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(shift in frequency), decimation,
equalization, and calibration. The
resulting narrowband signal then gets
demodulated and decoded, further
filtered, amplified, and stored to HDD
or any combination of these functions.
At a high level, typical streaming
and channelizing applications can be
divided as figure 2 shows.
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Wideband Streaming
To better understand the challenges
connectedwithwideband streaming,
one must first understand the
technical specifications of the IF
receiver. This paper focuses on the
PXIe-5624R module. IF receivers
are typically part of the vector
signal analyzer that comprises
the mixer, IF receiver, and signal
sources for LO. The architecture of
the example vector signal analyzer
is described in the Introduction to
the PXIe-5668R—High-Performance
26.5 GHz Wideband Signal Analyzer
white paper.
IF is characterized by the frequency
range from 5 MHz to 2 GHz and
bandwidth of 800 MHz typical
(see technical specs for details).
After adding a band-limited noise
(dither) signal, which helps reduce
the quantization effects of the ADC
and improve spectral performance,
the ADC samples the signal at up
to 2 GSa/s with 12-bit resolution.
The onboard FPGA processes
these samples and transfers the
data to other devices (PXI Express
controller, RAID) through the PCI
Express Gen 2x8, which allows for
data streaming with theoretical rates
of up to 4 GB/s. In the wideband
streaming case, the FPGA performs
only one digital downconversion
(DDC) for all incoming data, as
opposed to several independent
downconversions in the narrowband
case as mentioned later in the
document.
Figure 1.
Example Signals in 5 MHz–2 GHz Band
Figure 2.
Classification of Streaming Applications Covered in This Document
Bit Packing
When talking about wideband
streaming, one must consider
not only the theoretical available
bandwidth of the PCI Express bus
but also its practical limitations (that
is, control messages that travel over
the same bus). The first and more
simple implementation for sending
data over the PCI Express bus would
be to send 16-bit samples, one after
another, even if data from the ADC is
only 12 bit. However, this approach
leads to theoretical limitations of 4
GB/s per PCI Express link available
in the PXIe-5624R module (2
bytes/samples at 2 GS/s equals to
4 GB/s), which practically won’t
allow for continuous streaming.
However, there’s a clever solution:
bit packing. Using bit packing, four
12-bit samples are packed into
three 16-bit words. Consequently,
this method reduces the data rate
from 4 GB/s to 3 GB/s, enabling
continuous data streaming.
Intermodules
Synchronization
Often there is a need for continuous
streaming from several modules of
the same type. These multichannel,
synchronized RF systems enable certain
applications such as direction finding.
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