insertion loss at center frequency is high.

• The two filter configurations using DGS elements are compact and smaller in size as compared to conventional 3rd order parallel coupled line filter.

Conclusions

The filter configuration using Design II is chosen for final implementation because

It is able to suppress the spurious passbands with higher insertion loss.

It is smaller than filter configuration using Design I.

References

1. R. Levy, and S. B. Cohn, “A History of Microwave Filter Research, Design and Development,” IEEE Transactions On Microwave Theory And Techniques, Vol. MTT-32, pp. 1,055-1,067, Sept. 1984.

2. S. B. Cohn, “Parallel-Coupled Transmission-Line Resonator Filters,” IRE Transactions On Microwave Theory And Techniques, Vol. MTT-6, No. 2, pp. 223-232, Apr. 1958.

3. G. L. Matthaei, “Interdigital Bandpass Filters,” IRE Transactions On Microwave Theory And Techniques, Vol. MTT-10, pp. 479-491, Nov. 1962.

4. R. J. Wenzel, “Exact Theory of Interdigital Bandpass Filters and Related Coupled Structures,” IEEE Transactions On Microwave Theory And Techniques, Vol. MTT-13, pp. 559-575, Sept. 1965.

5. G. L. Matthaei, “Comb-Line Bandpass Filters of Narrow or Moderate Bandwidth,” Microwave J., Vol. 6, pp. 82-91, Aug. 1963.

6. R. J. Wenzel, “Synthesis of Comb-Line and Capacitively Loaded Interdigital Bandpass Filters of Arbitrary Bandwidth,” IEEE Transactions On Microwave Theory And Techniques, Vol. MTT-19, pp. 678-686, Aug. 1971.

References

7. D. M. Pozar, Microwave Engineering, John Wiley, New York, 1998, 2nd edition.

8. J. S. Hong, and M. J. Lancaster, Microstrip filters for RF/microwave application, Wiley, New York, 2001.

9. A. Riddle, “High performance parallel coupled microstrip filters,” IEEE MTT-S Int. Microwave Symposium Dig., pp. 427–430, 1988.

10. J. Kuo, S. Chen, and M. Jiang, “Parallel coupled microstrip filters with over-coupled end stages for suppression of spurious responses,” IEEE Microwave Components Letters, Vol. 13, No. 10, pp. 440–442, October 2003.

11. I. J. Bahl, “Capacitively compensated high performance parallel coupled microstrip filters,” IEEE MTT-S Int. Microwave Symposium Dig., pp. 679–682, 1989.

12. S. M. Wang, C. H. Chi, M. Y. Hsieh, and C. Y. Chang, “Miniaturized spurious pass-band suppression microstrip filter using meandered parallel coupled lines,” IEEE Transactions On Microwave Theory And Techniques, Vol. 53, No. 2, pp. 747–753, 2005.

References

13. C. K. Liao, and C. Y. Chang, “Modified parallel-coupled filter with two independently controllable upper stopband transmission zeros,” IEEE Microwave Wireless Comp. Letters, Vol. 15, No. 12, pp. 841–843, 2005.

14. M. C. Velazquez-Ahumada, J. Martel, and F. Medina, “Parallel coupled microstrip filters with ground-plane aperture for spurious band suppression and enhanced coupling,” IEEE Transactions On Microwave Theory And Techniques, Vol. 52, No. 3, pp. 1082–1086, 2003.

15. J. T. Kuo, M. Jiang, and H. J. Chang, “Design of parallel-coupled microstrip filters with suppression of spurious resonances using substrate suspension,” IEEE Transactions On Microwave Theory And Techniques, Vol. 52, No. 1, pp. 83–89,2004.

16. T. Lopetegi, M. A. G. Laso, J. H. M. Bacaicoa, D. Benito, M. J. Garde, M. Sorolla, and M. Guglielmi, “New microstrip wiggly-line filters with spurious passband suppression,” IEEE Transactions On Microwave Theory And Techniques, Vol. 49, pp. 1593–1598, 2001.

17. J. T. Kuo, W. H. Hsu, and W. T. Huang, “Parallel coupled microstrip filters with suppression of harmonic response,” IEEE Microwave Wireless Comp. Letters, Vol. 12, pp. 383–385, 2002.

References

18. P. Cheong, and K. W. Tam, “Novel folded-end parallel-coupled-line microstrip filter with 2nd and 3rd harmonic responses suppression,” Mediterranean Microwave Symposium, p. 115, 2004.

19. J. C. Liu, B. H. Zeng, J. M. Chang, C. H. Chien, C. C. Chang, and D. C. Chang, “Modified maximally flat parallel-coupled lines for band-pass filter applications and miniaturizations,” Microwave Opt. And Techniques Letters, Vol. 50, pp. 902–906, 2008.

