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Performance of high-temperature superconducting band-pass filters with high selectivity for

base transceiver applications of digital cellular communication systems

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2002 Supercond. Sci. Technol. 15 1147

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INSTITUTE OFPHYSICSPUBLISHING SUPERCONDUCTORSCIENCE ANDTECHNOLOGY

Supercond. Sci. Technol.15(2002) 11471150 PII: S0953-2048(02)34767-5

Performance of high-temperaturesuperconducting band-pass filters withhigh selectivity for base transceiverapplications of digital cellularcommunication systems

J S Kwak1, J H Lee1, C O Kim1, J P Hong1, S K Han2 and K Char2

1 Department of Physics, Hanyang University, Seoul 133-791, Korea2 RFtron Inc., Seoul 151-505, Korea

Received 12 March 2002Published 10 June 2002Online atstacks.iop.org/SUST/15/1147

AbstractHighly selective high-temperature superconducting band-pass filters basedon spiral meander line structures have been developed for base transceiverstation applications of digital cellular communication systems. The filtercomprised 12-pole microstrip line resonators with a circuit size of0.5 17 41 mm3. The filter was designed to have a bandwidth of 25 MHzat a centre frequency of 834 MHz. Particularly, the physical size of eachresonator was chosen not only to reduce far-field radiation, but also to have

reasonable tunability in the filter. Device characteristics exhibited a lowinsertion loss of 0.4 dB with a 0.2 dB ripple and a return loss better than10 dB in the pass-band at 65 K. The out-of-band signals were attenuatedbetter than 60 dB at about 3.5 MHz from the lower band edge, and 3.8 MHzfrom the higher band edge.

1. Introduction

Both the digital cellular communication system (DCS) and the

mobile personal communication service (PCS) increasingly

require high-performance filters in sensitivity and selectivity

in order to support mass growth in multi-media services,

and to cover larger numbers of subscribers. As is known,

conventional filters have various limitations for use in the next

generation communication systems, such as large insertion

loss, reduced signal-to-noise ratio and low selectivity [1].

Therefore, there have been many studies focusing on

the development of high-Tc superconducting (HTS) filters

with narrow bandwidths and sharp skirts under superior

performance [210]. Among the great advantages of the HTS

filter, a highly selectivity is of great importance in the base

transceiver systems or communication systems in order to

eliminate interfering signals in an available radio-frequency

(RF) frequency spectrum.

In this paper, we report on the simulated and experimental

results of the fabricated HTS YBa2Cu3O7(YBCO)band-pass

filters. The purpose of the work was to develop a highly

selective HTS filter, with a very low pass-band loss at a central

frequency of 834 MHz. The filter was based on twelve half-

wavelength resonatorsin whichthe resonatorconsists of spiral-

type meander line structures. The experimental properties of

the filter were compared with those of the filter simulated usinga full wave electromagnetic (EM) simulator, IE3D.

2. HTS filter design

The design of the HTS band-pass filter with high selectivity

was based on half-wavelength meander line resonators

producing multiple coupled microstrip line structures.

Figure 1 shows the typical layout and characteristics of a

half-wavelength resonator for use in a filter. The resonator

was initially designed to have a resonant frequency at a

centre frequency of 834 MHz. As shown in figure 1, the

whole resonatorstructure consists of a large coupling patch and

an inductively spiral meander line. Using the EM simulator,

0953-2048/02/071147+04$30.00 2002 IOP Publishing Ltd Printed in the UK 1147

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J S Kwaket al

Figure 1. Layout and dimensions of the HTS microstrip resonatorusing a spiral meander line structure.

0.20

0.15

0.10

0.05

0.000.0 0.5 1.0 1.5

S/H

Couplingcoefficient(K)

Figure 2. Simulated coupling coefficientKof a pair of resonatorsversus the ratio of a substrate thickness and coupling gap spacing.

we maintained the former with as low an internal impedance

as possible for the reduction of peak currents at coupling

edges, and the latter was determined and optimized to make

the resonator compact and to minimize far-field radiation. In

addition, the whole characteristic impedance of the resonator

was set to be exactly 50 and it was matched to 50 input and

output feed lines for electrical contact to the sub miniaturized

adaptor (SMA). Therefore, the determined dimensions of the

resonator were 2.15 mm in lateral size and 10.9 mm in vertical

length, which was a compact size resonator.

In order to meet the requirements of filter speculationin the DCS, at first the coupling coefficient Kof a pair of

resonators was calculated as a function of spacing between two

resonators using the EM simulator. Two peaks specified by

two resonators were tuned to be of equal height, and the valley

was set to be at least 10 dB lower. The coupling coefficient Kof a pair of resonators was determined as

K =2|f2 f1|

f2+ f1(1)

wheref1andf2are two resonant frequencies [11].

Figure 2 plots the coupling coefficient K of a pair

of resonators with various coupling gap spacings as a

summary. The simulated curve indicates particularly

comparable coupling strengths that we have calculated using

(a)

Frequency (MHz)

(b)

S11(dB)

S21(dB)

S21

S11

Figure 3. (a) Layout and dimensions of the 12-pole highly selectiveband-pass filter. The size of filter was 17 41 mm2. (b) Simulatedfrequency response of the 12-pole HTS band-pass filter using afull-wave EM simulator.

normalized low-pass prototype values. This curve made

it possible to realize the filter response in our experiment.

