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\newcommand{\atUCLA}{Dept. of Physics and Astronomy, Univ. of California, Los Angeles, Los Angeles, CA 90095.}
\newcommand{\atOSU}{Dept. of Physics, Ohio State Univ., Columbus, OH 43210.}
\newcommand{\atUH}{Dept. of Physics and Astronomy, Univ. of Hawaii, Manoa, HI 96822.}
\newcommand{\atNTU}{Dept. of Physics, Grad. Inst. of Astrophys., Leung Center for Cosmology and Particle Astrophysics, National Taiwan University, Taipei, Taiwan.}
\newcommand{\atUCI}{Dept. of Physics, Univ. of California, Irvine, CA 92697.}
\newcommand{\atKU}{Dept. of Physics and Astronomy, Univ. of Kansas, Lawrence, KS 66045.}
\newcommand{\atWashU}{Dept. of Physics, Washington Univ. in St. Louis, MO 63130.}
\newcommand{\atSLAC}{SLAC National Accelerator Laboratory, Menlo Park, CA, 94025.}
\newcommand{\atUD}{Dept. of Physics, Univ. of Delaware, Newark, DE 19716.}
\newcommand{\atUCL}{Dept. of Physics and Astronomy, University College London, London, United Kingdom.}
\newcommand{\atJPL}{Jet Propulsion Laboratory, Pasadena, CA 91109.}
\newcommand{\atCCAP}{Center for Cosmology and AstroParticle Physics, Ohio State Univ., Columbus, OH 43210.}
\newcommand{\atChicago}{Dept. of Physics, Enrico Fermi Institute, Kavli Institute for Cosmological Physics, Univ. of Chicago , Chicago IL 60637.}
\newcommand{\atGSFC}{Currently at NASA Goddard Space Flight Center, Greenbelt, MD, 20771.}
\newcommand{\atCalPoly}{Dept. of Physics, California Polytechnic State Univ., San Luis Obispo, CA 93407.}
\newcommand{\atMEPHI}{National Research Nuclear University, Moscow Engineering Physics Institute, 31 Kashirskoye Highway, Rossia 115409}
\begin{document}
\title{Low-power radio frequency amplification module with dynamic tunable notch filters for the Antarctic Impulsive Transient Antenna (ANITA)}
\author{P.~Allison}
\affiliation{\atOSU}
\affiliation{\atCCAP}
\author{O.~Banerjee\footnote{Corresponding author. \\\hspace*{1.8em} E-mail: oindreeb@gmail.com | Tel.: +1 (614) 800-4409}}
\affiliation{\atOSU}
\affiliation{\atCCAP}
\author{L.~Batten}
\affiliation{\atUCL}
\author{J.~J.~Beatty}
\affiliation{\atOSU}
\affiliation{\atCCAP}
\author{K.~Belov}
\affiliation{\atJPL}
\author{D.~Z.~Besson}
\affiliation{\atKU}
\author{W.~R.~Binns}
\affiliation{\atWashU}
\author{V.~Bugaev}
\affiliation{\atWashU}
\author{P.~Cao}
\affiliation{\atUD}
\author{C.~Chen}
\affiliation{\atNTU}
\author{P.~Chen}
\affiliation{\atNTU}
\author{J.~M.~Clem}
\affiliation{\atUD}
\author{A.~Connolly}
\affiliation{\atOSU}
\affiliation{\atCCAP}
\author{L.~Cremonesi}
\affiliation{\atUCL}
\author{B.~Dailey}
\affiliation{\atOSU}
\author{C.~Deaconu}
\affiliation{\atChicago}
\author{P.~F.~Dowkontt}
\affiliation{\atUCLA}
\author{P.~W.~Gorham}
\affiliation{\atUH}
\author{J.~Gordon}
\affiliation{\atOSU}
\author{B.~Hill}
\affiliation{\atUH}
\author{R.~Hupe}
\affiliation{\atOSU}
\author{M.~H.~Israel}
\affiliation{\atWashU}
\author{M.~Kovacevich}
\affiliation{\atOSU}
\author{J.~Kowalski}
\affiliation{\atUH}
\author{J.~Lam}
\affiliation{\atUCLA}
\author{J.~G.~Learned}
\affiliation{\atUH}
\author{K.~M.~Liewer}
\affiliation{\atJPL}
\author{T.~C.~Liu}
\affiliation{\atNTU}
\author{A.~Ludwig}
\affiliation{\atChicago}
\author{S.~Matsuno}
\affiliation{\atUH}
\author{C.~Miki}
\affiliation{\atUH}
\author{K.~Mulrey}
\affiliation{\atUD}
\author{J.~Nam}
\affiliation{\atNTU}
\author{R.~J.~Nichol}
\affiliation{\atUCL}
\author{A.~Novikov}
\affiliation{\atKU}
\affiliation{\atMEPHI}
\author{E.~Oberla}
\affiliation{\atChicago}
\author{S.~Prohira}
\affiliation{\atKU}
\author{B.~F.~Rauch}
\affiliation{\atWashU}
\author{J.~Roberts}
\affiliation{\atUH}
\author{A.~Romero-Wolf}
\affiliation{\atJPL}
\author{B. Rotter}
\affiliation{\atUH}
\author{J.~Russell}
\affiliation{\atUH}
\author{D.~Saltzberg}
\affiliation{\atUCLA}
\author{D.~Seckel}
\affiliation{\atUD}
\author{S.~Stafford}
\affiliation{\atOSU}
\author{J.~Stockham}
\affiliation{\atKU}
\author{M.~Stockham}
\affiliation{\atKU}
\author{B.~Strutt}
\affiliation{\atUCL}
\author{K.~Tatem}
\affiliation{\atUH}
\author{G.~S.~Varner}
\affiliation{\atUH}
\author{A.~G.~Vieregg}
\affiliation{\atChicago}
\author{S.~A.~Wissel}
