Comparative study of defected ground structures harmonics rejection ability in a compact hybrid coupler

International Journal of Electronics and Communication Engineering & Technology

(IJECET), ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 3, Issue 2, July-

September (2012), © IAEME

94

COMPARATIVE STUDY OF DEFECTED GROUND STRUCTURES

HARMONICS REJECTION ABILITY IN A COMPACT HYBRID

COUPLER

K. Annaram Professor, Department of Electronics and Communication Engineering,

Kamaraj College of Engineering and Technology,

Virudhunagar, 626001, Tamilnadu, India

[email protected]

ABSTRACT

In this paper defected ground structures (DGSs) featuring compact size and spurious free passband

in the context of 180° hybrid ring couplers are investigated. The design method for miniaturizing

the conventional 180° hybrid coupler is derived based on the fractal theory by replacing the

circumference of the hybrid ring coupler by Koch fractal curves. The advantage of this

miniaturization technique is no needs of any lumped elements, via hole or wire bond, but only

microstrip line. According to the fractal theory, almost the same frequency characteristics as those

of a conventional hybrid ring coupler are obtained. The experimental circuits were reduced to

32.85% in area with the good frequency characteristics. By etching various DGSs on the back-side

of compact hybrid coupler, the harmonics are rejected due to its slow-wave characteristics.

Furthermore, to demonstrate its practical viability, compact hybrid coupler with DGSs are designed,

simulated, fabricated and measured. This paper also compares the harmonic rejection ability of

various shapes of DGSs when these structures are etched in a compact hybrid coupler. Furthermore,

to show the improved harmonic rejection ability of the proposed hybrid couplers which are using

various shapes of DGSs are compared with the conventional hybrid ring coupler which one is not

having DGS by designing all the designs for the same design specifications. Finally the measured

and simulated results are compared for all the designs. The comparative study of these results

indicates that the harmonic rejection levels of the spurious resonance for the proposed couplers are

better than the conventional hybrid coupler at 3fo. It is also observed from the comparative study of

measured results the compact hybrid coupler with triangular DGS have better harmonic rejection

ability than other designs because it has minimum defected area than other DGSs. In addition, the

size of the RF-front-end becomes smaller by using the proposed couplers instead of using filters to

reject the unwanted harmonics. Hence this technique can be well suited to design any compact

microwave systems with low cost.

Keywords: Band-Gap Structure, Defected ground structure, Electromagnetic Interference, Fractals,

Frequency Selective Surfaces, Harmonic rejection, Hybrid coupler, Microstrip.

INTERNATIONAL JOURNAL OF ELECTRONICS AND

COMMUNICATION ENGINEERING & TECHNOLOGY (IJECET)

ISSN 0976 – 6464(Print) ISSN 0976 – 6472(Online)

Volume 3, Issue 2, July- September (2012), pp. 94-106

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International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN

0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 3, Issue 2, July-September (2012), © IAEME

95

1. INTRODUCTION Recently, there has been increasing interest in the use of Monolithic microwave wave integrated

circuits (MMICs) in microwave and millimeter-wave communication systems. In these systems,

cost effectiveness, low power consumption, and mass productions are much desired. Microwave

mixer is one of the important component in the above systems, which includes radio frequency

(RF), intermediate frequency (IF) and local oscillator (LO) stages. It is mainly used for the base-

band signal up-conversion and down-conversion of the received signal. In up-converting

microwave and millimeter wave communication systems, the most important parameter is LO-to-

RF isolation. Since the LO frequency is too close to even overlapping the RF frequency, the

rejection of the LO signal by a low pass filter is difficult or sometimes even impossible. Similarly

in down-converting microwave and millimeter wave communication systems, the super heterodyne

receiver is one of the important component which includes RF, IF (base-band) and LO stages. It has

the advantages of good stability and sensitivity, high gain and low noise. But it has the

disadvantages of high power consumption, complex circuitry and image frequency problems. In

this receiver band-reject filters are mostly used to reject the unwanted image frequencies. In both

the up-down converting mixers the radiation of harmonics generated by the nonlinear circuit has

been identified as a major problem for microwave and millimeter wave communication systems.

These harmonics could radiate freely into the surrounding environment at significant power levels

to interfere with continuous or harm the immediate inhabitants. Thus high performance low-pass

and band-reject filters are necessary to reject the unwanted harmonics. Finally the size of the

microwave and millimeter wave communication systems front-end becomes complex and bigger.

Such demand has a significant impact on a number of design issues both at component and system

levels, namely size-reduction, broadband operation, low power consumption, low noise, and

interference problems.

