Miniaturization and Bandwidth Enhancement of a Loose coupler by DGS Priyansha Bhowmik1, Tamasi Moyra2 and Partha Kumar Deb3 1, 2, 3
department of Electronics and Communication Engineering, National Institute of Technology, Agartala- 799046, West Tripura, India 1
[email protected];
[email protected]; and
[email protected] Abstract— A 10 dB branch line coupler with a dumbbell shaped defected ground structure (DGS) is proposed in this paper. The 150 ohm transmission line of the proposed BLC above the substrate is of much small size than the conventional due to the DGS in the ground layer. The equivalent LC circuit of the proposed 150 ohm line is obtained. The proposed 150 ohm transmission line is now used in the 10 dB branch line coupler. The proposed 10 dB coupler is now of reduced size and from the simulation result it’s seen that flatness of the response increases, in turn increasing the bandwidth. Keywords— DGS, defected ground structure, 10 dB branch line coupler, branch line coupler, BLC.
I.
INTRODUCTION
Recent researches shows different configuration in the structure of microwave circuits such as photonic band gap (PBG), defected microstrip structures (DMS), defected ground structures (DGS) etc for variation in characteristic impedance [4][6]. But implementation of PBG is difficult and increases the size. DGS have numerous applications among them increasing the impedance by changing the electrical length is reported in this paper. The defected ground structures are actualized by etching the shape of defect in the metallic ground plane of the component. Since the ground is etched the current distribution is disturbed, the charge now instead of moving in straight direction move across the defect structure which in turn increases the inductance and the electrical length of the transmission line resulting in an increase in effective impedance of the transmission line. As the high impedance transmission lines are thin sometime fabrication become difficult hence the high impedance line can be replaced by a low impedance transmission line with wider width and a DGS section [3].
II.
EQUIVALENT CIRCUIT OF DGS
A quarter-wavelength 150 ohm transmission line is replaced by a 120 ohm transmission line and a DGS section in FR4 substrate of dielectric constant 4.4 and substrate height of 1.59 mm at 2.5 GHz. A dumbbell shaped DGS is etched on the ground plane as shown in Fig. 1. As the length of DGS increases the electrical length of the structure also increases so a 120 ohm transmission line of 81.74° is taken arbitrarily to obtain a quarter-wavelength line after etching the DGS. The LC parameters of the transmission line with the dumbbell shaped DGS are extracted from the simulated output of the structure by [6]: L=
1 imag ( Y11 ) × 2πf
at center frequency
Fig. 1.
Proposed transmission line with dumbbell shaped DGS
Fig. 2.
LC equivalent circuit of proposed structure at a=5.85 mm
In this paper a loose branch line coupler of 10 dB with DGS section is proposed. The 150 ohm transmission line of width 0.2 mm is replaced by a 120 ohm transmission line of width 0.4 mm by implementing a dumbbell shaped DGS. The proposed 10 dB branch line coupler is of much reduced size with increment in bandwidth. The comparison between the conventional and proposed 10 dB branch line coupler is given to proof the result obtained.
(1)
C=
1 imag ( Z 22 ) × 2πf
at center frequency
(2)
From the (1) and (2) LC parameters are established from TABLE I. Impedance for different values of a
a (in mm) 5.4 5.6 5.8 6 6.2 6.4
ZDGS (in ohm) 146.873 147.596 148.346 149.102 149.88 150.826
TABLE II.
Dimension of the proposed DGS with h=1.59 mm and ∈r=4.4
Dimension of proposed DGS a b c d
Length 6.3 4 0.5 2
which the impedance of the structure is calculated by [3]: L ZDGS = C
(3)
For FR4 substrates at 2.5 GHz a 120 ohm transmission line, with a constant gap in the DGS (c x d) and constant height of dumbbell b the effective impedance increases on increasing the length a as shown in Fig. 3 and can be expressed in terms of a as:
ZDGS = 0.5417a 3 − 8.997a 2 + 53.396a + 35.614
(4)
The equation (4) obtained is strictly for FR4 substrate of dielectric constant 4.4 and substrate height 1.59 mm. Now keeping all the dimension constant (as in Table II) and with dielectric of 4.4 and varying the substrate height the relation between impedance and substrate height (h) is shown in Fig. 4. Fig. 3.
Relation between effective impedance versus length a
ZDGS = −8.2703h 3 + 27.421h 2 − 12.537h + 133.91
(5)
. Now keeping all the dimension constant (as in Table II) and with substrate height of 1.59 mm and varying the substrate relative permittivity the relation between impedance and permittivity (ϵr) is shown in Fig. 5.
