MMA Memo Abstract Template
 

MMA Memo Abstract Template


# abstract.template -- MMA Memo Series Electronic Abstract Template
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# Comments are preceded by the "#" symbol.  Anything following a "#" is
# ignored.
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# The following entry is for internal use only.  Do not fill it in.
#Number:


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# Enter the MMA memo title in the following section
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#Title:
A Digital BBC for the ALMA Interferometer

#
# List the MMA memo authors in the order that they appear on the paper.
# Use a comma to separate the names of multiple authors.  List each author's
# affiliation in parenthesis following the name.
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#Authors:

G. Comoretto (Osservatorio di Arcetri - Florence, Italy)

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# Enter the date that the memo was submitted in the format mm/dd/yy.
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#Date:
04/26/00

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# Enter a list of keywords which might be used to find this memo in an
# electronic search.  Multiple keywords should be separated by commas.
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#Keywords:
digital filter, quantization, DDS

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# Enter a plain ASCII abstract between here and the #END symbol
# 
#Abstract:
A digital baseband converter is proposed for the ALMA correlator.
The digital local oscillator/mixer can be implemented with lookup
tables, and increases only slightly the complexity of the digital filter
board.

Sideband rejection can be implemented either in the filter section, at
the expense of a roughly doubled number of filter taps, or adopting a
complex correlator. Several approaches are possible, that impact to a
various extent the existing correlator architecture. In the simplest
approach, only the memory board needs to be modified.

The digital nature of the mixer causes a loss in signal to noise ratio
of a few percent. More important is the presence of LO harmonics, that
create ghost images of the input lines. Spur level depends mainly on the
number of bits in the data path between the mixer and the filters. For
a 3-bit mixer, a spur level around -31 dBc (decibel with respect to the
fundamental) can be obtained. To achieve a -38 dBc spur level, a 4 bit
mixer is required.
#END