Dual-Band Superconducting Spiral Filters Including Narrow Bandstop Notches

Abstract

Dual-band filters designed as bandpass filters containing bandstop notches are described. The design formulation is simplified by considering the resonators in pairs. Two microstrip superconducting spiral filters are presented with 400-MHz center frequency, 15%-16% overall bandwidth, and notches of 1% and 0.1%, respectively. Internal dimensions of the enclosures are only 32 mm 30 mm 6 mm. Suitable spiral shapes and orientations enable the required wide range of coupling coefficients to be implemented, and also reduce unwanted coupling without using partitions. The remaining low level of unwanted coupling is cancelled using capacitive bridges. A qualitative understanding of spurious interactions between spirals is obtained via electromagnetic simulations.

Superconducting filter for radio astronomy using interdigitated, capacitively loaded spirals

“…Finally, superconductor Q factor decreases appreciably between 20 K and 60 K, and no significant change in S 21 was observed between these temperatures. The loss is therefore not believed to be due to the filter itself, which is consistent with previous work [6]. The ripple is somewhat higher than the 0.08 dB indicated by simulations, but the total loss, 0.21 dB, is close to the required 0.2 dB since the additional 0.01 dB is probably beyond the measurement accuracy.…”

“…As in previous work [6], the coupling k 14 between resonators 1 and 4 is primarily magnetic, and therefore it is of the correct polarity to provide a transmission zero that improves the selectivity of the filter. However, the size of the capacitors at the input and output to reduce the external coupling coefficients [6] has been reduced to zero as the maximum possible external coupling factor k e (or 1/Q e , where Q e is the "external quality factor") was required.…”

“…Without the innovations, the widest filter band width for the present 0.5 mm substrate thickness and 50 micrometer line width is presumed to be about 15% [6]. The present filter bandwidth of nearly 30% is probably the maximum using the improvements; all the coupling factors are near their limiting values.…”

“…However, the size of the capacitors at the input and output to reduce the external coupling coefficients [6] has been reduced to zero as the maximum possible external coupling factor k e (or 1/Q e , where Q e is the "external quality factor") was required. The line width of some parts of resonators 1 and 4 were also increased to increase the coupling, as already explained [4].…”

“…2 Left-hand half of the band-pass filter: dimensions are in mm; the 2.625 mm given is for HALF the capacitor pad input and resonator 2, and between the output and resonator 3 [6]. A transmission zero just below the pass band, expected of quasi-elliptic filters, is missing.…”