A high-performance narrowband high temperature superconducting filter

Abstract

Recent development of a high-performance narrowband high-temperature superconducting (HTS) filter is presented. A 10-pole quasi-elliptic function response filter with two pairs of transmission zeros (for high selectivity), which has a center frequency of 2.185 GHz and a 3 dB bandwidth of 10 MHz (a fractional bandwidth of 0.45%), is introduced. The filter was fabricated on a 40 mm×16 mm×0.5 mm MgO wafer with double-sided YBCO films. The measured results show that the filter has 0.15 dB insertion loss at the center of the passband and a return loss better than −22.5 dB at a temperature of 67 K. Band edge steepness reaches over 28.3 dB/MHz at both the low and high frequency edges. The out-of-band rejection is over 78 dB at 8 MHz away from the center frequency and close to 100 dB in a wideband range. high-performance, narrowband, HTS, filterCitation: Cui B, Zhang X Q, Sun L, et al. A high-performance narrowband high temperature superconducting filter. Recently, superconducting devices have been developed steadily in China. Superconducting materials have been widely used in the development of kinds of devices, such as filters [1-4], single photon detector [5] and hot electron bolometer (HEB) mixer [6]. Among these, an important aspect is the development of high-performance narrowband microwave filters. Generally speaking, the insertion loss of a filter is inversely proportional to its fractional bandwidth (FBW), and thus it is difficult to realize narrowband filters (for example FBW<0.5%) using normal metals. The hightemperature superconducting (HTS) films have a thousand times smaller microwave surface resistance than normal metal films. Therefore they are suitable for use in fabricating such narrowband filters and many works on narrowband HTS filters have been reported since early 1990s [7][8][9][10][11][12].In most cases, to realize high selectivity, HTS filters are constructed by numbers of resonators which are arranged within a testing house. The compact size of HTS filters will introduce some unwanted parasitic couplings between nonadjacent resonators, which makes it hard to realize a high out-of-band rejection. Besides, it is not easy to achieve better return loss (such as, better than −20 dB) due to the complexity of tuning at low temperature and in vacuum. There are, to our knowledge, only a couple of reports in the literature of HTS filters with high-performance specification of return loss (< −20 dB) and/or out-of-band rejection (> 95 dB) [13][14][15].In this work, we present a 10-pole high-performance narrowband HTS filter with a cascaded quadruplet coupling structure. The filter has an excellent return loss (−22.5 dB in the passband) and a high out-of-band rejection (close to 100 dB in a wideband range). A double-folded resonator structure [16], which has been developed in our laboratory for a filter with a center frequency of 8 GHz, is used in our design to realize the high out-of-band rejection level. A novel asymmetrical cross coupling structure is introduced to implement the filter with two pairs of t...

Thin‐Film‐Based Integrated High‐Transition‐Temperature Superconductor Devices

“…Using higher filter order, losses lower than 1 dB have also been demonstrated . In 2010, using improved filter design, a ten‐pole HTS band‐pass filter using YBCO at a center frequency of about 2.185 GHz with an insertion loss of 0.15 dB was demonstrated . Some applications require very low bandwidth for high selectivity.…”

Compact wide stopband superconducting bandpass filter using modified spiral resonators with interdigital structure

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