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NASA/TM-200 1-21 0512 

Fixed-Frequency and Frequency-Agile 

(Au, HTS) Microstrip Bandstop Filters 

for L-Band Applications 

Eileen M. Saenz 

Ohio University, Athens, Ohio 

Guru Subramanyam 

University of Dayton, Dayton, Ohio 

Fred W. Van Keuls 

Ohio Aerospace Institute, Brook Park, Ohio 

Chonglin Chen 

University of Houston, Houston, Texas 

Felix A. Miranda 

Glenn Research Center, Cleveland, Ohio 

April 2001 

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S 2~ ~JZ 

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i , 

L 

NASA/TM-200 1-21 0512 

Fixed-Frequency and Frequency-Agile 

(Au, HTS) Microstrip Bandstop Filters 

for L-Band Applications 

Eileen M. Saenz 

Ohio University, Athens, Ohio 

Guru Subramanyam 

University of Dayton, Dayton, Ohio 

Fred W. Van Keuls 

Ohio Aerospace Institute, Brook Park, Ohio 

Chonglin Chen 

University of Houston, Houston, Texas 

Felix A. Miranda 

Glenn Research Center, Cleveland, Ohio 

Prepared for the 

Applied Superconductivity Conference 

sponsored by the Institute of Electrical and Electronics Engineers 

Virginia Beach, Virginia, September 17-22, 2000 

National Aeronautics and 

Space Administration 

Glenn Research Center 

April 2001 

I 

J 

l 
NASA Center for Aerospace Information 

7121 Standard Drive 

Hanover, MD 21076 

Price Code: A02 

This report contains preliminary 

findings, subject to revision as 

analysis proceeds. 

Available from 

National Technical Information Service 

5285 Port Royal Road 

Springfield, VA 22100 

Price Code: A02 

Available electronically at http:! I gltrs.grc.nasa.gov I GLTRS 
Fixed-Frequency and Frequency-Agile (Au, HTS) 

Microstrip Bandstop Filters for L-Band Applications 

Eileen M. Saenz 

Ohio University 

Athens, Ohio 45701 

Guru Subramanyam 

Department of Electrical and Computer Engineering 

University of Dayton 

Dayton, Ohio 45469 

Fred W. Van Keuls 

Ohio Aerospace Institute 

Brook Park, Ohio 44142 

Chonglin Cben 

Texas Center for Superconductivity 

University of Houston 

Houston, Texas 77204 

Felix A. Miranda 

ational Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstracl--ln this work, we report on the performance of a 

highly selective, compact 1.83><2.08 cm2 (-{).72xO.82 in.2) 

microstrip line bandstop filter of YBa2Cu307-0 (yBCO) on 

LaAIOJ (LAO) substrate. The filter is designed for a center 

frequency of 1.623 GHz for a bandwidth at 3 dB from reference 

baseline of less than S.lS MHz, and a bandstop r ejection of 

30 dB or better. The design and optimization of the fIlter was 

performed using Zeland 's IE3D circuit simulator. The optimized 

design was used to fabricate gold (Au) and High-Temperature 

Superconductor (HTS) versions of the filter. We have also studied 

an electronically tunable version of the same fi lter. Tunability of 

the bandstop characteristics is achieved by the integration of a 

thin film conductor (Au or HTS) and the non-linear dielectric 

ferroelectric SrTiOJ in a conductor/ferroelectric/dielectric 

modified microstrip configuration. The performance of these 

fIlters and comparison with the simulated data will be presented. 

Index Terms--L-band Bandstop Filter, High Temperature 

Superconducting (HTS) thin films, LaAIOJ su bstr ates, 

Ferroelectric thin films, tunable filters. 

1. INTRODUCTION 

Since the discovery of High-Temperature Superconductors (HTS) in 1986, many HTS-based planar 

microwave components such as resonators and filters have 

been demonstrated. The use of these HTS films in place of 

normal conductors (e.g., gold and copper) has reduced 

conductor losses and consequently insertion loss. In recent 

years, HTS thin-film based filters have been seriously 

considered for applications in cellular and PCS 

communications, as well as for some specific radioNASAlTM- 200 1-210512 1 

astronomy applications [1]- [4]. For example, HTS 

pre-select filters are currently being investigated in cellular 

base-stations for improved selectivity [2]. 

