693
Ultra-Selective Constant-Bandwidth Electromechanically Tunable
HTS Filters
Genichi Tsuzuki, Matthew Hernandez, Eric M. Prophet, Silverio Jimenez, and Balam A. Willemsen
Superconductor Technologies, Santa Barbara, CA, 931 1 1, USA
Abstract - We report an ultra narrow band 3-pole tunable
filter. Thning from 770 MHz up to 930 MHz, which is about 20%,
was demonstrated. Insertion loss was 0.7 dB at 770 MHz and 1.6
dB at 930 MHz. Coupling bandwidth was kept almost constant
over the entire range. A novel filter configuration and tuning
algorithm is proposed and demonstrated.
Index Terms - Thning, Thnable Filter, HTS, Superconductor,
Switch, Cryogenic Electronics and Nanopositioning
I. INTRODUCTION
High Q narrow bandwidth tunable filters are desirable for
use in many applications, and many approaches have been
evaluated [1-3]. For certain signal collection scenarios it is
desirable to eliminate (or reject) nearby signals in order to
maintain the integrity of the receiver system. Because these
unwanted signals can be close in frequency to the desired
signal, it becomes necessary to create higher order, narrow
bandwidth filters in order to achieve the desired rejection. If
the desired receive signal frequency changes over time, then a
tunable filter is needed to track the signal. The filter must be
able to change its center frequency very quickly while
maintaining a good filter shape so as not to diminish the
receiver properties. The filter must tune over a broad range of
frequency spectrum to maximize the usefulness of the signal
collection. Finally, in order to maintain optimum receiver
sensitivity, it is necessary that the filter have a very high
unloaded-Q to minimize the impact of insertion loss on noise
allowing for greater access to the fields as required for tuning.
However, this also means that care must be taken when the
SISO resonators are aligned to compose a filter, since the
changes in the extended fields while tuning will affect the
coupling between resonators (i.e. filter bandwidth). The
resonator was tuned by changing the height between the
resonator and an HTS coated substrate placed above it. The
HTS tuner was sized to cover the entire resonator. Figure 1
shows the tuning range of the resonator as a function of the
tuner height calculated using a planar 3D electromagnetic field
solver, "momentum" provided by Agilent Technologies. Since
the fields extend broadly over its dielectric substrate, a wide
tuning range can be realized with little degradation of
unloaded-Q. Figure 2 shows the unloaded Q-factor measured
in the package for the demonstrated 3-pole filter. As the
resonator Q degrades, so does the filter insertion loss.
Qu>200,000 is required to maintain the insertion loss of the
filter to less than 2 dB, and we are able to maintain this level
for -20% tuning, up to 920MHz.
1090
1050
1010
970
2
figure.
This paper focuses on the design of the 3-pole tunable
filter we have developed. This includes; (1) high Q-factor
resonator over wide tuning range, (2) coupling control that
may affect on filter performance such as return loss
degradation and bandwidth change and (3) a tuning algorithm
that allows quick and simple tuning. The goal of the tunable
filter here is to achieve -30% tuning range around 1 GHz with
100 kHz filter bandwidth (-0.01% fractional bandwidth).
II. HIGH-Q RESONATORS
In this work, we use a microstrip full-wavelength spiral-inspiral-out (SISO) resonator which has demonstrated record
unloaded Qs at 77K (Qu>400,000). This design is very well
suited for broad tuning ranges because the electromagnetic
fields extend further than most other resonator designs,
0-7803-9542-5/06/$20.00 C2006 IEEE
930
°890
X
850
810
770
0
Fig. 1
500
1000
1500
2000
h (microns)
2500
3000
3500
Resonant frequency vs. tuning height over SISO resonator.
III. CONTROL OF FILTER COUPLINGS
Resonators were housed in individual compartments in order
to avoid the unwanted variations in filtering shape arising from
the tuner movement. Adjacent resonators are coupled by
means of additional transmission lines on a small substrate.
