Digital backends at Arcetri
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).
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
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.