A 77-GHz CMOS On-Chip Bandpass Filter With Balanced and Unbalanced Outputs

Abstract

This letter presents the design and implementation of a 77-GHz millimeter-wave on-chip bandpass filter with balanced output that was fabricated using a TSMC 0.18-μm standard CMOS process. The performances of the single-ended and single-ended-to-differential bandpass filters are simulated and measured. The chip size of the designed filter is 0.55 × 0.7 mm². The simulated and measured results of the proposed filter are found to be in good agreement. The filter has a 3-dB bandwidth of about 14 GHz at a center frequency of 77 GHz. The measured insertion loss of the passband is about 6 dB and includes a 3-dB splitting loss; the return loss is less than 10 dB within the passband.

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IEEE ELECTRON DEVICE LETTERS, VOL. 31, NO. 11, NOVEMBER 20101205
A 77-GHz CMOS On-Chip Bandpass Filter
With Balanced and Unbalanced Outputs
Cheng-Ying Hsu, Student Member, IEEE, Chu-Yu Chen, Member, IEEE, and
Huey-Ru Chuang, Senior Member, IEEE
Abstract—This letter presents the design and implementation of
a 77-GHz millimeter-wave on-chip bandpass filter with balanced
output that was fabricated using a TSMC 0.18-μm standard
CMOS process. The performances of the single-ended and single-
ended-to-differentialbandpassfiltersaresimulatedandmeasured.
The chip size of the designed filter is 0.55 × 0.7 mm2. The simu-
lated and measured results of the proposed filter are found to be in
good agreement. The filter has a 3-dB bandwidth of about 14 GHz
at a center frequency of 77 GHz. The measured insertion loss of
the passband is about 6 dB and includes a 3-dB splitting loss; the
return loss is less than 10 dB within the passband.
Index Terms—Balanced and unbalanced, balun filter, band-
pass filter, complementary metal oxide semiconductor (CMOS),
millimeter-wave, on-chip, open loop resonator, 77 GHz.
I. INTRODUCTION
B
systems. In many applications, baluns and bandpass filters
are directly cascaded. Fig. 1(a) shows that a bandpass filter
cascaded with a balun can provide a differential drive output to
the subsequent stage. Fig. 1(b) shows the balun used at the feed
point of a balanced antenna when an unbalanced feed line is
used.
In recent years, microwave balun filters fabricated using PCB
technology have been studied intensively. A 2.5-GHz wideband
balanced filter using a cross-coupled resonator has been pro-
posed [1], and the performance of the filter with integrated
quasi-Yagi antennas was measured. A 2.45/5.55-GHz dual-
band balun filter using a multicoupled line section has also been
proposed [2]. In addition, some balanced filters fabricated using
LTCC technology have been presented [3], [4].
The complementary metal oxide semiconductor (CMOS)
technology has high integration capability and is suitable for
low-cost mass production. With the advancements in process
technology, CMOS-related on-chip passive components, such
as bandpass filters, couplers, baluns, and antennas, operating
at millimeter-wave frequencies have been reported [5]–[9].
ALUNS and bandpass filters are widely used in micro-
wave and millimeter-wave communication RF front-end
Manuscript received June 23, 2010; revised August 3, 2010; accepted
August9,2010.DateofpublicationSeptember27,2010;dateofcurrentversion
October 22, 2010. The review of this letter was arranged by Editor A. Z. Wang.
C.-Y. Hsu and H.-R. Chuang are with the Institute of Computer and Commu-
nication Engineering, Department of Electrical Engineering, National Cheng
Kung University, Tainan 70101, Taiwan (e-mail: k1006849@ms18.hinet.net;
chuang_hr@ee.ncku.edu.tw).
C.-Y. Chen is with the Department of Electrical Engineering, National Uni-
versity of Tainan, Tainan 700, Taiwan (e-mail: cychen57@mail.nutn.edu.tw).
Color versions of one or more of the figures in this letter are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LED.2010.2068536
Fig. 1.
(b) Type II.
Architecture of conventional RF front-end circuit. (a) Type I.
However, for fully integrated CMOS consideration, the compo-
nent size is still a problem. Recently, multifunction millimeter-
wave components have been developed which can combine
different functions into one device and further simplify the
complexity of the RF front-end circuits.This letter presents a
CMOS millimeter-wave on-chip bandpass filter with balanced
output. Such a device can reduce not only the size of the balun
but also the additional insertion loss.
II. UNBALANCED TO BALANCED FILTER DESIGN
A lossless transmission line model for the unbalanced-to-
balanced bandpass filter is shown in Fig. 2. θn and Z0 are
the electrical length and characteristic impedance of the trans-
mission line section, respectively. It is composed of two open
loop resonators with coupled sections and three feeding lines.
The coupled-line section with an open-circuited terminal is
equivalent to a J-inverter, and the length of the open loop
resonators is nearly half a wavelength.
For the phase analysis of the balanced port, the three-port
network can be simplified into a two-port network when the
50-Ω load is terminated at one of the three ports. Furthermore,
the transmission coefficient between the two unloaded ports can
be calculated from the total Y -parameter matrix. The layout
of the presented filter is shown in Fig. 3. To operate at a
millimeter-wave frequency range, preventing the currents from
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1206IEEE ELECTRON DEVICE LETTERS, VOL. 31, NO. 11, NOVEMBER 2010
Fig. 2.Transmission line model of proposed millimeter-wave balun filter.
Fig. 3.Schematics and cross-sectional view of proposed CMOS balun filter.
being injected into the substrate and reducing the transmission
loss are crucial in a CMOS-based device. The ground plane is
arranged as the bottom metal (M1),and the singleline is used as
the top metal (M6); this forms a microstrip structure. Based on
the aforementioned analysis, a CMOS on-chip unbalanced-to-
balanced bandpass filter is proposed. Compared with a typical
square open loop resonator, its size can be reduced by about
30% when operating at the same resonance frequency.
In the balanced mode, the 0◦and non-0◦feeding structures
are used in the output ports. When a 0◦feeding line is used [10],
two transmission zeros can be excited to provide a quasi-elliptic
function that rejects the adjacent channel interference. How-
ever, the transmission zeros cannot be excited near the passband
when non-0◦feeding is used. It limits the suppression level of
the out-band rejection when operated in the balanced mode.
The designed filter can also operate as a single-input and
single-output two-port bandpass filter when port 2 is opened.
The input impedance Zin looking into port 2 can be ex-
pressed as
Zin= jZ2
ltanθltanθc− ZlZc
Zltanθc+ Zctanθl
(1)
where Zland Zcare the characteristic impedances of the θland
θcsections, respectively. When the length of the feed section
θlis sufficiently short, Zinbecomes very large. Therefore, the
performance of the typical single-ended bandpass filter can be
evaluated.
Fig. 4. Chip micrograph (chip size = 0.55 × 0.7 mm2).
Fig. 5.
balun filter. (a) Magnitude response. (b) Phase response.
Simulation and measurement results of designed millimeter-wave
III. SIMULATION AND MEASUREMENT RESULTS
The proposed filter implemented on a 0.18-μm CMOS multi-
layered structure is presented. Fig. 4 shows the chip micrograph
of the implemented CMOS filter. When the dummy metal is
included, the chip has a dimension of 0.55 × 0.7 mm2on a sub-
strate with a thickness of about 500 μm. The parameters of the
filterarel1= 127μm,l2= 137μm,l3= 192μm,l4= 62μm,

