Selected Papers

Third-order Intermodulation Measurements for Microwave Bandpass
Filters of Thin Film High-temperature Superconductors
Hisashi Sato† and Jose Kurian
Abstract
A major barrier to the application of high-temperature superconducting microwave filters is theirpower-handling capability. To clarify the key parameters for improving the power-handling capability ofrf (radio frequency) filters based on high-temperature superconductors with microstrip structures, we
synthesized bandpass filters with different layouts using several kinds of thin film high-temperature
superconductors and subjected them to third-order intermodulation measurements. The experimental
results indicate that increasing the film thickness and utilizing MBE-grown films of NdBa2Cu3O7 areeffective ways to obtain microstrip filters that can handle high power.

1. Introduction
For passive rf (radio frequency) filters, the use of
superconductors can improve filter performance andenable miniaturization [1]. Passive microwave filtersthat use thin films of high-temperature superconductors (HTS) are now being widely used in the receiversof wireless communication systems because of their
low insertion loss and high selectivity [1], [2]. Most
such filters are composed of many resonators withmicrostrip arrangements, which reduce filter size
while retaining superior performance. Currently,
however, HTS microstrip filters are not used in transmitters because of the power-handling limitations
imposed by nonlinear effects [3]. Several HTS filters
for high-power operation have been reported [4]-[6]but their configurations are very different from thosefor the microstrip filters successfully used in receiversystems [1], [2] and are not suitable for making highly integrated miniaturized filters with a few tens ofresonators.
For transmitter applications, the intermodulation
generated by the nonlinearity of the superconducting
filters should be suppressed to permit the use of highinput power. In particular, third-order intermodulation (IM3) is a serious problem because it produces† NTT Basic Research Laboratories
Atsugi-shi, 239-0847 Japan
E-mail: hisashi@will.brl.ntt.co.jp

Vol. 2 No. 6 June 2004

spurious signals within the passband of the filters. For
example, when two input signals with frequencies f1
and f2 are applied to a filter, we get only two output
signals with frequencies f1 and f2 if the filter has nononlinearity. These output signals are called fundamental signals. If the filter does have some nonlinearity, however, we get various output signals withfrequencies that differ from f1 and f2 in addition to thefundamental signals. The additional signals, which
are generated by intermodulation caused by the nonlinearity of the filter, include signals with frequenciesof 2f1 – f2 and 2f2 – f1 caused by IM3. The IM3 signals appear in the passband of the filter as spurious
signals if f1 and f2 are in the passband and if | f1 – f2 | issmall enough.
A convenient quantitative measure of nonlinearity
is the third-order intercept, IP3, which is defined as
the input power at which extrapolations of the fundamental and IM3 signal curves intersect [3]. High IP3
values indicate low nonlinearity and better powerhandling capability. Typical values of IP3 reported
for HTS microwave bandpass filters with the higher
selectivity are lower than 40 dBm at 70 K. For transmission filters in base stations for mobile telephoneservices, we need IP3 as high as 70 dBm. To expand
the usage of superconducting microstrip filters inmicrowave transmission components, it is essential
to clarify the key parameters that improve the IP3 of
HTS microstrip filters. In the present work, we took
two approaches.
37Selected Papers

The first approach was to investigate the effect ofresonator geometry on IP3. The geometry of the filter’s resonators is likely to affect the power-handlingcapability of the filters by varying the densities of the
rf currents in the resonators. We examined the effectof resonator geometry on IP3 by fabricating
microstrip bandpass HTS filters with different resonator line widths and film thicknesses and assessing
the IP3 of the filters via IM3 measurements.
The second approach was to investigate the effectof film quality on IP3. This approach was based on
the expectation that IP3 might be improved by using
thin HTS films with better properties than currently
available ones. Recently it has been reported thatlarge-area thin films of (RE)Ba2Cu3O7 with rareearth elements (REs) are being produced for
microwave applications by technique based on molecular-beam epitaxy (MBE) with metal-element beam
fluxes whose densities are strictly controlled by spectroscopic methods [7]. We fabricated sample filtersand performed IM3 measurements on the films
obtained by MBE growth and compared the resultswith values for commercially available films.
2. Experimental
2.1 IM3 measurements
IM3 measurements were carried out using two