Hybrid ring coupler is one of the fundamental component in both up-down converting mixers

which are mainly used to mix the RF/LO and IF signals. The most commonly used hybrid couplers

are branch-line (90°) and rat-race (180°) hybrid ring couplers. Among the above two a 180° rat-race

hybrid ring coupler has wider bandwidth and high isolation than the 90° branch-line coupler [1] -

[3].To reduce the size of the microwave and millimeter wave system front-end, the 180° hybrid ring

coupler is an ideal component. It consists of a ring that is 1.5λg in circumference at the center

frequency with four input/output ports. In order to achieve good port-to-port isolation, the 180°

hybrid couplers are widely used in balanced mixers which are mostly used in microwave and

millimeter wave systems. Also it can be used in balanced amplifiers, beam forming array antennas

and frequency multipliers, due to their simplicity, wide bandwidth in power dividing distribution,

and a high isolation between its two output ports. Numerous publications have addressed the topic

of how to reduce the size of the hybrid-ring coupler. Several design techniques have been proposed

to enhance the harmonic rejection ability and to reduce its size. In earlier days the hybrid ring

coupler is often implemented only by lumped capacitors and inductors. However, the appropriate

capacitor and inductor in the microwave frequency band are difficult to fabricate [4]. Hirota has

proposed a method for reducing the hybrid ring microstrip coupler length by adding lumped

elements for compensation. The advantage of this method is that the reduced size is arbitrary, but

the main limitation is that the capacitance or inductance which is calculated by this method is

usually not equal to the primary value available in the market. Moreover, the extra lumped elements

will increase the cost and complexity of manufacturing. The next category of the miniaturizing

technique is realized only by planar transmission lines such that the drawback of the method

described above can be avoided. Kim presented a strategy without any lumped elements, but the

total circumference is fixed at 1.25 λg or 7λg/6 and cannot be reduced further. Some approaches use

a phase-inverter structure to replace the half wavelength microstrip between ports 2 and 4, which is

the longest transmission line on a typical ring coupler [5] - [7]. Chang reduced the size of the rat-

race coupler by only using microstrip lines. It is achieved by connecting multiple open stubs on the

inside of the hybrid ring coupler. The main advantage of this method is that no lumped element or

extra transition circuit is needed. Thus, the minimum size is restricted. The meander line technique



96

has been used for the coupled line structure size reduction. Moreover, due to technology limitations,

the size reduction is limited by coupling effects between long parallel sections. To reshape the

circular rat-race coupler the space filling curves are used to reduce the occupied surface area while

keeping the performance unchanged [8]-[11]. A new rectangular geometry for the conventional

hybrid ring coupler, based on computer aided design (CAD) tool is proposed. This geometry is not

only easy to automatize in a CAD tool, it gives a better concordance between best-match and best-

isolation frequencies than the standard design. Generally low pass filter is placed for harmonic

rejection in microwave front-end systems. Otherwise; the coupler suffers from the presence of

spurious pass-band at the harmonics of the operating frequency. But this solution increases the

complexity of the circuit, and the insertion loss [12] - [14]. It is very difficult to integrate the

conventional rat-race 180° hybrid ring coupler with other adjoining devices such as filters and

amplifiers; etc which has the perfectly matching geometry because of its multiple non-collinearly

aligned ports. Therefore it becomes necessary to use a vertically oriented feeding point. This

initiates the use of rectangular shape 180° hybrid ring coupler geometry which overcomes the

divergence problem caused by the circular or radial geometry in view of the port alignment even

though the size was not miniaturized [15].

Therefore, attempts are continually being made to realize a rectangular shape 180° hybrid ring

coupler with vertically oriented feeding point with minimum in size, and rejection of odd

harmonics in microwave and millimeter-wave communication systems. Recently in order to

enhance the harmonic rejection ability split ring resonators (SRR), complementary split ring

resonators (CSRR), photonic band-gap (PBG), defected ground structure (DGS) techniques are

used in monolithic microwave integrated circuits (MMICs) which are mainly used in microwave

and millimeter wave communication systems [16].

Hence this paper experimentally compares the harmonic rejection ability of the various DGSs by

etching these structures in a compact 180° hybrid ring coupler. The design concept, compactness

and harmonic response of the proposed designs are illustrated in section 2 and 3. The compact 180°

hybrid ring coupler with harmonic rejection is then discussed and verified by measurement results

from a fabricated circuit in section 4 and 5. Finally the concluding remark is given in section 6.

