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Superior Data Quality (SFDR)
A key reason for the popularity of disk
recording for SIGINT applications is its outstanding data quality. For example,
Avalon disk recorders offer spurious free dynamic range (SFDR) results that are
far beyond anything that would typically be possible using magnetic tape.
Figures 1 and 2 (below) offer a direct comparison between the SFDR of a typical
12 MHz S-VHS tape recorder and that of a 12 MHz Avalon AE8222 disk
recorder. They demonstrate that disk recording offers an improvement of at
least 20dB over tape recording before even considering other system enhancements
such as DSP-based IF/baseband conversion. It would be hard to overstate the
importance of this quantum leap in performance for applications requireing to
detect low-level signals that would previously have been lost in the tape
recorder’s noise floor.
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| Figure 1: SFDR measurement
from a typical 12 MHz S-VHS tape recorder
(<40 dB). |
Figure 2: SFDR measurement
from a 12 MHz AE8222 baseband disk recorder (>63dB) |
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Wider Bandwidth
The bandwidth of S-VHS tape recorders
(the workhorse of SIGINT recording for many years) has evolved over time from 6
MHz, thru 8 and 12 MHz and finally to 18 MHz. But these increases came at the
cost of recording duration and signal quality. In 1998, Avalon introduced its
first 50 MHz disk recorder. This recorded to a group of eight computer hard disks
(HDDs) and became an immediate success in a range of critical SIGINT
applications. The maximum practical bandwidth of a disk recorder is no longer
media related since it is technically possible to write to as many parallel
disks as the bandwidth may require. So today, the pacing consideration is
the availability of suitable COTS analog/digital converters, digital/analog
converters and fast DSP processors.
Fortunately, COTS components now support the digitization and down-conversion of
Intermediate Frequencies (IFs) of up to 160 MHz and signal bandwidths as high as
80 or 100
MHz. It will soon be possible to make sustained recordings of a 1 GHz IF
at bandwidths of up to 500 MHz bandwidth using the same basic techniques.
Figures 3 and 4 are taken from a typical
Avalon 100 MHz baseband recording channel, demonstrating the level of
performance that is possible today.
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| Figure 3: SFDR measurement
from a typical 100 MHz Avalon baseband channel recording a single 1 MHz
tone (>63 dB). |
Figure 4: SFDR measurement
from a typical 100 MHz Avalon baseband channel recording a pair of tones
at 88.5 MHz and 91.5 MHz(>65 dB). |
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