Filter Designs for Next-Generation Analog Receivers

Abstract

A preliminary design for bandpass filters in the next-generation analog receivers of OVRO-LWA is described in [3] and was presented at the Preliminary Design Review of June 2019. The plan was to provide three remotely-selectable filters, all of which are intended to suppress high-power RFI in the bands below 20 MHz and above 88 MHz. One filter ("Wide") extends the bandwidth as much as practical on each end, while accepting some risk that observations will be RFI-contaminated or that gain and SNR will have to be reduced to avoid intermodulation. Another ("Safe") is intended to have low risk of RFI contamination and to maximize sensitivity in the center of the observing band, while accepting lower total bandwidth. The third ("Smooth") is designed to have a maximally-smooth bandpass, requiring still smaller total bandwidth to achieve adequate RFI suppression. The designs included selection of specific, available components and detailed simulations of performance, taking into account non-ideal behavior of practical inductors and parasitic effects of PC board layout. Since then, additional studies have been done via simulation of the performance of the selected filters in the measured RFI environment at OVRO, and some adjustments to the cutoff frequencies have been explored. This is partly in response to a suggestion in the PDR report of the Technical Advisory Committee [2] that frequency coverage be extended to lower frequencies than had previously been considered. This has led to changes in the filter selection topology and in the details of the filter responses. This memo describes the current design ("ARX filters revision 2") and the considerations that led to it. The new designs include specific component selection and detailed simulation, and they have been implemented on a test printed circuit board. Results of simulations and testing are reported here.

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