Evaluation Engineering - 20

AUTOMATED TEST

HARNESSING COTS
MODULAR POTENTIAL
How COTS, single-slot, PXIe NVMe SSD RAID
modules enable compact, wideband, modular
RF record, playback, and analysis systems
by Ross Q. Smith
Wideband RF RPA systems are widely
used in commercial, aerospace, and
defense markets. They digitize and store
RF data in I/Q format (typically 16-bit/16bit), support real-time and non-real-time
analysis of the data, and, in some cases,
convert it back to RF (playback) to stimulate devices under test (DUTs).
Wideband RF RPA systems are very
data-transfer- and storage-intensive. A
rule of thumb for data rate requirements
for RF RPA systems is 5 bytes/second of
I/Q data (16/16) per Hz of bandwidth. So,
a single channel (1-CH), 500 MHz bandwidth wideband RF RPA system must sustain 2.5 GB/sec write and read speeds for
the duration of the recording or playback
session. At a 2.5 GB/sec data rate-which
is about 10x the performance of a single
SATA III 6gbps HDD-the data storage
capacity requirements get very big, very
quickly. The 500 MHz bandwidth 1-CH RF
RPA system described above would need
2.5 GB/sec x 60 sec/min x 60 min/hour =

9 terabytes (TB) of storage to support just
one hour of recording duration. The required data rates and capacities increase
linearly with the number of channels and
the bandwidth per channel in the system.

RAID is essential for
wideband RF RPA
To sustain the multiple GB/sec data rates
and multiple TB capacities required, RF
RPA systems employ a redundant array
of inexpensive disks (RAID) storage subsystems that combine multiple HDDs or
SSDs operating in parallel to improve
storage performance and capacity. With
RAID level 0, data is striped across the
drives, which produces a nearly linear
increase in read/write performance
and capacity. However, RAID level 0
can also be somewhat risky since the
failure of a single drive can result in the
complete loss of data. Other RAID levels
(2, 5, 10, etc.) provide redundancy and
error correction, albeit at the expense

RADX' LibertyGT LGT1288 COTS 8-CH Wideband
RF RPA System with 4 x PXIe RAID Modules.

of throughput, capacity, SWaP, and additional cost.
Historically, the most popular interface for "inexpensive" HDDs and SSDs
has been SATA III, which supports a 6
Gbps data rate. SATA III HDDs can typically support a read/write data rate of
about 200 MB/sec, while SATA III SSDs
can typically support a rate of about 400
to 500 MB/sec. To achieve the 2.5 GB/
sec RF RPA system data rate described
earlier would notionally require a RAID
0 subsystem with 12 or more SATA III
HDDs, or six or more SATA III SSDs. And,
since most HDDs or SSDs are in a 2.5 or
3.5-inch form factor, a wideband RF RPA
RAID subsystem would physically require
a separate 1U to 4U rackmount subsystem
that cables to the RF RPA system via a

RF Record, playback and analysis (RF RPA)
systems require about 5 bytes per second for each
Hz of bandwidth: 500 MHz ÔÇÆ 2.5 GB/sec
RADX

EVALUATION ENGINEERING AUGUST 2019

Evaluation Engineering

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