input signals with frequencies of 2.000 and 2.010
GHz using the experimental setup shown in Fig. 1.
The two signals had equal amplitudes. The filter wasassembled in a gold-plated oxygen-free high-purity
copper package. The packaged filter is labeled as thedevice under test (DUT) in Fig. 1. The bandpass filter
BPF-1 cut the higher harmonics generated by thepower amplifiers. The center frequency and the 3-dB
bandwidth of BPF-1 were 2 GHz and 100 MHz,
respectively. The other bandpass filter, denoted BPF2, was placed to allow the spectrum analyzer to measure the IM3 signal at levels as low as –100 dBm
without saturation by the fundamental signal in themeasured range of the input power level. The sensitivity improvement achieved by BPF-2 in the measurement setup was crucial to the success of thisstudy. BPF-2 was composed of three tunable bandpass filters connected in series: each had a 3-dB bandwidth of 100 MHz. The center frequencies of thethree filters, between 1800 and 2000 GHz, were set to
optimize the sensitivity of the whole setup.
2.2 Filter fabrication
We designed 3-pole Chebyshev bandpass filtersthat use coupled hairpin resonators with line width w
and pairs of feed lines with characteristic impedanceof 50 Ω. A typical layout is shown in Fig. 2. The
50 Ω feedline

Synthesizer1: f1

Au electrode

Line width (w)Poweramplifier
Cryostat

Isolator

Isolator
Combiner

IsolatorPoweramplifierSynthesizer2: f2

DUT
Bandpass
filterBPF-1

Bandpass
filter
BPF-2

PCSpectrum
analyzer
Au electrode
Attenuator

Fig. 1. Schematic diagram of experimental setup for thirdorder intermodulation measurements of asuperconducting bandpass filter, labeled DUT(device under test).

38

50 Ω feedline

Fig. 2. Layout of a 3-pole microstrip bandpass filter
composed of resonators with line width w and feedlines with the characteristic impedance of 50 Ω;electrodes formed by Au evaporation.

NTT Technical Review

Selected Papers

10

T = 65 K

YBCO filtert = 0.67 µm
w = 1.0 mm

0–10|S12| (dB)design parameters of the filters were center frequency of 2 GHz, passband width of 100 MHz, and passband ripple of 0.2 dB. We designed filters with w of
0.5 and 1 mm by a conventional method [8] using acircuit simulator and electromagnetic analysis. The
filters were fabricated by a standard photolithographic process and Ar ion milling from double-sided films
of YBa2Cu3O7 (YBCO) and NdBa2Cu3O7 (NBCO)
on MgO substrates (35 mm × 35 mm); substrate
thickness h was 0.5 mm. The YBCO films were commercially obtained. The NBCO films were grown by
MBE [7]. On the ends of the filter feed lines, weformed ohmic electrodes with evaporated Au films(200 nm thick) as shown in Fig. 2. After the fabrication processes, w and film thickness t of the filters
were measured with a profile meter.

–20
(a)
–30
–40
–50
–60

1.6

1.8
2.0
2.2
Frequency (GHz)

2.4

2

1

0
|S12| (dB)

3.1 Filter characterization
The frequency dependencies of the magnitudes of
S12 and S11, the elements of the scattering matrix, theso-called S parameters, of an YBCO filter are plottedin Figs. 3(a) and (b) and (c), respectively. As shown
here, the transmission properties of the filters exhibited Chebyshev characteristics with insertion loss
under 0.1 dB, typical passband ripple of 0.5 dB, anda passband width between 110 and 120 MHz. These
values are consistent with the design parametersdescribed above. The reflection properties in Fig. 3(c)have three poles whose frequencies exhibit reasonable agreement with the results calculated using thecircuit simulator and electromagnetic analyzer. The
measured frequency responses verified the design
and fabrication procedures used in this study.

–1

(b)

–2

–3

–4
1.9

1.95

Vol. 2 No. 6 June 2004

2
2.05
Frequency (GHz)

2.1

10

|S11| (dB)0

3.2 IM3 measurements
Figure 4 shows a typical result for IM3 measurement. The amplitude of the input signal (denoted“power in”) was measured at the input port of DUT in
Fig. 1. The amplitude of the output signal (denoted“power out”) was obtained from the value measured
at the spectrum analyzer after correction of the insertion loss of the whole signal line between the output
port of DUT and input port of the analyzer. The inputpower dependences of the amplitude of the fundamental signal and the IM3 signal fit well to straightlines with slopes of 1 and 3, respectively, as expectedfrom a simple consideration [3]. To determine IP3,
we used slopes of 1 and 3 in extrapolating the fundamental and intermodulation signals, respectively [3],as seen in Fig. 4. The amplitude of the input power at