20. F. R. Yang, K. P. Ma, Y. Qian, and T. Itoh, “A Uniplanar compact photonic-bandgap (UC-PBG) structure and its applications for microwave circuits,” IEEE Transactions On Microwave Theory And Techniques, Vol. 47, pp. 1509–1514, 1999.

21. F. R. Yang, R. Coccioli, Y. Qian, and T. Itoh, “Analysis and application of coupled microstrips on periodically patterned ground plane,” IEEE MTT-S Int. Microwave Symposium Dig., pp. 1529–1531, 2000.

22. H. Sobol, “Application of Integrated Circuit Technology to Microwave Frequencies,” Proceedings of the IEEE, Vol. 59, pp. 1200-1211, 1971.

References

23. M. Kobayashi, “A dispersion formula satisfying recent requirements in microstrip CAD,” IEEE Transactions, MTT-36, Aug. 1988.

24. R. Garg, and I. J. Bahl, “Characteristics of coupled microstrip lines,” IEEE Transactions, MTT-27, pp. 700–705, July 1979. Corrections in IEEE Transactions, MTT-28, p. 272, March 1980.

25. S. Akhtarzad, T. R. Rowbotham, and P. B. Johns, “The Design of Coupled Microstrip Lines,” IEEE Transaction on Microwave Theory, Vol. MTT-23, No. 6, June 1975.

26. T. G. Bryant, and J. A. Weiss, “Parameters of microstrip transmission lines and of coupled pairs of microstrip lines,” IEEE Transactions On Microwave Theory And Techniques (1968 Symposium Issue), Vol. MTT-16, pp. 1021-1027, December 1968.

27. S. V. Judd, I. Whiteley, R. J. Clones, and D. C. Rickard, “An analytical method for calculating microstrip transmission line parameters,” IEEE Transactions On Microwave Theory And Techniques, Vol. MTT- 18, pp.78-87, Feb. 1970.

28. H. A. Wheeler, “Transmission-line properties of parallel strips separated by dielectric sheet,” IEEE Transactions On Microwave Theory And Techniques, Vol. MTT-13, pp. 172-185, March 1965.

References

29. W. M. Fathelbab, and M. B. Steer, “Parallel-coupled line filters with enhanced stop-band performances”, IEEE Transactions On Microwave Theory And Techniques, Vol. 53, No. 12, pp. 3774–3781, 2005.

30. J. R. Lee, J. H. Cho, and S. W. Yun, “New compact band-pass filter using l/4 resonators with open stub inverter”, IEEE Microwave Guided Waves Letters, Vol. 10, pp. 526–527, 2000.

31. M. Moradian, and H. Oraizi, “Optimum design of microstrip parallel coupled-line band-pass filters for multi-spurious pass-band suppression”, IET Microwave Antennas Propagation, Vol. 1, No. 2, pp. 488–495, 2007.

32. I. Chang, B. Lee, “Design of Defected Ground Structures for Harmonic Control of Active Microstrip Antennas,” IEEE AP-S International Symposium, Vol. 2, pp. 852- 855, 2002.

33. F. Karshenas, A. R. Mallahzadeh, and A. R. Rashed-Mohassel, “Size Reduction and Harmonic Suppression of Parallel Coupled-Line Bandpass Filters Using Defected Ground Structure”, 13th International Symposium on Antenna Technology and Applied Electromagnetics and the Canadian Radio Sciences Meeting, 2009.

References

34. E. G. Cristal, and S. Frankel, “Hairpin Line/Half-Wave Parallel-Coupled-Line Filters,” IEEE Transactions On Microwave Theory And Techniques, Vol. MTT- 20, pp. 719-728, Nov. 1972.

35. U. H. Gysel, “New Theory and Design for Hairpin-Line Filters,” IEEE Transactions On Microwave Theory And Techniques, Vol. MTT-22, pp. 523-531, May 1974.

36. G. L. Matthaei, L. Young, and E. M. T. Jones, Microwave Filters,Impedance Matching Networks and Coupling Structures, Norwood, MA: Artech House, 1980.

37. A. I. Zverev, Handbook of filter Synthesis, Publisher John Wiley & sons, 1967.

38. B. S. Kim, J. W. Lee, and M. S. Song, “Modified microstrip filters improving the suppression performance of harmonic signals,” IEEE MTT-S Int. Microwave Symposium Dig., pp. 539–542, 2003.

39. D. Ahn, J. Park, C. Kim, J. Kim, Y. Qian, and T. Itoh, “A design of the low-pass filter using the novel microstrip defected ground structure,” IEEE Transactions On Microwave Theory And Techniques, Vol. 49, pp. 86–93, 2001.

Coupled Line Bandpass Filter

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