\affiliation{\atCalPoly}
\author{F.~Wu}
\affiliation{\atUCLA}
\author{R.~Young}
\affiliation{\atKU}
\medskip
\date{\today}
\medskip
\medskip
\begin{abstract}
The Antarctic Impulsive Transient Antenna (ANITA) is a NASA long-duration balloon mission with the primary goal of Askaryan radio detection of ultra-high-energy neutrinos. The fourth ANITA mission, ANITA-IV, recently flew from Dec 2, 2016 through Dec 29, 2016. The most significant change in signal processing in ANITA-IV was the inclusion of the Tunable Universal Filter Frontend (TUFF) boards. The TUFF boards had a three-fold purpose as follows: a) second-stage amplification by 45~dB to boost the $\sim\,\mu\mbox{V-level}$ radio frequency (RF) signals to $\sim$ mV-level for digitization; b) mitigation of narrow-band, anthropogenic noise with tunable, switchable RLC notch filters and c) supplying power via bias tees to the first-stage, antenna-mounted amplifiers. To accomplish this with a $<1\,\mbox{kW}$ total power budget and highly constrained space and weight requirements of a balloon mission, the TUFF boards needed to be low-power, compact and light. Sixteen TUFF boards were built to serve six ANITA channels each with a per-channel power consumption of 330~mW. Eight pairs of TUFF boards were each assembled into a final 12-channel aluminum housing to provide heat-sinking, structural support, and RF isolation. During the ANITA-IV mission, these eight, 12-channel modules, for a total of 96 channels, were successfully operated throughout the flight. In this paper, we outline the design and performance of the TUFF boards during the ANITA-IV flight and plans for improving the boards for ANITA-V.
\end{abstract}
\maketitle
\section{Introduction}
The Antarctic Impulsive Transient Antenna (ANITA) is a
NASA long-duration balloon-borne radio experiment for the detection of ultra-high energy
(UHE) neutrinos using the Antarctic ice as its detection volume. UHE
neutrinos would interact in the ice, producing an electromagnetic cascade that develops a charge asymmetry and emits a coherent Cherenkov
radio impulse extending to $\sim1\,\mbox{GHz}$ via the Askaryan effect \cite{askaryan}.
The radio impulses received at the payload are at the level of thermal emission from the Antarctic ice ($\sim20\,\mu\mbox{V}$).
Following thermal radiation, ANITA's dominant source of noise is human-made narrow-band transmissions,
henceforth referred to as continuous-wave (CW) interference.
While Antarctica itself is relatively free of CW transmissions, except for bases of human activity, transmissions from geosynchronous satellites are continuously in view because
the beam of the ANITA antennas extends to the horizontal and
above.
The Antarctic science bases such as McMurdo Station, South Pole Station, etc. are much radio-louder than the rest of the continent, producing CW interference, for example, in the $\sim430-460\,\mbox{MHz}$ band.
CW interference due to military satellites has been problematic for all past flights of ANITA, but especially detrimental to the ANITA-III flight.
ANITA-I and ANITA-II observed CW interference primarily in the $240-270\,\mbox{MHz}$ band, peaking
at $\sim260\,\mbox{MHz}$. This frequency range is predominantly used by the aging Fleet Satellite (FLTSAT) Communications System
and Ultra High Frequency Follow-On (UFO) System, both serving the United States Department of Defense. Lesser interference was also seen
at approximately $\sim380\,\mbox{MHz}$, which is presumed to be the
newer Mobile User Objective System (MUOS) satellites. Because of the design
of the ANITA-I and ANITA-II trigger, which required coincidences among different frequency bands, the CW interference did not overwhelm
the acquisition system. However, the ANITA-III trigger, which was redesigned to trigger on full-bandwidth signals
for improved sensitivity,
produced trigger rates far in excess of the acquisition system's readout
capabilities ($\sim50\,\mbox{Hz}$) for thresholds comparable to those used in previous flights. This forced us to increase our thresholds in the presence of CW interference and sacrifice some neutrino sensitivity during those periods.