T = 65 K

YBCO filtert = 0.67 µm
w = 1.0 mm

3. Results and discussion

YBCO filtert = 0.67 µm
w = 1.0 mm

T = 65 K

–10
(c)
–20

–30

–40

1.6

1.8
2
2.2
Frequency (GHz)

2.4

Fig. 3. Frequency dependences of (a) |S12|, (b) |S12| in the
passband region and (c) |S11| measured at 65 K fora 3-pole microstrip bandpass filter fabricated on afilm of YBa2Cu3O7 (YBCO) with thickness t = 0.67
µm and line width w = 1.0 mm.

39

Selected Papers

120

T = 59 K

YBCO filtert = 0.67 µm
w = 1.0 mm

80

40Fundamental

IP3 (dBm)

Power out (dBm)

80

100
YBCO filtert = 0.80 µm
w = 0.5 mm

0SLOPE = 1
–40

60

40

SLOPE = 3
–80

IM3

20

IP3 =
67.32 dBm
040
Power in (dBm)80

Fig. 4. Input-power dependences of the amplitude of thefundamental signal (open circles) and the thirdorder intermodulation signal (IM3) (open squares)
measured at 59 K for a 3-pole microstrip bandpass
filter fabricated on a film of YBa2Cu3O7 (YBCO)
with thickness t = 0.80 µm and line width w = 0.5
mm. The solid straight lines are the fitting results
for the fundamental signal data and IM3 data withslopes of 1 and 3, respectively. The amplitude of
the input power at the crossing point of the twolines is 67.32 dBm, which represents the thirdorder intercept IP3 for the filter.

the crossing point of the two lines gives IP3 for thefilter.
A typical temperature dependence plot of IP3 for a
YBCO filter is shown in Fig. 5. As the temperature
decreased from the superconducting transition temperature to 70 K, IP3 increased rapidly. Below 70 K,
it was basically saturated. The highest value of IP3
observed was +67.32 dBm at 59 K, which is about 30
dB higher than typical values reported for microstrip
hairpin filters based on superconducting films [9].
Note that this comparison may not be completely
valid because of differences in the passband widths,
namely external Q values, and the numbers of poles.
For example, relatively high IP3 values have beenreported for several filters designed for high-power
handling purposes with small numbers of poles [10],[11].
Figure 6 plots the thickness dependences of IP3 for
YBCO filters with w = 0.5 and 1.0 mm by closed andopen squares, respectively. The measured IP3 for thefilters with w = 0.5 mm at 70 K increased from +53 to+65 dBm as t was increased from 0.62 to 0.80 µm.
The data for the filters with t around 0.6 µm indicate
40

0
50

60

70

80

90

100

Temperature (K)Fig. 5. Temperature dependence of the third-orderintercept IP3 for a 3-pole microstrip bandpass filter
fabricated on a film of YBa2Cu3O7 (YBCO) withthickness t = 0.67 µm and line width w = 1.0 mm.
70

IP3 (@70K) (dBm)

–120

60

w = 1.0 mm

w = 0.5 mm

50
–2.5

–2

–1.5

–1

–0.5

10 log (t /1 µm)

Fig. 6. Plot of measured values of IP3 at 70 K as afunction of film thickness t for 3-pole microstrip
bandpass filters fabricated on films of YBa2Cu3O7(YBCO) with line widths w = 0.5 and 1.0 mm. The
data for the filters with w = 0.5 and 1.0 mm aredenoted by closed and open squares, respectively.
The solid lines represent the calculated values for
w = 0.5 and 1.0 mm normalized by the measured
value for the filter with t = 0.62 µm and w = 0.5 mm.
The calculation is based on the theoretical work by
Dahm and Scalapino [12].

