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2
demonstrates lower loss than identically patterned circuits
using single sided films. A similar filter has been reported, but
made using postannealed films.’
MILLER, D. A. B., CHEMLA, D. S., DAMEN, T. C., GOSSARD, A. C., WIEG-
MA”, w., WOOD, T. H., and BURRUS, C . : ‘Electric field dependence
of optical absorption near the band gap of quantum-well structures’, Phys. Rev. E , 1985,32, pp. 1043-1060
3 SCHMITT-RINK, s., CHEMLA, D. s., and MILLER, D. A. B.: ‘Theory of
transient excitonic optical nonlinearities in semiconductor
quantum-well structures’, Phys. Rev. E, 1985,32, pp, 660-6609
4 NOJIMA, s., LARSEN, P. D., LARSEN, A.-D., MITOMI, o., WAKITA, K., and
NAGANUMA, M.: ‘Effects of photoexcited carriers on dynamic
response of quantum-well optical modulators’, Electron. Lett.,
1991,27, pp. 1387-1389
5 WOOD, T. H., PASTALAN, J. Z., BURRU$ C. A., JOHNSON, B. C., MILLER,
B. I., DEMICUEL, J., KOREN, U,, and YOUNG, M. G . : ‘Electric field
screening by photogenerated holes in multiple quantum wells
(MQWS): a new mechanism for absorption saturation’, Appl.
Phys. Lett., 1990,57, p. 1081
6 OBATA, K, VAMANISHI, M., YAMAOKA, Y., KAN, Y., HAYASHI, I., and
SUEMUNE, I.: ‘Observation of optical bistability by charge-induced
self-feedback in biased AlGaAs multiple quantum well structures’,
Appl. Phys. Lett., 1990,57, pp. 419-421
7 FOX, A.M., MILLER, D. A. B., LIVFSCU, C., CUNNINGHAM, J. E., HENRY, J.
E., and IAN, w . Y.: ‘Exciton saturation in electrically biased
quantum wells’, Appl. Phys. Lett., 1990,57, pp. 2315-2317
8 V A N ECK, T. E., NIKI, s., CHU, P., CHANG, w. s. c., WIEDER, H. H.,
MARDINGLY, A. J., ARON, K., and HENDERSON, G. A.: Presented at the
Quantum Wells for Optics and Optoelectronics Topical Meeting,
Salt Lake City, Utah, 1989, paper WA3-1
9 MILLER, D. A. B., CHEMLA, D. S., DAMEN, T. C., WOOD, T. H., BURRUS, C.
A., GOSSARD, A. C., and WIEGMANN, w.: ‘The quantum well selfelectrooptic effect devices: optoelectronic bistability and oscillation, and self-linearized modulation’, IEEE J. Quantum Electron.,
1985, QE-21, p. 1462
10 WAKITA, K., KAWAMURA, Y., NAKAO, M., and ASAHI, H . : ‘Long wavelength waveguide multiple quantum well optical modulators’,
IEEE J. Quantum Electron., 1987, QE-23, p. 2210
Experiment: A coupled resonator microstrip filter was
designed for O.ldB ripple, 5 % bandwidth and a centre frequency of 6.2GHz. T o fit the four-resonator design onto an
MgO substrate of readily available size a folded resonator
geometry was used. The filter pattern is shown in Fig. 1; the
substrate size is -23 x 19 x 0 4 m m . MgO was chosen as the
substrate because of its good dielectric properties (E, = 9.65,
tan 6 15
at 77K), and freedom from twinning. The
dielectric constant of MgO is comparable to that for materials
such as alumina and sapphire which are commonly used for
conventional planar microwave thin film circuits. Hence the
design equations and circuit models developed for conventional circuit design can still be used along with standard
connectors and transitions. This is an advantage compared to
using LaAIO, which has a high dielectric constant (E, = 23)
outside the range of several CAD models and which places
increased demands on patterning tolerances.
DEVELOPMENT OF PLANAR MICROWAVE
FILTERS FROM DOUBLE SIDED YBCO THIN
FILMS ON MgO SUBSTRATES
Fig. 1 Pattern for microstripfilter
Details of the film growth process have been described else~ h e r e . 4 Briefly,
,~
the films were grown by co-evaporation of
the metals from electron beam heated hearths in the presence
of atomic oxygen. The substrates were (100) oriented MgO
held a t a temperature of 670°C. The oxygen pressure at the
substrate was estimated as 10-4mbar, of which 5-10% was
atomic. This pressure was increased to -5 x lO-’mbar
during cooling after growth. The films were 0.35pm thick and
had T, s of 86 K. J , was measured by D C magnetisation to be
2.8 x lo6 A/cm’ at 77 K. The reduced T, of the films has since
been identified as due to insuflicient oxygen activity during
growth.
