Incorporating the ddc, Storing data in memory, Incorporating the ddc -4 storing data in memory -4 – National Instruments NI PXI-562X User Manual
Page 18: Storing, Data in memory
NI PXI-562x User Manual
Incorporating the DDC
Optionally, you can route the data through the DDC before storing it in
The DDC is a digital signal processing (DSP) chip, the Intersil
HSP50214B. The first stage uses a digital quadrature mixer that shifts a
signal to baseband from any frequency within the range of the digitizer.
The next stage decimates (reduces the sample rate) by an integer from
4–16,384. A series of programmable digital lowpass filters prior to each
stage of decimation prevents aliasing when the sample rate is reduced. You
can retrieve the decimated data as in-phase and quadrature, or as phase and
magnitude. A discriminator allows you to take the derivative of the phase
to demodulate an FM signal.
By mixing, filtering, and decimating the sampled data, the DDC allows you
to zoom in on a band of frequencies much narrower than the Nyquist band
of the ADC. The lower sample rate means that signals of longer duration
can be stored in the same amount of memory. For spectral analysis, you can
use a smaller, faster FFT to look at only the band passed through the DDC.
Refer to the NI-SCOPE VI Reference Help for specific DDC attributes you
can use to program your NI 562x. For more information on using the
onboard DDC with LabVIEW, refer to the online help included with
NI-SCOPE and the Spectral Measurements Toolset software.
Storing Data in Memory
Samples are acquired into onboard memory on the NI 562x before being
transferred to the host computer. The minimum size for a buffer is
approximately 256 samples although you can specify smaller buffers in
software. When specifying a smaller buffer size, the minimum number
of points are still acquired into onboard memory, but only the specified
number of points are retrieved into the host computer memory.
During the acquisition, samples are stored in a circular buffer that is
continually rewritten until a trigger is received. After the trigger is received,
the NI 562x continues to acquire posttrigger samples if you have specified
a posttrigger sample count. The acquired samples are placed into onboard
memory. The number of posttrigger or pretrigger samples is limited only by
the amount of onboard memory.