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Superconductor Science and Technology

Miniaturized superconducting microwave filters

M R Rafique, Thomas Ohki, Biddut Kumar Banik, Henrik Engseth, Peter Linnér, Anna Herr

Published Apr 16, 2008• 14 citations
Publisher

Abstract

In this paper we present methods for the miniaturization of superconducting filters. We consider two designs of seventh-order bandpass Chebyshev filters based on lumped elements and a novel quasi-lumped element resonator. In both designs the area of the filters, with a central frequency of 2–5 GHz, is less than 1.2 mm 2 . Such small filters can be readily integrated on a single board for multi-channel microwave control of superconducting qubits. The filters have been experimentally tested and the results are compared with simulations. The miniaturization resulted in parasitic coupling between resonators and within each resonator that affected primarily the stopband and increased the bandwidth. The severity of the error depends on the design in particular, and was less sensitive when a groundplane was used under the inductances of the resonators. The best performance was reached for the quasi-lumped filter with central frequency of 4.45 GHz, quality factor of 40 and 50 dB stopband.

DOI
10.1088/0953-2048/21/7/075004

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In this paper we present methods for the miniaturization of superconducting filters. We consider two designs of seventh-order bandpass Chebyshev filters based on lumped elements and a novel quasi-lumped element resonator. In both designs the area of the filters, with a central frequency of 2–5 GHz, is less than 1.2 mm 2 . Such small filters can be readily integrated on a single board for multi-channel microwave control of superconducting qubits. The filters have been experimentally tested and the results are compared with simulations. The miniaturization resulted in parasitic coupling between resonators and within each resonator that affected primarily the stopband and increased the bandwidth. The severity of the error depends on the design in particular, and was less sensitive when a groundplane was used under the inductances of the resonators. The best performance was reached for the quasi-lumped filter with central frequency of 4.45 GHz, quality factor of 40 and 50 dB stopband.
We present techniques to miniaturize superconducting ultra narrowband filters for on-chip applications. The filters are designed for 2 GHz, 5 GHz and 10 GHz operating frequencies. The expected bandwidths of the designed filters are 2-20 MHz. The designs are of 7 stages Chebyshev filters having maximum passband ripples of 0.5 dB. On-chip filters are particularly interesting as they are applicable for monolithic design with the RSFQ circuit aiming qubit applications. One of the designs is a quasi-lumped filter where the effective wave propagation constant has been increased by the addition of lumped components to the superconducting microstrip line (SMSL). Additionally, this design provides the optimum width of an SMSL without violating design rules. The other design consists of distributed and lumped components. The area of the filter is in the range of 1-2 square mm. We present the filter topologies and corresponding experimental results for the frequency response of these on-chip filters designed for Hypres 4.5 kA/cm2 fabrication process.
Abstract This paper presents a new class of sharp-selectivity low-temperature superconducting filter that incorporates lumped element resonant couplings. Dependent on a novel synthesis approach, the proposed filter exhibits great advantages such as: (1) a very simple in-line topology (without any cross coupling), (2) extremely compact size based on lumped inductor-capacitor (LC) elements, and (3) multiple transmission zeros (TZs) independently generated and controlled (via each resonant coupling). To facilitate the physical implementation, a group of lumped element circuit models are detailed, where series LC units are adopted for both the resonators and the resonant couplings. Considering an in-line topology here, the entire filter layout is then designed by cascading the lumped models one after another. For verification, a 5th-order bandpass filter centered at 5 GHz, with 500 MHz bandwidth and 3 TZs, is designed, simulated, and tested at cryogenic temperature (4.2 K). Moreover, preliminary simulations of the presented filter in series with an on-chip rapid single-flux-quantum microwave pulse generator are discussed for superconducting quantum applications.
We propose a mechanically tunable superconducting microwave filter based on lumped elements composed of planar meander-line inductors (L) and inter-digital capacitors (C). As the first step, we have designed and evaluated a 3-pole Chebyshev bandpass filters with lumped elements using a circuit simulator. It is found that the center frequency, bandwidth and skirt rejection of the filter can be controlled by changing only the capacitance. The L and C elements values are estimated using an electromagnetic simulator. Furthermore, we have fabricated the inter-digital capacitor designed with the above procedure. We have evaluated change of the C values using mechanical tuning as a step toward a tunable superconducting microwave filter.