Also , HTS components have been integrated with 

ferroelectric thin films to develop low loss, frequency, and 

phase-agile microwave components [5]-[7]. For radioastronomy applications, the L-band 21-cm hydrogen line 

(- 1.428 GHz) is heavily utilized for deep space exploration. 

Because the spectrum is crowded at these frequencies, 

suppression of spurious signals is extremely necessary to 

avoid degradation of the probing frequency band of interest. 

Therefore, bandstop filters that offer small size, low loss, and 

ease of integration into the receiver subsystems while 

maintaining stringent specifications are highly desirable. 

In this paper, we report on the performance of a highly 

selective, compact 1.83x2.08 cm2 (-D.72xO.82 in?) bandstop 

microstrip line L-band filter fabricated using gold (Au) or 

YBa2Cu30 H (YBCO) thin films on LaAl03 (LAO). Two 

versions of the filter have been studied: a fixed-frequency 

design and a tunable design using a (Au, YBCO)I SrTi03/LAO 

superconducting/ferroelectric/dielectric multilayer structure. 

II. DESIGN, F ABRICATIO , AND T ESTING 

The bandstop filter was designed for symmetrical stopband rejection at the center frequency of 1.623 GHz, with the 

stop-band 3 dB bandwidth (from reference base line) of 

5.15 MHz or better. The stop-band rejection needed was 

~ 30 dB at the center frequency, and the passband insertion 
loss less than 1.0 dB below 1.614 GHz and above 1.634 GHz. 

To meet the above stringent specifications, the bandstop filter 

was designed from the traditional Chebychev low-pass 

prototype synthesis and transformation technique [8]. Due 

to the higher quality factor (Q) possible with 

superconductors, the minimwll number of poles required was 

detelmined to be 3. 

The bandstop filter was implemented using the microstrip 

edge-coupled resonators coupled to a through line. Desired 

band-rejection characteristics were obtained by optimizing 

the spacing between the through line and the resonators, and 

the length of the coup led sections, with the microstrip lines 

designed for 50 Q characteristic impedance on 508 /.l.m (20 

mil) LAO substrates. The final optimization of the filter was 

perfOlmed using Zeland's IE3D electromagnetic simulators 

[9]. The schematic of the optimized filter is shown in Fig. I. 

The performance of the fi lter as modeled using IE3D is 

shown in Fig. 2. The modeled data comply with all the 

aforementioned specifications for this filter. 

The YBCO/LaAl03 filter considered in this work consisted 

of a 508 /.l.m thick LaAl03 substrate with an area of 

1.83x2.08 cm2 coated with a 600 run thick YBCO HTS film. 

The filter was fabricated using standard photolithography and 

chemical etching techniques. A gold (Au) version of the filter 

was also fabricated by depositing a Au thin film (- 2.5 /.l.m 

thick) on a LaAIO) substrate using e-beam evaporation. For 

both versions of the filter a 2.5 /.l.m thick gold ground plane 

film was deposited on the side of the substrate opposite to the 

circuit. 

...... 0------- 1.735 --- ---< .. ~ 

- el- W - el- INPUT UT _e4 _e4 _ OUTP 

---. r--

~ e! IS 

I-Gl 

e2 , f6 

les 

W'" f.... I-.J~ I L..,: 

03:::::: 

~1 I 

I 

e6 I 

GIel II-ec 1--

~ 

e7 Ie vt IS 

- er 

fl 

e2 

e3 

f4 

es 

f6 

e7 

es 

w 

GI 

G2 

G3 

=0.25 

= 1.33 

= 1.20 

= 0.25 

= 0.35 

= 1.175 

= 1.1 8 

=0.30 

= 0.Q75 

= 0.0675 

= 0.0425 

= 0.0475 

Fig. l.- Schematic representation of the 

YBCOILaAI03 bandstop filter. All dimensions 

are in centimeters. 

