Progress In Electromagnetics Research Symposium Proceedings, Xi’an, China, March 22–26, 2010
1795
A Study on Equivalent Circuit of Short Wavelength Microstrip Line
Employing PPGM on GaAs MMIC
Jang-Hyeon Jung, Bo-Ra Jung, Young-Bae Park, Se-Ho Kim, Jeong-Gab Ju, Suk-Youb Kang,
Dong-Woo Kang, Mi-Jung Kim, Byeong-Su Lim, Cheol-Hee Do, and Young Yun
Department of Radio Sciences and Engineering, Korea Maritime University, Korea
Abstract— In this work, equivalent circuit of short wavelength microstrip line employing periodically perforated ground metal (PPGM) were investigated using theoretical analysis. Equivalent circuits for the PPGM cell were extracted, and all lumped circuit parameters were expressed
by closed-form equation. For application to miniaturized on-chip passive components, Wilkinson
power divider and imped-ance transformer employing microstrip line with PPGM were fabricated on GaAs MMIC. The size of power divider and impedance transformer were 6 and 0.64%
of the conventional ones. Above results reveal that the transmission line employing PPGM is a
promising candidate for a development of matching and passive elements on MMIC.
1. INTRODUCTION
Recently, demands for highly integrated and miniaturized monolithic microwave integrated circuit
(MMICs) have increased in wireless communication systems market. However, the signal coupling
between adjacent lines have been an obstacle for chip size reduction, because a large spacing between
adjacent lines is required to suppress the signal coupling in higj frequency.
In this work, we propose the short wavelength microstrip line employing PPGM (Periodically
Perforated Ground Metal), and its equivalent circuit was thoroughly studied for application to
circuit design. Using a microstrip line employing PPGM on GaAs MMIC, a highly miniaturized
and broadband on-chip impedance transformer was developed for application to low impedance
matching in broadband.
2. STRUCTURE OF MICROSTRIP LINE EMPLOYING PPGM AND ITS
WAVELENGTH CHARACTERISTIC
Figure 1 shows the structure of the microstrip line employing PPGM, and its cross-sectional view according to Y -Y direction. As shown in Fig. 1, PPGM was inserted at the interface between SiN film
and GaAs substrate, and it was electrically connected to backside GND metal through the via-holes.
As is well known, conventional microstrip line without PPGM has only a periodical capacitance Ca
(Ca is shown in Fig. 1) per a unit length, while the microstrip line employing PPGM has additional
capacitance Cb as well as Ca due to PPGM. Therefore, as shown in Table 1, the microstrip line
with PPGM exhibits much shorter guided-wavelength (λg ) than conventional one, because λg is
inversely proportional to the periodical capacitance, in other words, λg = 1/[f · (LC)0.5 ]. The
characteristic impedance Z0 and guided-wavelength λg for the conventional microstrip line. Above
results indicate that highly miniaturized passive circuits can be realized by using the microstrip
line employing PPGM. We can deduce that the above structure shows high isolation characteristics
from equivalent circuit.
r
1
L
λg = √
Z0 =
(1)
C
f LC
r
L
1
Z0 =
(2)
λg = p
Ca + Cb
f L(Ca + Cb )
Equation (1) is characteristic impedance and wavelength of conventional microstrip line. L and
C correspond to the periodical inductance and capacitance of the LC equivalent circuit of the
conventional microstrip line. Equation (2) is characteristic impedance and wavelength of PPGM
structure. From (1) and (2), we can see that the microstrip line employing PPGM will exhibit
lower Z0 and λg than the conventional one. We can control the value of the additional capacitance
Cb by changing the spacing T of Fig. 1, which enables an adjustment of values for Z0 and λg .
PIERS Proceedings, Xi’an, China, March 22–26, 2010
1796
Figure 1: Structure of microstrip line employing PPGM.
Table 1: Measured wavelength (λg ) for the microstrip line employing PPGM and conventional one at 20 GHz.
