Chapter2 HardwareOverview
NIPXI-562x User Manual 2-4 ni.com
Incorporating the DDC
Optionally, you can route the data through the DDC before storing it in
onboard memory.
The DDC is a digital signal processing (DSP) chip, the Intersil
HSP50214B. The firststag eu ses a digital quadrature mixer that shifts a
signal to baseband from anyfrequ encywithin t her ange of the digitizer.
The nextstage decimates (reduces the sample rate) by an integer from
4–16,384.A series of programmable digital lowpassfilters prior to each
stageof decimation prevents aliasing when the sample rate is reduced. You
canretrieve the decimated data as in-phase and quadrature, or as phase and
magnitude. A discriminator allows you to take the derivativeof the phase
to demodulate an FM signal.
Bymixing, filtering, and decimating the sampled data, the DDC allows you
tozoom in on a band of frequ encies much narrowerthan the Nyquist band
oft he ADC. The lowersample rat em eans that signals of longer duration
canbe stored in the same amount of memory. For spectral analysis, you can
usea smaller, faster FFT to look at only the band passed through the DDC.
Referto the NI-SCOPE VI Reference Help for specific DDC attributes you
can uset o program your NI 562x. Formore information on using the
onboard DDC with LabVIEW,refer to the online help included with
NI-SCOPEand t he Spectral Measurements Toolsetsoftware.
Storing Data in Memory
Samples are acquired into onboard memory on the NI 562xbefore 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 bufferthat is
continuallyrewritten until a trigger is received. After the trigger is received,
theNI 56 2xcontinuesto acquire posttriggersamples if you have specified
ap osttrigger 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.