2. MINIATURIZATION

Fractals: In recent years the beauty of fractals has attracted wide interest among mathematicians

and microwave researchers. A number of techniques for generating fractal shapes were developed

and used to produce miniaturized circuits. The most commonly used fractal shapes are Koch curve,

Sierpinski gasket, Cantor dirt and Hilbert curves. These fractal shapes are self similar in structure, a

portion of the fractal geometry always has the same shape as that of entire structure. The

microwave circuit size can be reduced by increasing the perimeter of the shape as the iteration

increases, while still being confined in the same area due its space filling property. By using these

properties the most commonly used techniques are construction and iteration function which are

popularized by Mandelbrot, for generating fractal shapes. This paper focused only Koch fractals

which are very useful for designing MMICs because these are simpler than other fractals because of

its structure.

According to Mandelbrot’s generalization, the Koch construction consists of recursively

replacing edges of an arbitrary polygon (called the initiator) by an open polygon (the generator),

reduced and displaced so as to have the same end points as those of the interval being replaced. For

example in figure.1, ao is length of the initiator and Ko is the generator transformation. The ad-hoc

iterative function system (IFS) algorithm is used to construct the Koch curves as follows:

12 (1)o

ab

a=



97

The initiator length ao corresponds to the λg/4 length. In order to construct the generator, n

arbitrary transformations Ko, K1, K2…..Kn are applied successfully as follows

( ) ( )n+1K (2)o n p

a K a K n= = U

where p=1, 2, 3…..n;

( ) ( ) ( )n+1 1 2K .... (3)n n n n np n

a K a K a K= U U

After applying the above transformations the Koch curves generators physical parameters

should satisfy the equation 11 12 13a a a= = which are shown in Figure.1 because fractals are self

similar. In this paper, Koch fractal with iteration factor (b) 0.25 is chosen to design the proposed

miniaturized hybrid coupler. The geometrical properties of Koch fractals for various iterations are

shown in Figure.2 [17] [18].

Figure.1. Initiator and Generator

Figure.2

Koch fractal for various iterations

3. HARMONIC REJECTION

DGS: Many techniques were reported in order to suppress the unwanted harmonics such as PBG,

SRR, CSRR and DGSs in microwave and millimeter wave communication systems, which has

periodic array of defects. These periodic and non-periodic defects enhance the harmonic rejection

ability of a microwave and millimeter wave circuits. This harmonic rejection enhancement is vey

much needed in many microwave and millimeter wave circuits such as power amplifier, planar

antennas, power divider, hybrid couplers and filters. The research on PBG and EBG structures were

originally used for enhancing the harmonic rejection ability in the stop-band. However, these

structures are very difficult to model because of its too many design parameters also causes

radiation from the periodic defects in microwave and millimeter wave communication systems.

Hence DGSs have gained significant interest in microwave and millimeter wave circuits for

harmonic rejection.

Various shapes of DGSs are shown in Figure.3 which is very useful for designing proposed coupler.

The DGSs should be etched in the proposed compact hybrid coupler ground plane to reject the

harmonics. An etched defect in the ground plane disturbs the shield current distribution in the

ground plane. This disturbance can change the characteristics of a transmission line such as line

capacitance and inductance. It can be seen that employing the proposed DGSs increases the series

inductance to the microstrip line. This effective series inductance introduces the cut-off

characteristic at a certain frequency. In addition, the DGS has a self resonant frequency at the stop-

band. Due to this self resonant characteristic of the DGS section, the proposed coupler can provide



98

an attenuation pole in the upper stop-band. Due to this attenuation pole, the stop-band is wider than

that of conventional coupler. These attenuation poles can be explained by parallel capacitance with

the series inductance. This capacitance depends on the etched gap below the conductor line [19].

The physical parameter extraction method for various DGSs and its equivalent are described by LC

circuit as shown in Figure. 4.

a) Square-head DGS (b) Dumb-bell DGS

c) Arrow-head DGS (d) Fractal DGS

Figure 3. Schematic of DGSs

Figure.4. Equivalent Circuit of DGS

The capacitance Cp and inductance Lp are computed by

2 2

5= (4)c

p

o c

fC F

f fπ −

( )2

250= (5)

p

p o

L HC fπ

wherecf is the cut-off frequency of the band reject and

of is its pole frequency. At any frequency

of f< , the parallel circuit behaves as an inductor and its value is

( )2

6

1

p

eq

o

LL H

f

f

= −

It is observed that Leq is frequency dependent, which is inconvenient for the design of any

microwave circuits. However, variation in Leq is not very rapid for the frequency below fc. The

parameter extraction method is used to find the equivalent physical parameter of the DGSs. The



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shunt capacitance and series inductance will be implemented by employing the open stub and short

stub respectively. The length reduction of long DGS slot is achieved by creating the slot heads at

both ends DGSs. The following parameters are used to characterize the band-stop performance of

the DGSs

(1) Linear dimension of a slot

(2) Area/radius of a slot head

(3) Relative control of cut-off frequency fc and attenuation pole frequency fo by changing the

dimension of a slot

(4) Sharpness factor fc/fo

The slot-head area basically controls the inductance Lp whereas; the width (s) of connecting DGS

slot controls the capacitance. The separating distance (d) between the slot heads has influence on

both inductance and capacitance. Various dimensions involved in formation of various DGSs heads

have a different degree of control on fc and fo [20] - [22].