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transmission lines need not be HTS. Figure 3 is a diagram of
the coupling structure and its equivalent circuit. The coupling
value is determined by the series capacitor (C12) between the
line and resonator, shunt capacitor (C22) to ground and
property of transmission line, the characteristic impedance and
the phase length (Z, 69. ClI and C12 are functions of tuner
height so we must compensate for this variation by adjusting
the phase length and impedance of the transmission line so that
the overall coupling will remain constant. Figure 4 shows the
coupling change vs frequency. Measured and calculated
couplings agreed very well.
One advantage of this coupling structure is that different
target bandwidth filters can be designed with the same
configuration and dimension by simply changing the coupling
magnitude between the resonators and transmission lines.
Otherwise, if the resonators were directly coupled, the distance
between resonators and its layout would have to be varied for
different bandwidths. Hence, both RF and mechanical redesigning would be required for different target bandwidths.
Another advantage of this structure is that the
compartmentalized housing eliminates undesired parasitic
couplings (e.g. between the first and the third resonators).
These parasitic couplings can affect filtering shape, especially
for a narrow band filter such as the one presented here. Yet
another advantage of the compartment filter structure is that it
enables the tuning approach described in the following section.
-0
400,00
350,000 300,000 250,000 0 200,000 -
150,000
-
100,000
50,0000
750
800
850
900
950
1000
1050
Frequency [MHzJ
Fig. 2
Unloaded Q-factor vs. tuning frequency measured at 77 K.
90
-
Measurement
Calculation
N
I
s 80
(a)
70
0
60
750
800
850
900
950
Frequency [MHz]
Fig. 4
Resonator 1
IV. TUNING APPROACH
Resonator 2
Several approaches to filter tuning have been proposed.
Filters could be tuned using a look-up table [4] or by analyzing
a filter response such as SI1 and actively make improvements
[5]. From a sensitivity analysis based on Monte Carlo method,
a 100 kHz bandwidth filter requires 2 kHz accuracy to
maintain good return loss. From Fig. 1, the frequency slope at
the high frequency end of the tuning range is 0.5 kHz/nm of
tuner movement, so the tuner height is required to be set to
(b)
Coupling structure of the filter (a) and its equivalent
circuit (b). Series capacitor C12 and Shunt capacitor C22 are
functions of both frequencyf and tuner height, Ht.
Fig. 3
Since these
are
low
Q
Coupling bandwidth of the 3-pole filter.
portions of the circuit, these coupling
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difficult to bring all of the resonators together to the same
frequency. So a combination of look-up table for coarse tuning
and optimization for fine tuning can be a better approach.
However, computation time for the optimization may limit the
tuning speed in that case.
The compartmentalized filter described in this paper
provides a new approach for tuning. The circuit has two
operational states. In the tuning state, the resonators are
isolated from each other and the resonators are tuned
independently. One very appealing feature of this approach is
that only a single reference signal at the frequency where the
filter is to be tuned is necessary, as we are only measuring
single resonator responses. Hence, the filter can be tuned
quickly using a very simple single resonator algorithm,
regardless of the order of the filter.
In order to implement this tuning approach, we introduce
bypass switches, which are composed of two SPDT switches,
into the transmission line between the resonators. The bypass
switches change the filter between its two states, as shown in
Fig. 5. In the filter state, where the switches are ON, the
transmission line works as a coupling structure and couples
resonators to function as a filter. In the tuning state, where all
the switches are OFF, the resonators are isolated from each
other and they can be tuned independently. Each resonator can
then be coupled to a source at the tuning input and a detector
at the tuning output.
The resonators are tuned using a single reference signal at
the target frequency where the resonator is to be tuned. The
tuner is then moved up and down until the detector receives
the maximum power. At that point, the resonator will be tuned
at that reference frequency which is usually the target
frequency of the tunable filter. Looking at the phase of the
transmitted signal near the resonance provides even more
sensitivity and allows for the resonators to be more precisely
aligned. After all of the resonators are tuned in this way, the
bypass switches are switched ON to return to the filter state.
1I
1
Filter
Input
Resonator I
Resonator 2
(a)
I
I
Q-o
Filter
Input
IR
I
Resonator I
Resonator 2
(b)
Fig. 5
(a) Filter state: All bypass switches connect resonators
through coupling structure and forms filter. (b): Tuning state:
resonators are isolated from each other when the bypass switches
disconnect the coupling structure.