ASAlTM-200 1-21 0512 2 

o 

iO -10 

:s 

J;' - 20 Ql 

D 

3 'c 

g' -30 

::;;; 

-40 

~ 1.W ~ 1.~ .~ ~ 1.~ 

Frequency (GHz) 

Fig. 2.-Plot of simulated data for the YBCOILAO 

filter. The filter has a center frequency of 

1.623 GHz, a bandstop rejection of 39 dB, and 

a bandwidth of 5 MHz at 3 dB. The simulation 

was performed using Zeland's IE3D electromagnetic simulator. 

Fig. 3.- The L-band AulLAO filter mounted 

on experinlental test fixture. The dimension 

scale is in centimeters. 

The microwave characterization of the filter was 

performed by mounting the filter with silver paint on a 

custom-made brass test fixture. The microwave signal was 

launched to the circuit via coaxial SMA connectors. A picture 

of the filter mounted in the test fixture is shown in Fig. 3. For 

cryogenic testing of the YBCO filter, the fixture was bolted 

to the cold finger of a helium gas, closed-cycle refrigerator 

modified with a vacuwn shroud with coaxial feedthroughs for 

input and output connections to an HP 8510C network 

analyzer. Measurements were taken under a vacuum of less 

than 1 mtOIT. 
f 

A tunable version of the filter was also studied. This 

version was optimized based on the multilayer structure 

shown in Fig. 4(a), for a ferroelectric thickness of 300 nrn, 

dielectric constant of ErSTO= 1500 and loss tangent of 

tanO=O.O I, which are conservative parameters for STO 

ferroelectric thin films [10]. Because of the ferroelectric, the 

optimized design for this version shown in Fig. 4(b) differs 

from that shown in Fig. I for the YBCO/LAO filter. The 

performance of the YBCO/STO/LAO filter obtained using 

IE3D is shown in Fig. 5. ote that the filter can be tuned by 

2 percent to both sides of the center frequency (1.623 GHz) 

without degrading the specifications outlined earlier in the 

paper. Beyond these range, although the filter is tunable up to 

5 percent with respect to the center frequency, the impedance 

mismatches introduced particularly for values of ErsTo>2000 

result in filter performance degradation. For the experimental 

characterization of this filter, dc voltage was applied through 

gold wire bonds directly connected to the poles of the filter 

while keeping the transmission line grounded. 

Au, YBa Cu ..z~ !-W_ I Au, 2.5 /lm , _,_ ...... YBCO 600 nm -- T ~_~"«_~:::~"o __ ~'" -+-

sr t:::====================:::=j _2._S, fl.!Tl _ 

Gold Ground - t- (a) 

.... ~t--;;-}-_----1.73 ----_--,;;-<~~ 

INPUT OUTPUT 

G} 1:6 e6 G} 6 e} =0.25 

e, = 1.33 

0- e; = 1.20 r-: e4 = 0.25 

es = 0.35 

e6 = 1.06 

e7 = 1.05 

ci)" e3 D:e. 

eg = 0.30 

w =0.0175 

;lL e7 G} = 0.135 

G, = 0.118 

G; = 0.0975 

(b) - eg- - es 

Fig. 4.-(a) Modified rnicrostrip line structure for 

ferroelectric-base L-band tunable filter. (b) 

Schematic representation of the YBCO/STOILAO 

bandstop filter. All dimensions are in (b) are in 

centimeters. 

NASAlTM-200 1-21 0512 3 

-10 

CD 

~£ 500da 

~E 'OOOde -'--~ £ ,-1250 dB ----, £,-1 500 dB 

................ £ ,-1750 ... _ ... .......-- Er_2oo0 

- E,_2500 ~-E .3000 

:g -15 . 

~N -20 

] '" -25 

c:: 

g' -30 

~ 

-35 

-40 

-45 L-~ __ ~_~ _____ ~_~_~~ 

1.52 1.54 1.56 1.58 1.60 1.62 1.64 1.66 1.68 1.70 

Frequency (G Hz) 

Fig. 5.-Plot of simulated data for the YBCO/ 

STOILAO filter for different values of STO's 

dielectric constant and for tano = 0.01. 