To design a circuit with any one of DGSs, the physical dimension of the DGS unit is obtained from

Lp and Cp values by full wave simulation and optimization is performed by using Agilent ADS. In

this paper, DGSs which are used for rejection of harmonics in a compact 180° hybrid coupler,

because

• The structures are simple to design and fabricate

• The stopband is very wider and deeper

The researchers have commented that for the equal area of slot head, any shape of slot can be

used. However, an equal area only ensures equal equivalent inductance and not the identical

response of the DGS circuit elements. The shape, size, and orientation of a slot can have an

influence on performance of the coupler and other neighboring circuits. In this paper, we

investigated the performance of a compact hybrid coupler by etching various shapes of DGSs on

the ground plane of a compact hybrid coupler.

4. DESIGN AND FABRICATION

By considering the above advantages of DGSs, these DGSs are used to construct a compact hybrid

coupler. In order to validate our idea, a rat-race hybrid coupler with various shapes of DGSs are

designed, fabricated and measured. The layout of the proposed hybrid coupler with DGSs which are

generated by using Agilent advanced design system software is illustrated in Figure.5. The center

frequency of the stopband (3fo) for the DGS is determined by the resonance frequency of each DGS

units. The proposed compact coupler with DGSs is designed at the operating frequency 2.44GHz

(fo) and the center frequency of stopband (3fo) at 7.32GHz. Both conventional and compact hybrid

couplers were designed and implemented on an inexpensive FR4 substrate (εr = 4.3, h = 1.6mm). A

prototype of a conventional and proposed hybrid couplers with DGSs were fabricated which are

shown in Figure.6. The optimized physical dimensions of the hybrid couplers and DGSs are

summarized in Table.1 and Table.2 to 5.



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a) With square-head DGSs (b) With dumb-bell DGS

(c) With arrow-head DGSs (b) With fractal DGSs

Figure .5. Layout of the compact hybrid couplers with DGSs

Table 1. Dimensions of the hybrid couplers

Table 2. Dimensions of square-head DGSs

Parameter Dimensions of DGS (mm)

Area of the square-head DGS (a)

Spacing length between square-head (d)

Spacing width between square-head (s)

a = 1.5

d = 4.3

s = 0.5

Table 3. Dimensions of dumb-bell DGS


Radius of dumb-bell DGS (r)

Spacing length between dumb-bell heads (d)

Spacing width between dumb-bell heads (s)

r = 1.5

d = 4.3

s = 0.5

Table 4. Dimensions of an arrow-head DGS

Type of coupler Width (mm) Length (mm) Area mm2

Feed Ring Feed Ring

Conventional hybrid coupler 2.909 1.497 16.899 17.417 68.35×59.19

Proposed hybrid couplers 2.909 1.497 7.5 9.826 30 ×44.293


0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 3, Issue 2, July

Area of an arrow-head DGS (b)

Spacing length between an arrow

Spacing width between an arrow-

Area of the fractal DGS (a)

Spacing length between fractal (

Spacing width between fractal

b) Back-side view with square- head DGSs dumb


6472(Online) Volume 3, Issue 2, July-September (2012), © IAEME

101


between an arrow-heads (d)

-heads (s)

b = 3

d = 4.3

s = 0.5

Table 5. Dimensions of fractal DGSs


Spacing length between fractal (d)

Spacing width between fractal edges (a3)

a = 3.1

d = 4.3

a3 = 0.5

(a) Front-side view

(c) Back-side view with umb-bell DGSs


September (2012), © IAEME

Dimensions of DGS (mm)

Dimensions of DGS (mm)



102

d) Back-side view with (c) Back-side view with arrow- head DGSs fractal DGSs

Figure 6. Fabricated prototypes of compact hybrid couplers with DGSs

5. RESULTS AND DISCUSSION

To compare the harmonic rejection ability of the DGSs, compact hybrid coupler with DGSs and the

conventional hybrid ring coupler were designed and fabricated which one is not having DGSs with

the same design specifications. Finally the measured and simulated results are compared for both

the structures. Before the prototypes fabrication, full wave EM simulation results have been

obtained with the aid of Agilent advanced design system software. The S-parameter measurements

were performed by using Agilent N5230 PNA series vector network analyzer.