Therefore, in all of the measurements, much attention was

paid to the K values with high accuracy since the use of

many standard Kwith different dimensions would result ina more accurate curve and thus a more accurate fitting would

be possible. With the aid of these coupling coefficients, the

HTS filter was designed to have a bandwidth with 25 MHz at

a centre frequency of 834 MHz for the DCS on the receiver

side.

Figure 3(a) presents the filter layout that consisted of 12-

pole meander line resonators as discussed above. The physical

size of the filter in a ladder form was only 17 41 mm2,

which could provide at least two filters in a 2 inch wafer. For

the best filter simulation, the phenomenological model of an

HTS thin film was at first used as a simulation factor of full

wave EM simulation [12]. Then the optimization procedure

was applied to improve the performance of the filter with thebenefits of the fitting curve presented in figure 2. As shown in

figure 3(b), the EM simulation data of the filter have a less than

0.1 dB insertion loss and a 25 MHz bandwidth at a resonant

frequency of 834 MHz. In addition, the frequency response of

the filter exhibited asymmetrical quasi-elliptic characteristics.

The asymmetrical frequency response might be caused by

unwanted coupling effects between each resonator or any

complex behaviour of EM waves in a microstrip line structure.

In the simulation, the LaAlO3substrate was approximated to

have a low dielectric loss (tan = 1 104) and a relative

dielectric constant of 23.5 at microwave frequency ranges.

Figure 4 shows a real photograph of the packaged 12-pole

HTS filter.

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Performance of HTS band-pass filters with high selectivity

Figure 4. Photograph of the rf packaged 12-pole HTS filter.

3. Fabrication and measurement

The HTS filter was fabricated using a double-sided YBCOthin film of a thickness of 600 nm. The dimensions of LaAlO 3substrate were 2 inches in diameter and 0.5 mm in thickness.The film used in this experiment had zero resistance at88.6 K. The filter was mounted in the aluminium test package.

All the packages were directly mounted on to the cold stage ofa closed-cycle cryogenic system for the filter measurement at

65 K. Transmission and reflection parameters of the filter were

measured using a HP 8510C vector network analyser with aninput power of +10 dB m. The calibrations were performed

at room temperature before measurements in the frequencyrange between 800900 MHz.

Figure 5(a) shows the measured transmission and

reflection coefficients of the filter. Although it was a highlycompact size, the 12-pole filter presented a very promising

performance without any tuning treatment or tuner at 65 K.The filter exhibited low insertion losses of about 0.4 dB and

return losses better than 10 dB in the pass-band with out-of-band rejection losses of about 70 dB. The out-of-band

signals were attenuated better than 60 dB about 3.5 MHz

from the lower band edge, and 3.8 MHz from the higher bandedge without any artificial transmission zeros. In addition,

it had a small pass-band ripple of about 0.2 dB. The filteralso indicated slightly asymmetric frequency responses in two

ports, as expected by the simulation. However, the cause

of the asymmetry in reflection loss response cannot yet beexplained.

For comparison, experimental and simulated results of

the 12-pole filter are shown in figure 5(b). As shown inthis figure, the centre frequency in the filter performance

was slightly shifted down about 1 MHz. The discrepancyin the centre frequency seemed critically to be due to non-

uniformity in the substrate thickness and variation in the filmthickness [4]. In addition, the over-etching effect on the filter

pattern mainly allowed the decrease in the bandwidth when thepatterned filter was investigated. Therefore, since a tolerancein the fabrication process could have a considerable impact

on the filter performance, resulting in a slightly differentcoupling effect between coupled resonators, the tuning process

is currently under process. However, up to now, although

no tuning procedure was carried out, the whole frequencyresponse of the filter did verify the simulated results very well,

except for a frequency shift of 1 MHz.

Frequency (MHz)

(a)

Frequency (MHz)

(b)

Measured

EM simulated

S

21(dB)

S

11(dB)

S21(d

B)

S21

S11

Figure 5. (a) Frequency response of the 12-pole filter fabricated bya 600 nm thick double-sided YBCO film on a LaAlO3substrate.The filter was measured using a HP8510C with an input power

of +10 dB m at 65 K. (b) Comparison of transmission coefficients ofthe HTS filter measured experimentally and simulated by the EMsimulator.

4. Conclusion

Highly selective HTS band-pass filters formed by 12-pole

spiral meander line structures have been developed for the

application of DCS base transceiver systems. With a small

size of 0.5 17 41 mm3, the filter exhibited a very low

insertion loss of 0.4 dB and a return loss better than 10 dB at a

centre frequency of 834 MHz. The out-of-band signals were

attenuated better than 60 dB about 3.5 MHz from the lower

band edge, and 3.8 MHz from the higher band edge at 65 K.

Acknowledgments

The work was supported by the Korea Research Foundation

Grant (KRF-99-D00211) and in part by the Ministry

of Information and Communication for the Advanced

Technology Project (2002-S-069).

References

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