III. RESULTS 

The performance of the AulLAO version for the fixedfrequency L-band filter is shown in Fig. 6. The filter exhibits a 

rejection greater than 32 dB at 1.625 GHz (only 2 MHz away 

from specifications) and insertion losses of less than 

1 dB away from the rejection band (markers 2 and 3, Fig. 6). 

However, the 3 dB bandwidth of the filter is -47.5 MHz, 

which is 9 times wider than the specifications. ot 

withstanding, these are impressive results given the fact that 

the optimization process was done for a superconducting filter. 

This filter exhibited a loaded Q-650 at room temperature . 

Figure 7 shows the performance of the YBCO/LAO filter. 

Data were taken at 60 K, which was the temperature at which 

the filter exhibited the best performance, possibly due to 

nonoptimal film quality. In this case the filter exhibits a 3 dB 

band-width (markers 5 and 4) of - 5.5 MHz, which is very 

close to the specified bandwidth . However, it shows a 

rejection of only 24 dB at 1.6471 GHz and an insertion loss 

away from the bandstop frequency worse than those for the 

Au-filter counterpart, again possibly due to film quality 

limitations. This filter exhibited a loaded Q- 3660. The 

performance of this filter should be improved by using highquality, double-sided HTS films. 

Figure 8(a) shows the performance as a function of 

temperature of an Au/STO/LAO version of the filter shown in 

Fig. 4. ote that the reject frequency shifted by 47.5 MHz from 

298 to 45 K. Note that the better performance of the filter is 

realized at 45 K where the value of Er should be above 1000. 

Figure 8(b) shows the performance of this filter when applying 

300 Vdc to the filter's poles (Emax = 0.63 V/llm) , which 

resulted in a frequency shift of nearly 2 percent, consistent with 

simulations. Although the tunable filter did not live up to all 

the specifications of the simulated performance shown in Fig. 

5, its performance could be improved by using STO films with 

higher Er and larger dc bias. 
l 

$ 

:B. 

I/)N 

'0 

Q) 

"0 

3 

0 

-

-8 -

- -16 -

-

~) 1 

5 4 

Marker 1 

1.625 GHz 

-32.352 dB 

Marker 2 

1.525 GHz 

-0.7211 dB 

'§, -24 -

Marker 3 

1.725 GHz 

-0.8953 dB 

Marker 4 

1.65 GHz 

-3.822 dB 

~ '" 

N 

I/) 

-

-32 -

I I 

1.0 1.2 

I 

1.4 1.6 

Frequency (GHz) 

IL 

1.8 2.0 

Marker 5 

1.6025 GHz 

-3.8412 dB 

Fig. 6.- Experimental performance of the AulLAO 

L-band, 3-pole bandstop filter at room temperature. 

o~------~==----~~==~ 

Marker 1 

1.6471 GHz 

-24.037 dB 

Marker 2 

1.6525 GHz 

- 1.3579 dB 

'0 -12 Q) 

Marker 3 

1.6382 GHz 

-1 .5307 dB 

"0 

3 

'§, -18 Marker 4 

1.6491 GHz 

-4.3809 dB 

~ '" 

-24 

Marker 5 

1.620 1.628 1.636 1.644 1.652 1.660 1.6436 GHz 

Frequency (GHz) -4.5039 dB 

Fig. 7.- Experimental performance of the YBCOILAO 

L-band, 3-pole bandstop filter at 60 K. 

:g, 

co 0 ~""~-~~~~;:3~~~~ 

vl - 2 

Q) 

~ - 4 'c 

01 

'" 

~ -6 

(a) 

T(K) 

300 

150 

100 

77 

4 5 

Marker 1 

1.2 GHz -0.5611 dB 

Marker 2 

1.6725 GHz 

-4.2148 dB 

Marker 3 

1.9012 GHz 

-0.6484 dB 

Marker 5 

1.2 1.4 1.6 1.8 

Frequency (GHz) 

2 0 1.625 GHz . -6.9553 dB 

0F=r======= ~==~~ r- ~ \\ (.. ~ co \ f -2 r i I/)N r- de bias (V) i 

'0 --- 0 Q) - 4 r- , -g _. __ . __ .__ ..300 (Emax = :: 