Microstrip line employing PPGM
Conventional microstrip line
Figure 2: An equivalent circuit for a unit cell of the
microstrip line employing PPGM.
2.2 mm
5.6 mm
Figure 3: The equivalent circuit for microstrip line
employing PPGM.
3. EQUIVALENT CIRCUIT OF MICROSTRIP LINE EMPLOYING PPGM
Figure 2 shows the equivalent circuit of adjacent two lines, which corresponds to the equivalent
circuit of the N th unit section of the periodic structure surrounded by rectangular box in Fig. 1.
Cb corresponds to the capacitance between top line and PPGM, which is shown in Fig. 1, and it is
proportional to the cross area W · T of line of line and PPGM (As shown in Fig. 1, W and T are
the width of top lines and the periodic strips of PPGM, respectively). Rg and Lg are resistance and
inductance originating from the loss and current flow of the periodic strip of PPGM with width
T , respectively. Cg corresponds to the capacitance between PPGM and backside metal of GaAs
substrate. Lind is parasitic inductance originating from via-holes.
Figure 3 shows an equivalent circuit of the microstrip line employing PPGM. As shown in this
figure, a number of the equivalent circuits of unit section are connected to each other, and via-hole
was expressed as lumped inductor. The capacitance and inductance of the equivalent circuit are
given by,
"
#
µ ¶
µ ¶2
T
T
Lind = 0.0267 −
× 0.776 +
× 0.0533 nH
(3)
W
W
"
#
µ ¶
µ ¶2
T
T
Cb = 0.0933 +
× 6 × 10−4 −
× 1.33 × 10−6 pF
(4)
di
di
where, W , T and di are top line width, width of periodic metal strip and the thickness of SiN (See
Progress In Electromagnetics Research Symposium Proceedings, Xi’an, China, March 22–26, 2010
1797
Figure 4: Measured and calculated insertion loss S21 for microstrip line employing PPGM.
Fig. 1). The whole equivalent circuit is shown in Fig. 3. As shown in this figure, a number of
the equivalent circuits of unit section are connected to each other, and via-hole was expressed as
lumped inductor.
Figure 4 shows measured and calculated insertion loss S21 for microstrip line employing PPGM.
For the calculation result, equivalent circuit of Fig. 3 and above closed form equations were used.
As shown in this figure, we can observe a fairly good agreement between calculated and measured
results.
4. CONCLUSIONS
In this work, equivalent circuit of microstrip line employing PPGM were investigated using theoretical analysis. Above result indicates that the transmission lines employing PPGM is very useful
for application to compact signal lines of highly integrated MMIC requiring a high isolation characteristics between lines. According to results, a much better isolation characteristic was observed
from the adjacent microstrip lines employing PPGM compared with conventional microstrip lines,
and the frequency range for high isolation was easily controlled by changing the PPGM structure.
For simplification of design process, equivalent circuits for the PPGM cell were extracted, and all
lumped circuit parameters were expressed by closed-form equation. The calculated results showed
a comparatively good agreement with measured ones.
For application to miniaturized on-chip passive components, Wilkinson power divider and impedance
transformer employing microstrip line with PPGM were fabricated on GaAs MMIC. The size of
power divider and impedance transformer were 6 and 0.64% of the conventional ones. Above results
reveal that the transmission line employing PPGM is a promising candidate for a development of
matching and passive elements on MMIC.
ACKNOWLEDGMENT
This research was supported by the MKE(The Ministry of Knowledge Economy), Korea, under the
ITRC (Information Technology Research Center) support program supervised by the NIPA (National IT Industry Promotion Agency) (NIPA-2009-C1090-0903-0007). This work was financially
supported by the Ministry of Knowledge Economy (MKE) and the Korea Industrial Technology
Foundation (KOTEF) through the Human Resource Training Project for Strategic Technology.
This work was partly sponsored by KETI (Korea Electronics Technology Institute). This work was
also partly supported by ETRI SoC Industry Promotion Center, Human Resource Development
Project for IT SoC Architect.
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