Digital backends at Arcetri

Multibeam spectropolarimeter

A spectrometer with 16 input channels (8 dual polarization), based on the ALMA Tunable Filter Board. The board has been reprogrammed to implement the spectrometers and associated digital filters. The instrument is composed of two TFB boards, two sampler boards, and a control PC.

The main characteristics of the instrument are:

16 (8 pairs) input channels
Input bandwidth: 125 MHz nominal, 80 MHz including antialiasing filter. Antialiasing filter band: 140-220 MHz
Quantization: 8 bit (12 input channels) or 6 bit (16 input channels)
A total of 16 dual channel polarimetric spectrometers, in four groups. Each spectrometer can select two input channels among 4 possible group inputs
Each spectrometer analyzes two independently positioned regions of the input band (same region in polarimetric modes). In polarimetric modes all 4 Stokes parameters are computed for each spectral point
Region bandwidth from 125 MHz to 0.5 MHz, in binary steps
Spectral resolution of 4096 spectral points across selected bandwidth. Reduced resolution possible using hardware spectral binning
Minimum integration time limited by data bandwidth (currently 100 Kbyte/s)

The instrument has been tested using water masers in Orion (left) and W49 (right).

References:

A. Russo, G. Comoretto: Spettrometri a correlazione e a FFT per il radiotelescopio SRT, Arcetri technical report 1/2005
G. Comoretto, A. D'Ambrosi, R. Nesti, A. Russo, F. Palagi: A modular multichannel spectrometer - design study Arcetri technical report 4/2006
G. Comoretto, A. Russo: Software di comunicazione con il correlatore Altera Arcetri technical report 2/2007
A. Tani, G. Comoretto: Progetto di un Filtro CIC su Stratix FPGA Arcetri technical report 6/2007
G. Comoretto, F. Palagi: ARCOSWEB - interfaccia grafica per il controllo remoto di ARCOS IRA-FI tech. rep. (2006). This report describes the software used for the first observations. Source code can be found here.

Digital BBC

The Digital Baseband Converter (DBBC) is a project of the Noto radiotelescope together with other institutes, in order to develop a all-digital replacement for the VLBI data acquisition terminal. The DBBC analyzes up to 4 IF signals, typically 2 polarizations with a total band of 1 GHz per polarization, divided in 2 chunks of 500 MHz each. A number of channels each up to 32 MHz is extracted from the IF signals and sent to a MK5 (disk based) recorder.

The hardware is quite flexible. It is composed of a stack of different modules (ADC; processing; electric interface to the recorder) and systems with different complexity can be obtained adding more modules.

The Arcetri group has acquired a DBBC in a minimal configuration, and has started a study to explore the possibility to use it as a general purpose backend.

A general framework logic has been studied for the processing module (CORE1), based on a Xilinx Virtex2 FPGA. It includes the interface to the high speed data busses, clock distribution and a general purpose programmable interface to the control computer. This structure will be ported to the new Virtex4 board (CORE2).

A porting of the FFT spectrometer described above is in progress.

A FFT architecture to split the input band into 16 32MHz channels has begun, using the experience gained with the ALMA correlator.

References:

G. Comoretto, G. Tuccari: Reference design for the Digital BBC Architecture Arcetri technical report 2/2008