Mitigation of CW interference, especially due to military satellites, is critical to the ANITA experiment.
In ANITA-III, "phi-masking" was used during noisy periods to veto triggers from
approximately half of the payload field-of-view to keep the
trigger rate at or below $50\,\mbox{Hz}$. To restore triggering efficiencies
in the presence of CW interference, tunable notch filters were built for the $\sim260\,\mbox{MHz}$ (Notch 1), $\sim380\,\mbox{MHz}$ (Notch 2) and $\sim460\,\mbox{MHz}$ (Notch 3) bands.
\section{ANITA-III AND IV TRIGGER SYSTEMS}
The ANITA-III and IV payloads primarily consist of the following components: a) 48 dual-polarized, highly directional (on-axis gain of $\sim10\,\mbox{dBi}$, $3\,\mbox{dB}$ point averaged over in-band frequencies is $\sim30^{\circ}$) horn antennas, b) an Instrument Box containing different units for signal processing (illustrated for ANITA-IV in Figure~\ref{system}) c) a NASA Science Instrument Package, d) a Battery Box, e) three GPS systems and f) photovoltaic cells for solar-powering the payload. The 12-channel TUFF modules reside inside four Internal Radio Frequency Conditioning Modules (IRFCMs) inside the ANITA-IV Instrument Box. Figure~\ref{anita} shows the ANITA-IV payload just prior to launch at NASA Long Duration Balloon (LDB) Facility near McMurdo Station.
\begin{figure}
\centering
\includegraphics[width=1.0\textwidth]{anita.png}
\caption{The ANITA-IV payload just prior to launch at NASA LDB Facility near McMurdo Station, Antarctica.}
\label{anita}
\end{figure}
For each ANITA channel, RF signal is processed and digitized in the Instrument Box as illustrated in Figure~\ref{system} (ANITA-IV). Since the ANITA-III and IV payloads each has 48 dual-polarized antennas, there are total 96 RF channels for these flights.
A tunnel diode acts as a square-law power detector for each RF channel. Following the tunnel diode, the RF signal is sampled by four LABRADOR chips (buffers) each consisting of a 260-element switched capacitor array, in the Sampling Unit for Radio Frequency (SURF). The tunnel diode and the SURF are together responsible for issuing the Level 1 trigger when the power in a channel exceeds a certain threshold. When the Level 1 trigger is issued, one LABRADOR chip digitizes the signal, while the remaining three continue to sample.
\begin{figure}
\centering
\includegraphics[width=1.0\textwidth]{anita4systemdiagram.png}
\caption{The ANITA-IV signal processing chain for a single RF channel.}
\label{system}
\end{figure}
The ANITA antennas (48 total) are mounted on three rings: bottom, middle and top, forming 16 azimuthal sectors or phi sectors of antennas (as shown in Figure~\ref{anita}). If RF signal in a top ring antenna exceeds the threshold, then the middle and bottom antennas are checked for excess in the previous $8$ and $12\,\mbox{ns}$ respectively, or if a middle ring antenna has signal exceeding the threshold, then the bottom ring antenna is checked in the previous $4\,\mbox{ns}$. This process could issue Level 1 triggers for an entire phi sector of antennas consisting of a top, a middle and a bottom ring antenna. A Level 3 trigger is issued when Level 1 triggers are issued in adjacent phi sectors of antennas. This is how the trigger works in ANITA-III.
In ANITA-IV, there is an additional requirement that the signal must contain equal parts of left and right circular polarization (LCP and RCP) components, before a Level 1 trigger can be issued. This would increase the probability of the signal being linearly polarized, and thus it would be more likely to come from a neutrino or cosmic-ray source.
During the ANITA-III flight, when the payload was strongly hit with anthropogenic noise or CW interference from a particular direction, the triggers from phi sectors on that side of the payload, covering approximately half of the payload field-of-view, were vetoed, and this was called "phi-masking".
\section{TUFF Board Design}
The total power budget of the ANITA payload is $<1\,\mbox{kW}$. Moreover, there are severe restrictions on weight and space for a balloon mission. Thus, the TUFF boards needed to be low-power, compact and light. Figure~\ref{tuff_channel} shows a single TUFF channel next to a quarter USD coin for size comparison.
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