. ,€ r 0.63 /~ ) !i 01 -6 r- ~ 

'" I 

~ -8~~( b) I -~~I I I ~~~ ~~~-~~ V3 ~ ~ = ~5K 1.2 1.4 1.6 1.8 2.0 

Frequency (GHz) 

Marker 1 

1.2 GHz 

-0.2537 dB 

Marker 2 

1.9 GHz 

-0.2923 dB 

Marker 3 

1.6437 GHz 

- 7.4063 dB 

Marker 5 

1.625 GHz 

- 6.9204 dB 

Fig 8.- Experimental performance of an Au/STOILAO 

L-Band, 3-pole bandstop filter as a function of 

temperature (a) and dc bias voltage (b). 

ASAlTM-200 1-21 0512 4 

IV. CONCLUSIONS 

In this work, a compact, highly selective (Au,YBCO)/LAO 

microstrip bandstop filter has been demonstrated for L-band 

applications. Both the HTS and gold versions of this filter 

show promising performance to comply with the stringent 

specifications upon fulther optimization. The HTS circuit had 

a 3 dB band rejection bandwidth of 5 MHz, compared to 

--47.5 MHz for the gold counterpmt, and a rejection of 24 dB, 

which could be improved using very high-quality, doublesided YBCO thin films. We have also shown the feasibility of 

a frequency tunable ver ion for this filter. Simulated data for 

a YBCO/STO/LAO filter predict at least 2 percent tunability 

while maintaining specifications. Attainment of this level of 

tunability has been demonstrated with an Au/STO/LAO 

version of the filter. Experimental work is currently 

wlderway for optimization of the YBCO/STO/LAO tunable 

bandstop filter. 

REFERENCES 

[I] J-S. Hong, M.1. Lancaster, R.B. Oreed, D. Jedamzik, 

lC. Mage, and H.J. Cbaloupka, "A High-Temperature 

Superconducting Duplexer for Cellular Base Station 

Applications," IEEE Transactions on Microwave 

Theory and Techniques, Vol. 48, pp. 1336- 1343, 

August 2000. 

[2] R Keenan, "Superconductors Improve Base-Station 

Sensitivity," Wireless Systems Design, pp. 32-37, July 1997. 

[3] R. Simon, "Expanding opportunities for HTS 

technology in the wireless industry," Superconductor 

and Cryoelectronics, pp. 9-14, Spring 2000. 

[4] M.W. Sinclair, W.E. Wilson, 0.0. Moorey, P.B. Sykes, 

R.1. Bolton, "A cryogenically cooled, thirteen beam, 

21 cm receiver front end," Electrochemical Society 

Proceedings, Vol. 97-2, pp. 336-346, 1997. 

[5] F.A. Miranda, C.H. Mueller, C.D . Cubbage , 

K.B. Bhasin, R.K. Singh and S. Harkness, 

"HTSlFerroelectrics Thin Films for Tunable Microwave 

Components, IEEE Trans. on Appl. Supercond., Vol. 5, 

pp. 3191-3194, June 1995. 

[6] R.A. Chakalov, Z.O. Ivanov, Yu. A. Boikov, P. Larsson, 

E. Carlsson, S. Oevorgian, and T. Claeson, "Fabrication 

and investigation of YBa2Cu30 7.ofBao.osSro.95 Ti03 thin 

film structures for voltage tunable devices," Physica C, 

vol. 308, pp. 279-288, 1998. 

[7] Yu. A. Boikov and Z.O. Ivanov, "YBa2Cu30 7..o/SrTi03 

epitaxial heterostructures on sapphire substrates," 

Journal of Alloys and Compounds, Vol. 251, pp. 

193- 195,1997 . 

[8] K. Chang, Handbook of Microwave and Optical 

Components, Vol. 1, John Wiley, NY, 1989. 

[9] Zeland 's Software, Inc. 29676, Mission Blvd. Fremont, 

CA 94539. 

