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For
many years, most SIGINT data was captured and analyzed in the analog domain. The
receiver's IF was down-converted in an external analog mixer and the resulting
'baseband' signal was recorded on a multi-channel analog tape recorder at a
bandwidth of maybe 2 or 4 MHz. Typically, the replayed signal was analyzed in the
analog domain. Then came new classes of tape recorder that processed (and maybe
recorded) the analog signal as digital samples. But wideband digital tape recorders
tended to be very costly so the less expensive hybrid technology of S-VHS tape
recorders resulted in these becoming the product of choice for many SIGINT
applications around the world. Eventually, the bandwidths,
performance and features required of data recorders by the user community let to
the introduction of a new generation of system that used hard disk drives rather
than magnetic tape as their storage medium. Suddenly it was possible to
extend bandwidths to 50 MHz and beyond, while SFDR performance jumped from
around 40 dB to >60 dB. Interfacing to computer networks also became
much simpler. Meanwhile, as mainframes and
software programs became faster and more sophisticated SIGINT agencies turned increasingly to digital analysis techniques.
It is interesting
to note that many of the technologies that underpinned the growth of digital
analysis were equally important to the new generation of disk-based data
recorders. During the past few years we have seen a fusion
of these various strands:
-
direct sampling of IF frequencies of up to 160 MHz at
bandwidths of up to 80 MHz
-
direct digital analysis and display of these signals
as they are recorded
-
fast DSP-processing to down-convert the IF to
baseband for recording
-
replay at full bandwidth in the analog domain at
baseband or IF for 'quick look' or analysis
-
exporting the data as digital files to a high-end
workstation for detailed analysis
Unlike their tape-based forebears, disk recorders are
generally easy to integrate into data capture and networked computer
environments, thereby simplifying data collection, distribution, analysis and
archiving.
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