NTT Technical Review

Selected Papers

Vol. 2 No. 6 June 2004

70
NBCO (0.55 µm)
YBCO (0.63 µm)
65

w = 1.0 mm

60

IP3 (dBm)

that the filters with w = 1.0 mm yield IP3 about 4 dB
higher than that offered by the filters with w = 0.5
mm. These results suggest that the power-handlingcapability of superconducting filters can be improvedby increasing t and w.
For superconducting microstrip resonators, Dahm
and Scalapino theoretically deduced that IP3 increases as the line width and film thickness increase [12].
Based on their study, we calculated the thickness
dependences of IP3 values for w = 0.5 and 1.0 mm
normalized by the measured value for the filter with t= 0.62 µm and w = 0.5 mm. Following their theoretical study [12], we used a penetration depth λ = 0.22
µm for the calculation, which we considered to be an
appropriate value for YBCO films at around 77 K.
The calculation results are represented by the solidlines in Fig. 6. Here, the increase of about 4 dB in IP3
with the increase in w from 0.5 to 1.0 mm is generally consistent with the theoretical expectation. On theother hand, the increase of about 10 dB in IP3 with
the increase in t from 0.62 to 0.80 µm is much larger
than theoretically expected 1 dB increase. As shownin Fig. 6, we found that the measured IP3 at 70 K
increased from +53 to +65 dBm with an increase in t
from 0.62 to 0.80 µm, which gives an increase of only
1 dB in IP3 in the theoretical analysis. Our results
suggest that the power-handling capability of superconducting filters can be quite effectively improvedby increasing t. It may be noteworthy that an IP3value as large as 65 dBm at 77 K at 10 GHz has beenreported for an YBCO film with a thickness as large
as 3 µm [11]. At the present stage, the origin of theunexpectedly strong enhancement of IP3 with theincrease in t is not clear.
Figure 7 compares the temperature dependences of
IP3 for filters made from MBE-grown NBCO films
with t = 0.55 µm with those created using commercial
YBCO films with t = 0.63 µm. It should be noted thatIP3 for the NBCO filter was 2–5 dB higher than thatfor the YBCO filters as seen in Fig. 7 while t for theformer was smaller than that for the latter. This result
suggests that improving film quality is also effectivein improving the power capability of rf filters basedon thin HTS films. It is also noteworthy that the
improvement in IP3 increases with temperature. This
result is consistent with the fact that the superconducting transition temperature for NBCO is higher
than that for YBCO. The improvement in power-handling capability at higher temperatures may raise theoperating temperatures of HTS filters for both receiving and transmission, resulting in smaller cryocoolers
in the filter systems.

55

50

45

60

70
Temperature (K)80Fig. 7. Temperature dependences of the third-orderintercept IP3 for 3-pole microstrip bandpass filterswith line width w = 1.0 mm fabricated on a film of
NdBa2Cu3O7 (NBCO) grown by molecular-beam
epitaxy with thickness t = 0.55 µ m and acommercially obtained film of YBa2Cu3O7 (YBCO)
with t = 0.63 µm.

4. Conclusion
We performed third-order intermodulation measurements on microstrip bandpass filters with different layouts made from several kinds of thin film hightemperature superconductors. The improved sensitivity of the measurement setup described here achievedby selective reduction of the fundamental signals
allowed us to successfully measure the IM3 signals
for the filters. We found that IP3 for the filters was
effectively enhanced by increasing film thickness t.
For the YBCO filters, IP3 at 70 K increased from +53
to +65 dBm as t was increased from 0.62 to 0.80 µm.
Moreover, filters made from MBE-grown NBCO
films yielded IP3 values 2–5 dB higher than filters
made from commercially obtained YBCO films.
5. Acknowledgments
We acknowledge collaborations with Professor
Michio Naito at Tokyo University of Agriculture andTechnology and Dr. K. Cho at Wireless Laboratories,
NTT DoCoMo, Inc. We thank Kenjiro Nishikawa,
41

Selected Papers

Yoh Yamaguchi, and Dr. Tadao Nakagawa for their
help in microwave measurements at NTT Network
Innovation Laboratories (NIL), as well as Dr. Yoshio
Ebine at Wireless Laboratories, NTT DoCoMo, Inc.
for his helpful support and stimulating discussions.
We are grateful for the help in the experiments
offered by Takahide Ishibashi of Toyohashi University of Technology. We acknowledge the helpful advice
and encouragement of Dr. Masahiro Umehira at NTT
NIL.
References[1]

