Project QUIJOTE TGI Instrument’s Data Acquisition System for the Study of the Big Bang’s Cosmic Microwave Background

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"NI hardware and software helped us develop a complex system in a relatively short amount of time. We addressed the big challenges for the system, such as the synchronization and treatment of high volumes of data, rapidly and efficiently using NI solutions."

- Yolanda Martin Hernando, Instituto de Astrofísica de Canarias

The Challenge:
Acquiring the data obtained in the 31 polarimeters that make up the TGI instrument for the QUIJOTE telescope, simultaneously sampled at 160 KHz and synchronized with the switches and the telescope’s control signal for preprocessing in real time and delivery to an external storage terminal.

The Solution:
Using the NI PXI platform to accurately synchronize the sampling of multiple channels.

Author(s):
Yolanda Martin Hernando - Instituto de Astrofísica de Canarias
Miguel Nuñez Cagigal - Instituto de Astrofísica de Canarias
Noemí Gonzalez Cobos - Instituto de Astrofísica de Canarias
Teodora Viera Curbelo - Instituto de Astrofísica de Canarias

QUIJOTE (QUI JOint TEnerife) is an experiment that studies cosmic microwave background (CMB). Its objective is to characterize the polarization of CMB and other galactic and extragalactic emissions within 10–40 GHz frequency ranges and within large angular scales.

QUIJOTE covers a sky area of 10,000 degrees square, with a sensibility of 1–2 μ Kelvin and an angular resolution of 1°. The telescope features several instruments working within several frequency bandwidths that go from 11 to 40 GHz. These measurements complement, at low frequencies, those of the Plank satellite to correct the satellite data’s galactic contamination. Moreover, the measurements obtained with QUIJOTE are the more sensitive measurements obtained for the characterization of synchrotron emissions and the anomalous microwave emissions in our galaxy at those frequencies.

The second QUIJOTE instrument, the TGI (or 30 GHz instrument), mainly studies the B modes of primordial origin. With the detection of the B modes we can confirm the idea of inflation, which says that the universe, after the Big Bang, went through an initial and brief phase of accelerated expansion. The TGI is equipped with 31 polarimeters that function within the frequency ranges of 26–36 GHz. The current design of a TGI’s polarimeter includes a fixed polarizer and phase switches of 90° and 180° to generate four polarization statuses.

Objectives

The acquisition system of the TGI instrument needs to simultaneously sample, at 160 KHz, the four output channels of each one of the 31 polarimeters to ensure a precise synchronization between the 124 (4 x 31) sampled channels and the telescope. Also, the system generates the control signals of the phase switches, which in turn need to be correctly synchronized with the data acquisition. On the other hand, the high volume of data acquired forces us to carry out some processing in the actual acquisition system so that it reduces the rate of data sent to the external storage terminal.

Architecture

To implement the TGI instrument’s data acquisition system, we selected an architecture based on three subsystems:

  • PXI RT host, for data acquisition, processing, and sending
  • PXI FPGA to control the phase switches
  • PC with a user interface

We used LabVIEW 2013 software to develop the various subsystems. We opted for a queued message handler architecture for the user and the RT host’s interfaces. This architecture is flexible and provides a simple increment of system options and commands in the instrument’s future updates.

Network streams communicate between RT host and user interface. This communication method is designed for loss-free transmissions in communications with a high level of data transfers, which are required in this project. In terms of data transfer and latencies, the network streams characteristics are similar to TCP’s, but its implementation is simpler, optimizing the solution’s development time.

RT Host

The RT host is the core of the system and manages the acquisition, processing, and transfer of the scientific data obtained by the polarimeters. The hardware for this subsystem includes a PXI-1044 rack with an embedded PXI-8109 processor and eight NI 4495 acquisition modules. Using this hardware, together with LabVIEW software and the NI-DAQmx driver, we can sample 128 channels at 160 KHz with a low level of noise and an accurate synchronization. The two cores of the embedded PXI-8109 controller deliver a huge processing capacity so the data can be pretreated in real time and decrease the rate of data transfer per channel from 160 KS/s to 4 KS/s. We synchronize the eight NI-4495 cards in ‘reference clock’ mode. We carry this out in a transparent way for the developer and so we can implement, in a relatively short amount of time, a high-precision solution with no temporal derivations. Lastly, we synchronize the RT host with the telescope’s NTP server through TimeSync for SNTP.

FPGA: Control of Switches

This subsystem generates the control signals of the phase switches of each one of the 31 polarimeters at a frequency of 16 KHz or 8 KHz, as per the user’s configuration. Each switch has 16 possible statuses, which translates into a control signal of 4 bits per switch, needing 124 (31 x 4) control outputs. For its implementation, we selected a PXI-7813R module with up to 160 digital outputs and an FPGA controller. We use the PXI rack’s lines to synchronize this module with the data acquisition.

PC and User Interface

The user interface delivers a graphical user interface and the option of sending commands and receiving maintenance and scientific data sent by the RT host. Also, this subsystem saves the scientific data to disk and stores it in the required format. The solution that we selected for this subsystem is a PC with a custom developed LabVIEW application.

Results

NI hardware and software helped us develop a complex system in a relatively short amount of time. We addressed the big challenges for the system, such as the synchronization and treatment of high volumes of data, rapidly and efficiently using NI solutions.

Author Information:
Yolanda Martin Hernando
Instituto de Astrofísica de Canarias
Via Lactea S/N
La laguna 38205
Spain
Tel: 922 605 200
Fax: 922 605 230
ymartin@iac.es

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