ZDGS = 0.355ε3r − 1.7177εr2 − 15.942ε r + 223.23 III.
(6)
DESIGN OF 10 DB BRANCH LINE COUPLER
A 10 dB branch line coupler consist of two series quarterwavelength transmission line of impedance, ZOS and two parallel quarter-wavelength transmission line of impedance, ZOP [2]. The design equations are: Fig. 4.
Relation between impedance and substrate height
ZOS = Zo S21 = Zo 1− S31 Z OP =
2
(7)
Z os S31
(8)
By using equation (7) and (8) ZOS=35.3 ohm and ZOP=150 ohm are calculated, by using the AutoCAD package the dimensions of the transmission line for the corresponding impedance are obtained. A 10 dB coupler without DGS is shown in Fig. 6.
Fig. 5. Relation between impedance and relative permittivity of material
In the conventional 10 dB branch line coupler the parallel quarter-wavelength transmission line of 0.2 mm width is sometime difficult to fabricate so the thin line is replaced by a 0.4 mm width transmission line of electrical length 81.74o and a dumbbell shaped DGS section on the ground plane such that
Fig. 6.
Conventional 10 dB branch line coupler
Fig. 7.
Fig. 8.
S-parameters of conventional 10 dB branch line coupler
Fig. 9.
S parameters of proposed 10 dB branch line coupler
Fig. 10.
Angle of S21 and S31 of conventional 10 dB coupler
Fig. 11.
Angle of S21 and S31 of proposed 10 dB coupler
Proposed 10 dB branch line coupler
electrical characteristic matches with the conventional 150 ohm line. A transmission line of electrical length less than 90° is taken such that by adding DGS the total electrical length is approximately 90o. The DGS pattern is designed by using above design procedure described above (1) – (4). The dumbbell shaped DGS should not be very close to the series transmission line and the DGS section of parallel transmission line, else undesirable coupling will be produced. IV.
RESULTS AND DISCUSSION
The proposed branch line coupler is designed in IE3D at center frequency of 2.5 GHz and the substrate used is FR4 of dielectric 4.4 and substrate height of 1.59 mm. By following the above design procedure, the proposed 10 dB branch line coupler gives an approximate result with respect to the conventional coupler. The simulated result of conventional TABLE III.
S parameters S11 S21 S31 S41 TABLE IV.
Angle S21 S31 Phase difference
S parameters of proposed and conventional
Proposed 10 dB branch line coupler -21.65 -0.5579 -9.894 -29.47
Conventional 10 dB branch line coupler -21.79 -0.6079 -9.845 -30.35
Phase difference of proposed and conventional
Proposed 10 dB branch line coupler 87.19 -2.497 89.687
Conventional 10 dB branch line coupler 87.82 -2.174 89.994
The bandwidth of conventional 10 dB coupler ranges from 1.8 to 3.05 GHz as observed from the Fig. 8 and Fig. 10. The bandwidth of proposed loosed coupler ranges from 1.85 to 3.35 GHz as observed in Fig. 9 and Fig. 11. Therefore a 20% bandwidth increment is noticed due to the addition of DGS in the loose coupler. The area occupied by the conventional 10 dB coupler is 0.86λg × 0.42λg and the area occupied by the proposed structure is 0.86λg × 0.39λg. As observed the proposed 10 dB coupler have a size reduction of 7% due to the addition of DGS. V. Fig. 12.
Phase difference between port 2 and port 3
and proposed 10 dB branch line coupler is given in Table III and IV. The simulated graphs of conventional 10 dB coupler are Fig. 8 and Fig. 10 and that of the proposed 10 dB coupler are Fig. 9 and Fig. 11.
REFERENCES
The bandwidth region is measured by following the criteria described below: a)
The phase difference between Port 2 and Port 3 is approximately 90°
b) The coupled output (S31) has maximum deviation of 1.5 dB from -10 dB c)
The reflection coefficient (S11) at port 1 is less than 20 dB and
d) The isolation at port 4 is less than -15 dB.
CONCLUSION
A new 10 dB coupler is presented with 7% reduced size and 20% more bandwidth from the conventional. All slots widths are more or equal to 0.4 mm so it can be fabricated easily in the substrate. The performance of the 10 dB coupler is effectual in the bandwidth region.
[1] [2] [3] [4] [5] [6]
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