The films used in the filters were chosen to be slightly Cu
rich ( I10%). The films were not extensively studied but
similar films have been shown to be highly c-oriented (50.5%
a). with good rotational epitaxy, rather small (often
undetectable) fractions of 45” misoriented grains and have low
surface resistance.6
The filters were patterned using standard photolithography
and ion beam milling. Silver was evaporated at the ends of the
input lines to allow contact with the coaxial launchers, and
around the edges of the ground plane to make electrical
contact with the housing. After patterning and metallisation,
the films were annealed at 500°C in 1 bar oxygen to improve
the adhesion of the silver, reduce contact resistance and reoxygenate the milled edges of the superconducting lines.
Three filter circuits were made from the same mask on
MgO substrates. These were a single sided thin film gold
circuit with separate gold ground plane, a single sided YBCO
circuit with separate YBCO ground plane and the double
sided YBCO circuit. All the circuits were measured in the
same housing. Connections to the coaxial launchers (K
connectors) were made via stress relief contacts secured with
gold epoxy to the silver contact pads. The transmission loss of
the filters was measured in a closed cycle cryocooler. The
insertion loss of the double sided filter at 77 K is shown in Fig.
2. The circuit has not been optimised and some mistuning is
clearly evident. Table I shows the measured minimum pass-
Indexing terms: Microwave filters, Microwave devices and
components, Superconducting devices
Planar microwave filters patterned from thin film HTS on
low loss substrates can offer waveguide-like performance at a
fraction of the component mass and volume. First results are
presented for a microstrip filter at 6.2GHz fabricated from
YBCO thin films deposited onto both sides of an MgO substrate. The measured results are compared with identically
patterned filters assembled from gold and YBCO single sided
films.
Introduction: Planar circuits are in many ways ideal candidates for fabrication from high temperature superconductor
(HTS) materials. When fabricated from normal conductors
they have a relatively high loss, most of which is due to the
conductors, but when fabricated from HTS they can use epitaxial thin films and hence benefit from the lowest loss
materials.’ The Q of conventional planar circuits of around
500 can be increased with HTS materials to the loo00 or so
readily achieved with waveguide, and waveguide performance
achieved in a fraction of the size. This is important for high Q
devices such as narrowband filters which hitherto have been
unable to be realised in planar form. The most popular form
of planar circuit is microstrip, which consists of a patterned
conductor and a ground plane either side of a dielectric substrate. Up to now, superconducting thin films have only been
grown ‘in situ’ on a single side of the substrate so that microstrip has to be formed from a sandwich of two substrates. This
both increases the mechanical complexity of the component,
and introduces an unreproducible air gap between the substrate and the ground plane.
This Letter presents first results for a microstrip filter fabricated from in situ double-sided deposition of YBCO thin films
on an M g O substrate. Although mistuned, the circuit still
231 2
ELECTRONICS LETTERS 5th December 1991
~~
~
Vol. 27
No. 25
band loss and 3 d B bandwidth for the filters at 77 and 24K.
The insertion loss is reduced by replacing normal metal with
superconductor both in the circuit and in the ground plane,
55
LYONS, W. G., BONETTI, R. R., WILLIAMS, A. E., MANKIEWICH, P. M.,
O’MALLEY,
M. L., HAW,
I. M., ANDERSON,
A. c., WITHERS,
R. s.,
MEULENBERG, A., and HOWARD, R. E.: ‘High Tc superconductive
microwave filters’, IEEE Trans., 1991, MAG-27, (2), pp. 25372539
MAG% J. c.: ‘Theoretical and experimental study of the resonant
frequency of a cylindrical dielectric resonator’, IEEE Trans., 1983,
MTT-31, (lo),pp. 844-848
CHEW,
N. G., GOODYEAR,
s. w., EDWARDS,
I. A.,SATCHELL,
J. s., and
HUMPHREYS, R. G.: ‘Eflect of small changes in composition on the
electrical and structural properties of YBa,Cu,O, thin films’,
Appl. Phys. Lett., 1990.57, (19), pp. 2016-2021
HUMPHREYS, R. G., SATCHELL, J. S., CHEW, N. G., EDWARLX, 1. S.,
GOODYEAR, S. W., BLENKINSOP, S. E., WSSER, 0.D., and CULLIS, A. G . :
‘Physical vapour deposition techniques for the growth of
YBa,Cu,O, thin films’, Superconductor Sci. Technol., 1990, 3, (l),
pp. 38-52
HEIN, M., HENSON, S., MULLER, G., ORBACH, S., PIEL, H., STRUPP, M.,
CHEW,
N. G., EDWARDS,
I. A., GOODYEAR,
s. w., SATCHELL, I. s., and
HUMPHREYS,
R. c.: ‘Low microwave loss at high surface magnetic
fields in large epitaxial YBa,Cu,O, thin films’. Presented at
ICAM ’91, Strasbourg, 27th-31st May 1991
75
EZE
frequency.GHz
Fig. 2 Measured insertion loss of double-sided YBCOfilter at 77 K
and is further reduced when the ground plane is integrated
with the circuit. The losses in the Table include some extra
loss (0.13dB a t room temperature) associated with the connectors.