Figure 7 presents the measured and simulated S11 parameters for a compact hybrid coupler with, as

well as without DGSs. A passband seen in the operating resonance frequency 2.44GHz and

stopband is centred around 7.32GHz. It is observed that the harmonic rejection in the stopband

extends up to 10GHz for the proposed couplers with DGSs over the conventional 180° hybrid ring

coupler. Also it can be clearly seen in Figure.7, the S11 happens to be much better for arrow-head

DGSs. For fo and 3fo, all the designs return loss is below -20dB and above -1.5dB respectively. It

can be noticed that the optimal harmonic rejection happens to be much better than which one is not

having DGSs. It is observed that the compact hybrid coupler with triangular DGS has slightly better

harmonic rejection ability than other designs. The experimental S13 comparison response is shown

in Figure.8. It was found to be isolation is better than -18dB for the whole band for all the designs

which are having DGS.

Figure 7. Return loss (S11)



103

Figure 8. Isolation loss (S13)

Figure 9. Coupling loss (S12)

Figure 10. Coupling loss (S14)

Figure 9 and 10 shows the coupling responses of S12 and S14 respectively. It is clearly observed that,

the proposed couplers have the coupling ratio of -3.91dB at 2.44GHz and below -10 dB up to

10GHz for all the designs except conventional coupler. It is clear from the results that the third

harmonics of the proposed coupler were perfectly suppressed. Therefore the harmonic rejection

ability of the proposed coupler have improved lot and better than the conventional hybrid coupler,

while maintaining 3dB power dividing performance. Figure 11 and 12 shows the simulated and

measured phase differences between its output ports of the compact hybrid couplers with DGSs and

conventional hybrid coupler. It is worth noting that the good out-of-phase (180º ± 6

º) characteristics

between the output ports are obtained at the desired frequency 2.44GHz (in the passband), while

rejecting harmonic elements effectively only for the designs which are having DGSs. Finally it is

observed that the proposed compact hybrid coupler with triangular DGS has better harmonic

rejection ability compared to other designs because it has only minimum defected area.



104

Figure 11. Phase response (S12)

Figure 12. Phase response (S14)

6. CONCLUSION

It can be concluded that a proposed couplers can be implemented in a few mm2 area without

any lumped element and will be easy to integrate with other devices. Usually low pass filter and

band reject filters were used in the microwave and millimeter wave front-end to reject the unwanted

harmonics. However, this approach increases the overall RF-front-end size and insertion loss. To

overcome the above limitations the proposed coupler can be used because it has the ability to mix

the RF and LO signal as well as rejects the unwanted harmonics. Hence there will be no need of

separate filters for harmonic rejection. Thus the size of the microwave and millimeter wave-front-

end size becomes smaller by using the proposed couplers. So it is well suited for designing any

compact and low cost MMICs circuits which can be used in microwave communication systems. It

is also observed that variation in the measured performance is mainly due to imprecise fabrication,

simulation mesh density and also by the junction discontinuities. It is believed that better quality

(harmonic rejection) results can be obtained by optimizing the DGS section and by using other

fractal curves. This task is left for further investigations, which can be used for further size

reduction and harmonic rejection.



105

ACKNOWLEDGEMENT The author wish to acknowledge the fabrication and testing support of the TIFAC-CORE in

Wireless Technologies, Thiagarajar Advanced Research Center, RF Systems Lab, Thiagarajar

College of Engineering, Madurai, Tamilnadu, India.

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K. Annaram was born in India in the year 1977. She completed B.E and

M.E degree in Electronics and communication engineering and

Communication systems from Thiagarajar College of Engineering, Madurai

Kamaraj University, India in 1999 and 2001 respectively. She received her

Ph.D degree from Anna University Chennai in 2010. She is now working as

a Professor in Kamaraj College of Engineering and Technology, Department

of electronics and communication engineering. She is a life member in ISTE, ATMS and

IEICE. Prior to coming to Kamaraj College of Engineering and Technology, she was a

research associate for the Thiagarajar advanced research center, Madurai and RF design

engineer for Quasar Innovations limited, Bangalore. Her research interest includes Meta

materials, defected ground structures and miniaturized RF and microwave circuits and

systems.

Documents

Comparative study of defected ground structures harmonics rejection ability in a compact hybrid coupler