Transcript
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
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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,
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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.
694 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.
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IV. TUNING APPROACH
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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
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695 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.
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(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
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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.
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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 superconducting resonators and filters," IEEE Trans. Applied Superconductivity., vol. 9, no. 2, pp. 4170-4175, 1999. [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).
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Fig. 7 Measurement results at three different center frequencies at (a) 770 MHz, (b) 860 MHz and (c) 920 MHz.
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