V. FABRICATION AND MEASUREMENT
Fig. 6
A
performance
high
cryogenically
compatible
electromechanical reed switch developed at STI was used for
the bypass switches. The measured insertion loss of the switch
was less than 0.1 dB at 77 K per switch so that it should have
little impact to the overall insertion loss of the filter. GaAs
PIN-Diode SPDT switches were also used in order to
distribute the tuning input signal from one port and gather the
tuning output signal into another port. The RF characteristics
(such as insertion loss and IP3) of these PIN-Diode switches
are largely irrelevant as they remain outside the circuit in filter
mode. A small nano-motor [6] actuator was used to move the
tuner over the resonator. This actuator has sub-nanometer
precision and millimeter travel and is thus adequate to cover
the entire tuning range with the required accuracy. Figure 6
shows base of the filter package.
3-pole filter package base. Left and right side
connectors are input and output for filter and top and bottom
connectors are for tuning.
within 4 nm. A look-up table approach is unlikely to be
successful with this positional accuracy requirement.
Optimization based tuning may be another approach, however
if the filter becomes significantly detuned it may be very
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696
All measurements were taken at 77K in a liquid nitrogen
cooled microwave enclosure. Figure 7 shows a set of
measurement data tuned at 770, 830, 860 and 920 MHz center
frequencies. The filter was designed to have Butterworth shape
so that a single deep notch at the band center is recognizable
for all the data. This would not have been possible in the
presence of the undesired parasitic coupling from the first
resonator to the third.
VI. CONCLUSION
We have demonstrated an approach for a highly selective
low-loss tunable filter with 20 % tuning range. The resonators
maintain high Q-factor across the wide range and the
bandwidth of the tunable filter is held nearly constant. We
have proposed a novel approach to the tuning algorithm based
on single resonator tuning. The three-resonator prototype filter
we developed and presented in this paper is still in the first
stages but we plan to expand it to a five-resonator filter to
further improve the rejection performance.
(a)
ACKNOWLEDGEMENT
The authors would like to acknowledge G.L. Matthaei, G.L.
Hey-Shipton, A. Cardona, K.F. Raihn, J. Fuller, D. Amezquita,
T. Jones, R. Orozco, J. Costa, S. Bilski, K.E. Kihlstrom, P.E.
Blumenfeld, R.C. Eden, and R.B. Hammond for many useful
interactions relating to this work.
This work was supported by the Defense Advanced
Research Projects Agency, Defense Sciences Office, Totally
Agile RF Sensor Systems, issued by DARPA/CMD under
Contract #MDA972-00-C-0010.
(b)
REFERENCES
[1] D. E. Oates, G. F. Dionne, " Magnetically tunable
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[2] B. H. Moeckly, L. S. Peng and G. M. Fischer, " Tunable HTS
microwave filters using strontium titanate thin films," IEEE
Trans. Applied Superconductivity., vol. 13, no. 2, pp. 712-715,
2003.
[3] B. A. Willemsen, "Tunable HTS Filters with Constant
Bandwidth, " IEEE International Microwave Symposium 2004
Workshop digest WFE02, Jun, 2004.
[4] E. M. Prophet, J. Musolf, B. Zuck, S. Jimenez, K.H. Kihlstrom,
B. A. Willemsen, "Highly-Selective Electronically-Tunable
Cryogenic Filters Using Monolithic, Discretely-Switchable
MEMS
Capacitor Arrays," IEEE Trans. Applied
Superconductivity., vol. 15, no. 2, pp. 956-959, 2005.
[5] V. Borzenets, S. J. Berkowitz, P. E. Blumenfeld, N. Maltsev,
"Resonator tuning assembly and method", US Patent application
2003/0122635A1 (2003).
[6] S. Kleindiek, "Development and Applications of a miniaturized
Linear Drive with Sub-Nanometer Precision and Millimeter
Travel", Dissertation University of Tuebingen (1996).
A
A
(c)
Fig. 7 Measurement results at three different center frequencies
at (a) 770 MHz, (b) 860 MHz and (c) 920 MHz.
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