The ALMA project at Arcetri: Digital filtering
The second generation correlator
An improved correlator for ALMA has been proposed by an European
consortium including Astron (NL), Bordeaux University (FR), IRAM (FR)
The basic approach is that of a hybrid correlator. The input bandwidth
is decoposed in a number (34 in the present design) of separate
subbands, that can be independently correlated by a standard (non
time-multiplexed) correlator. This approach is intermediate between a
XF time multiplexed nstrument, like the first ALMA correlator, and a FX
correlator, like the one proposed by the Japanese NRO.
The work in Arcetri has focused on the efficent design of a digital
receiver, that exploits the peculiarities of the astronomical signal,
i.e. the extremely coarse quantization and the stochastic nature of the
Three designs have been proposed, with decreasing resource usage.
- Arcetri technical report n. 5/2002. A complex
mixer followed by a two stage bandpass filter. The SSB rejection of
the undesired band is performed in the first filter. The second filter
operates on a bandpass decimated real signal.
- Ver. 2 of report 5/2002
The complex mixer is followed by a two stage complex low pass filter.
A low pass filter uses much less resources for comparable sharpness, and
this compensates for the use of a complex signal processing. The signal
is translated to real before the output stage.
Analytic design approach.
A complex band pass filter is followed by a
decimation stage and a complex mixer. The mixer rotates the filter
oputput in order to place the desired subband around frequency zero. A
second low pass complex filter produces the desired bandpass response,
followed by a complex-to-real conversion, as in the previous design.
- Phase effects in a hybrid
correlator. It is shown that overlapping the sub-bands in a hybrid
real correlator is necessary to prevent large phase errors at the band
- Sub-band calibraton errors and
alignment in a hybrid correlator Simulations and theorical analysis
of quantization effects and their effects in alignin sub-bands. (draft)
- Quantization of the FIR coefficents for the
two-stage ALMA digital filter Analysis of the bandshape of the
digital filter proposed for ALMA as a function of the number of bits
used in the tap weight representation.
All these designs have been extensvely simulated, including truncation
effects and finite coefficient precision. Design implementation is
currently in progress at the Bordeaux University using Altera Stratix
FPGA. The design is used in a filtercard that will be included in the
baseline ALMA correlator.
See also the Digital filter activity page.
Simulations of the "analytic approach" filter. From top to bottom, left
to right we have: a) the input signal, composed of stochastic noise plus
three monochromatic lines; b) the undecimated output of the
first (broad) filter; c) The same, after decimation; d) The output of
the complex mixer; e) The signal after the second (sharp) filter; f) The
real output. All vertical scales are in dB. Click on the images to
display them full scale.
- G. Comoretto: A Digital BBC for the ALMA Interferometer
ALMA memo n. 305. This report deals with general quantization effects
for a digital LO with a few-bit representation.
- G. Comoretto: Possible architectures for an hybrid
Arcetri internal report n.8/2000. Some
alternative architectures for a hybrid correlator are presented and
compared in term of resource usage.
- B. Quertier, G. Comoretto, A. Baudry, A. Gunst, A. Bos:
Enhancing the Baseline ALMA Corrrelator Performance with the Second
Generation Correlator Digital Filtercard
ALMA memo n. 476. A filtercard with 32 digital filters is described.
- P.Camino, B. Quertier, A.Baudry, G.Comoretto, D.Dallet:
The new 3-stage, low dissipation digital filter of the ALMA Correlator
ALMA memo n. 579. An alternative architecture for the ALMA filterboard,
with lower power consumption, is described.
- G. Comoretto: Algorithms and formulas for hybrid correlator data reduction
- Alma memo 583