[10] M.1. Dalberth, R.E. Stauber, lC. Price, C.T. Rogers, and 

D. Oalt, "Improved low frequency and microwave 

dielectric response in strontium titanate thin films grown 

by pulsed laser ablation," Appl. Phys. Lett., 

Vol. 72, pp. 507- 509 (1998). 

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REPORTTYPEANDDATESCOVERED 

April 2001 Technical Memorandum 

4. TITLE AND SUBTITLE 5 . FUNDING NUMBERS 

Fixed-Frequency and Frequency-Agile (Au, HTS) Microstrip 

Bandstop Filters for L-Band Applications 

6. AUTHOR(S ) 

VfU-297- 60-OO-00 

Eileen M. Saenz, Guru Subramanyanl, Fred W. Van Keuls, 

Chonglin Chen, and Felix A. Miranda 

7. P ERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION 

REPORT NUMBER 

National Aeronautics and Space Administration 

John H. Glenn Research Center at Lewis Field E-12464 

Cleveland, Ohio 44135-3191 

9 . SPONSORINGIMONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSORINGIMONITORING 

AGENCY REPORT NUMBER 

National Aeronautics and Space Administration 

Washington, DC 20546-0001 NASA TM- 2001-210512 

11. SUPPLEMENTARY NOTES 

Prepared for the Applied Superconductivity Conference sponsored by the Institute of Electrical and Electronics Engineers, Virginia Beach, Virginja, 

September 17-22, 2000. Eileen M. Saenz, Ohio University, Athens, Ohio 45701 (work funded by a NASA/OAl Summer Internshjp Program for Summer 

2000 at NASA Glenn Research Center); Guru Subramanyam, Department of Electrical and Computer Engineering, University of Dayton, Dayton, Ohjo 

45469-D226 (work funded by a NASA/OAl Summer Faculty Fellowship Program for Sununer 2000 at NASA Glenn Research Center); Fred W. Van 

Keuls, Ohio Aerospace Institute, Brook Park, Ohio 44135; Chonglin Chen, Texas Center for Superconductivity, University of Houston, Houston, Texas 

77204; and Felix A. Miranda, NASA Glenn Research Center. Responsible person, Felix A. Miranda, organization code 5640. 216-433-6589. 

12a. DISTRIBUTION/AVAILABILITY STATEMENT 12b. DISTRIBUTION CODE 

Unclassified -Unlimited 

Subject Categories: 33 and 76 Distribution: Nonstandard 

Available electronically at htt;p:llgltrs.grc.nasa.gov/GLTRS 

This publication is available from the NASA Center for AeroSpace Information, 301-621-0390. 

13. ABSTRACT (Max imum 200 w ords) 

In this work, we report on the performance of a highly selective, compact 1.S3x2.0S cm2 (-0.72xO. S2 in.2) micros trip line 

bandstop filter ofYB~Cu307-O (YBCO) on LaAl03 (LAO) substrate. The filter is designed for a center frequency of 

1.623 GHz for a bandwidth at 3 dB from reference baseline of less than 5.15 MHz, and a bandstop rejection of 30 dB or 

better. The design and optimization of the filter was performed using Zeland's IE3D circuit simulator. The optimized design 

was used to fabricate gold (Au) and High-Temperature Superconductor (HTS) versions of the filter. We have also studied 

an electronically tunable version of the same filter. Tunability of the bandstop characteristics is achieved by the integration 

of a thin film conductor (Au or HTS) and the nonlinear dielectric ferroelectric SrTi0 3 in a conductor/ferroelectric/dielectric 

modified microstrip configuration. The performance of these filters and comparison with the simulated data will be 

presented. 

14. SUBJECT TERMS 15. NUMBER OF PAGES 

L-band bandstop filter; High-temperature superconducting thin films; YBCO HTS films; 10 

LaAI03 substrates; Ferroelectric thin films; SrTi03; Tunable filters 

16. PRICE CODE 

A02 

17. SECURITY CLASSIFICATION 18. SECURITY CLASSIFICATION 19. SECURITY CLASSIFICATION 20. LIMITATION OF ABSTRACT 

OF REPORT OFTHIS PAGE OF ABSTRACT 

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