Superconducting microelectronics (SME) technology has the potential of realizing very high speed digital receivers capable of performing direct digitization of radio frequency signals with very low power consumption. The SME receiver is implemented on a single chip using Niobium based low temperature superconductive (LTS) Josephson Junction (JJ) technology by HYPRES. Analogue RF filters are still required at the receiver front end and are key components of the overall superconductor digital receiver. SME receivers usually require two types of RF filters; a wideband bandpass filter and a bandstop filter (a notch filter). The notch filter is required to eliminate interference and unwanted signals in the passband. In this thesis, design of highly miniaturized lumped element wideband and bandstop filters is investigated and some challenges are addressed. The filters are fabricated by the HYPRES process and therefore can be integrated with the SME receiver on the same chip. \nIn a wideband filter, the coupling between the adjacent resonators is high. Achieving such a strong coupling is one of the challenges of designing wideband filters. The wideband filters realized with distributed elements usually suffer from very low spurious frequency. As the bandwidth of the filter becomes wider, the spurious peak of the second harmonic gets closer to the passband of the filter. In the first part of this work, the possibility of realizing lumped element superconducting bandpass filters (BPF) with a relative bandwidth of 80% is investigated. \nIn the second part of the thesis, design and realization of lumped element superconducting bandstop filters (BSF) is discussed. The challenge for designing a bandstop filter is providing a good match over a wide frequency range. So narrowband inverters cannot be used. Instead, usually λ/4 matched transmission lines provide 90° phase shift between the resonators of a notch filter. The possibility of replacing the long transmission line with other means or eliminating the inverters and using both shunt and series resonators are investigated. Having both series and shunt resonators introduces some new challenges that are addressed in the thesis and discussed thoroughly. \nA tunable notch resonator is presented. The tunability is provided by a superconducting MEMS varactor that is realized in our group by doing some post processing on the device fabricated by HYPRES. The tunability range of the device at cryogenic temperatures is investigated. A 3-pole tunable BSF is also designed that uses the same tunable resonators. The tunability of the filter is investigated through simulation.
A superconductor bandpass filter with tunable central frequency in the range of 2-3.5 GHz has been implemented using superconducting quantum interference devices (SQUIDs). The filter is designed using two pi-network resonators connected by a transmission line. Both resonators have a SQUID inductor with a tuning range of 65-200 pH, controlled by current magnetically coupled to the SQUIDs. Over a frequency tunability of 1.5 GHz, the filter has a mid-band insertion loss of 0.5-3.0 dB and corresponding maximum unloaded quality factor of 40. Due to the presence of active elements, tunability of the filter depends on the power of the microwave signal. A maximum power of -52 dBm corresponds to a frequency tuning range of 15%. Spectral measurements by controlling the central frequency of the filter with variable pulsewidth shows that the filter can be tuned at a rate of 120 GHz/mus.
The opacity due to water vapor in the Earth's atmosphere obscures portions of the sub-THz spectrum (mm/sub-mm wavelengths) to ground based astronomical observation. For maximum sensitivity, instruments operating at these wavelengths must be designed to have spectral responses that match the available windows in the atmospheric transmission that occur in between the strong water absorption lines. Traditionally, the spectral response of mm/sub-mm instruments has been set using optical, metal-mesh bandpass filters [1]. An alternative method for defining the passbands, available when using superconducting detectors coupled with planar antennas, is to use on-chip, superconducting filters [2]. This paper presents the design and testing of superconducting, lumped element, on-chip bandpass filters (BPFs), placed inline with the microstrip connecting the antenna and the detector, covering the frequency range from 209-416 GHz. Four filters were designed with pass bands 209-274 GHz, 265-315 GHz, 335-361 GHz and 397-416 GHz corresponding to the atmospheric transmission windows. Fourier transform spectroscopy was used to verify that the spectral response of the BPFs is well predicted by the computer simulations. Two-color operation of the pixels was demonstrated by connecting two detectors to a single broadband antenna through two BPFs. Scalability of the design to multiple (four) colors is discussed.