Table 1 MINIMUM PASSBAND LOSS MEASURED FOR
DEMONSTRATED FILTERS
COHERENT AND LIMITER-DISCRIMINATOR
DETECTION OF GMSK I N INTERFERENCE
LIMITED CONDITIONS
Minimum passband
insertion loss (dB)
3 0
77
Indexing terms: Detectors, Noise, Interjerence, Digital communication svstems
24
~
K
Gold/MgO filter
with gold ground plane
2.07
YBCO/MgO filter
with gold ground plane
-
YBCO/MgO filter
with YBCO ground plane
Double-sided
YBCO/MgO filter
~
-
K
1.15
(326)
~
0.80
0.76
(362)
0.67
0.37
(357)
0.22
0.27
(248)
0.13
Figures in brackets are 3dB bandwidths in MHz
Conclusions: First results for a microstrip filter using YBCO
films deposited by co-evaporation on both sides of a substrate
have been presented which showed a useful improvement over
identical single sided gold and YBCO circuits. A double sided
approach gives more control over the mechanical and electrical properties of the component significantly enhancing the
repeatability of assembly and tuning between circuits.
Although the insertion loss values for the single sided circuit
are already at a level acceptable for applications, the integral
ground plane is expected to be essential for meeting narrow
band filters for the high Q, narrow bandwidth designs envisaged for communications applications.
Acknowledgment: This was carried out under Contract
RP4-230 and supported by RSRE and GEC-Marconi
Research Centre.
S. J. HEDGES
27th September 1991
GEC-Marconi Research Centre
Chelmsford, Essex, United Kingdom
R. G. HUMPHREYS
N. G . CHEW
S. W . GOODYEAR
DRA Electronics Division (RSRE)
Malvern, Worcs., United Kingdom
References
1 RITTTR, I. C., NISENOFF, M., PRICE, G., and WOLF, S. A.: ‘High temperature superconductivity space experiment (HTSSE)’, IEEE
Trans., 1991, MAG27, (2), pp. 2533-2556
ELECTRONICS LETTERS 5th December 1991
V o l . 27
~~
The bit error rate performance of GMSK (BT product of0.5)
in a single interferer environment is considered. It is shown
that the superiority of coherent over LD detection is not as
marked as for the noise only case, and depends strongly on
receiver filtering. Comparative estimates of spectral efficiency
suggest that, for small cell cordless and mobile networks with
irregular layouts, the adoption of coherent receiver techniques may provide small system gains only.
K
Introduction: It is generally expected that coherent detection
of FSK-type schemes with modulation index of 0.5 (such as
GMSK) should provide an advantage of about 3 d B with
respect to noncoherent detection of the same schemes under
noise conditions. However, the choice (explicit or not) of a
particular detection method in the design of new cordless and
mobile radio networks has often been dictated by other
factors, such as the desired level of receiver complexity, the
requirement for channel equalisation and the ease of TDMA
burst-mode synchronisation. At the same time, the implications of this choice on system capacity tend to be addressed
through use of the known noise performance of each scheme.
The noise limited case can be a good indicator of required
cochannel protection ratios in planned systems such as a classical large cell cellular system. In such a system, the worst case
corresponds to that of multiple interferers in different cochanne1 cells, providing noise-like statistics for the perceived
impairment. However, as the size of cells diminishes and their
shape is more affected by local topography, the layout of base
stations becomes rather irregular so that the case of several
interferers with similar power levels will be less common. In
addition, the use of dynamic channel assignment techniques
(DCA), particularly in cordless systems, makes it even less
likely that the distribution of channels among cells will fit a
regular pattern. These considerations lead to the conclusion
that the radio environment perceived a t any point within such
systems will consist mainly of dominant users within each
channel, so that the limiting factor on capacity will be the
ability to reuse a channel with a strong single interferer. In
other words, the cochannel protection ratio of interest should
be that for the single interferer case.
This issue is addressed here through assessment of GMSK,
with relevance to its single interferer performance and how it
differs from the noise only case. In particular a relatively wide
BT product (0.5) for the premodulation Gaussian filter is
examined. This still allows simple polarity decisions to be
NO. 25
231 3
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