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HSU et al.: A 77-GHZ CMOS ON-CHIP BANDPASS FILTER 1207
Fig. 6.Simulation and measurement results of single-ended bandpass filter.
l5= 192 μm, w1= 15 μm, w2= 25 μm, w3= 15 μm, and the
coupling spacing s = 2 μm. The 110-GHz on-wafer measure-
ment system was used. However, it can only measure the two-
port S-parameters arranged in a face-to-face layout. Therefore,
only S21and S11were measured and plotted. The simulated
and measured results are shown in Fig. 5. The magnitude of
S21at the center frequency is about −6 dB, including a 3-dB
splitting loss, and the magnitude of S11is better than −10 dB.
The simulation results for the amplitude and phase differences
of the balance port between 75 and 85 GHz are 1.4 dB and
within 180◦± 4◦, respectively. The performance of the ampli-
tude balance cannot be perfectly matched since the structure
is not symmetric. The simulated and measured results of the
magnitude and phase of S11and S21are all in good agreement.
When port 2 was opened, the two-port bandpass filter was
constructed. Fig. 6 shows the simulated and measured results.
As observed in this figure, the filter can generate two transmis-
sion zeros near the side of the intended passband within 56–
98 GHz. The fractional bandwidth is 18%. The insertion loss is
about 3.5 dB, and the return loss is better than 10 dB within the
passband. The insertion loss variation of the designed bandpass
filter ranged from 72 to 80 GHz is less than 1.5 dB. The out-
band rejections are greater than 20 dB at 0–63 GHz.
IV. CONCLUSION
A 77-GHz CMOS on-chip bandpass filter with balanced
and unbalanced output is designed and fabricated. Dummy
metals and parasitic effects are considered and included in the
design. The full-wave EM solver, IE3D, is used for the design
simulation. Due to the limitation of the on-wafer arrangement
of the used 110-GHz measurement system, only the two-port
S-parameters (S21and S11) arranged in a face-to-face layout
can be measured. The measured insertion loss at the passband
of the single-ended bandpass filter is about 3.5 dB, and the
return loss is better than 10 dB. The simulated amplitude and
phase differences of the balance port between 75 and 85 GHz
are 1.4 dB and within 180◦± 4◦, respectively. The simulated
and measured results of the magnitude and phase of S11and
S21are all in good agreement. The designed millimeter-wave
on-chip bandpass filter with balanced and unbalanced output
configuration is useful for the integrated design of a 77-GHz
CMOS single-chip RF transceiver.
ACKNOWLEDGMENT
The authors would like to thank the Chip Implementation
Center, National Science Council, Taiwan, for supporting the
TSMC CMOS process.
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