Chapter2 HardwareOverview
NI5620 User Manual 2-4 ni.com
Incorporating the DDC
You may optionally 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
as ignal to baseband from any frequencyw ithin the digitizer’srange.
The nextstage decimates (reduces the sample rate) by an integer from 4
to 16384. A series of programmable digital lowpassfilters prior to each
stage ofd ecimationprevents aliasing when the sample rate is reduced.
The decimated data may beretrieved as in-phase and quadrature, or as
phase and magnitude. A discriminator allows you to take the derivative
oft he phase to demodulate an FM signal.
By mixing, filtering, and decimating the sampled data, the DDC allows
you to zoomin on a band of frequencies much narrowerthan the Nyquist
band of the ADC. The lowers ample rate means that signals of longer
durationcan be stored in the same amount of memory. For spectral analysis,
asmall er,faster FFT may be used to look at only the band passed through
the DDC.
Refert o the NI-SCOPE VI Reference Help for specific DDC attributes
you can use to program your NI 5620. If you installed the included
measurements oftware,there is also onlinehelp for LabV IEW users using
the DDC.
Storing Data in Memory
Samples are acquired into onboard memory on the NI 5620 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’s memory.
During the acquisition, samples are stored in a circular bufferthat is
continuallyrewritten until a trigger is received. After the trigger is received,
theNI 5620continues to acquire posttrigger sam ples if you havespecified
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.