M. J. Lancaster, “Passive Microwave Device Applications of High
Temperature Superconductors,” Cambridge University Press, 1997.
[2] B. A. Willemsen, “HTS Filter Subsystems for Wireless Telecommunications”, IEEE Trans. Appl. Supercond., Vol. 11, No.1, pp.60-67,
2001.
[3] D. E. Oates, “Nonlinear behavior of superconducting devices,” in“Microwave Superconductivity”, eds. H. Weinstock and M. Nisenoff,
Kluwer Academic Publishers, Dordrecht, pp. 117-148, 2001.
[4] R. R. Mansour, “High temperature superconductor multiplexers forsatellite applications” in “Microwave Superconductivity,” eds. H.
Weinstock and M. Nisenoff, Kluwer Academic Publishers, Dordrecht, pp. 417-446, 2001.
[5] K. Setsune and A. Enokihara, “Elliptic-Disc Filters of High-Tc Superconducting Films for Power-Handling Capability Over 100 W,” IEEE
Trans. Microwave Theory Tech., Vol. 48, No. 7, pp. 1256-1264,
2000.
[6] M. Barra, A. Cassines, and A. Prigiobbo, “Superconducting filters
based on mixed resonators,” Supercond. Sci. Technol., Vol. 16, pp.
407-411, 2003.
[7] M. Naito, S. Karimoto, H. Yamamoto, H. Nakada, and K. Suzuki,“Production of Double-Sided Large-Area High-Tc Wafers by Molecular Beam Epitaxy,” IEEE Trans. Appl. Supercond., Vol. 11, No. 1,
pp. 3848-3851, 2001.
[8] G. Matthaei, L. Young, and E. M. T. Jones, “Microwave filters,
impedance-matching networks, and coupling structures,” Artech
House, Inc., Norwood, 1980.
[9] G. Tsuzuki, S. Ye, and S. Berkowitz, “Ultra-Selective 22-Pole 10Transmission Zero Superconducting Bandpass Filter Surpasses 50Pole Chebyshev Filter,” IEEE Trans. Microwave Theory Tech., Vol.
50, No. 12, pp. 2924-2929, 2002.
[10] G-C. Liang, D. Zhang, C-F. Shih, M. E. Johansson, S. W. Richard, D.
E. Oates, A. C. Anderson, P. Polakos, P. Mankiewich, E. Obaldia Ede, and R. E. Miller, “High-Power High-Temperature Superconducting Microstrip Filters For Cellular Base-Station Applications,” IEEE
Trans. Microwave Theory Tech., Vol. 43, No. 2, pp. 3020-3029,
1995.
[11] S. Miura, J-G. Wen , K. Suzuki, T. Morishita, T. Yoshitake, G. Fujii,
and S. Suzuki, “Microwave Properties of Liquid Phase Epitaxially
Grown YBa2Cu3Ox Films,” in “Advances in Superconductivity XI,”
Springer-Verlag Tokyo, pp. 1239-1242, 1998.
[12] T. Dahm and D. J. Scalapino, “Theory of intermodulation in a superconducting microstrip resonator,” J. Appl. Phys., Vol. 81, No. 4, pp.
2002-2009, 1997.

42

Hisashi Sato
Senior Research Scientist, Material Science
Research Department, NTT Basic Research Laboratories.
He received the B.E., M.E., and Dr. Eng.
degrees from the University of Tokyo, Tokyo in
1986, 1988, and 1991, respectively. In 1991, hejoined NTT Basic Research Laboratories, where
he has been engaged in studies of thin films ofhigh-temperature superconductors. He is currently interested in microwave applications of superconductors.

Jose Kurian
Senior Research Associate, Material Science
Research Department, NTT Basic Research Laboratories.
He received B.Sc, M.Sc, and Ph.D. degrees inphysics from the University of Kerala, Trivandrum, India in 1989, 1991, and 1997, respectively. In 1992, he began his research career as a
Junior Research Fellow at Regional ResearchLaboratory, Council of Scientific and Industrial
Research (CSIR), Trivandrum, India in the fieldof high-temperature superconductivity. From
1992–2000 he was with the Regional ResearchLaboratory, where he was involved in the development of new substrate materials for HTS andthe growth of superconductor thin/thick films onthese new substrates. From Nov. 2000 to Oct.
2001, he was with the Superconductivity
Research Laboratory, International Superconductivity Technology Center, Tokyo as STA Fellow. Since 2001, he has been a Research Associate with the Material Science Research Department of NTT Basic Research Laboratories,
where he is involved in the growth of high-quality HTS films by MBE for microwave applications. He was the recipient of the 1991 JuniorResearch Fellowship and Lectureship awarded
by the Council of Scientific and Industrial
Research, India and STA Fellowship awarded byJST, Japan. He is a member of Japanese Societyof Applied Physics.

NTT Technical Review