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Chapter 2 Hardware Overview
NI 5620 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 first stage uses a digital quadrature mixer that shifts
a signal to baseband from any frequency within the digitizer’s range.
The next stage decimates (reduces the sample rate) by an integer from 4
to 16384. A series of programmable digital lowpass filters prior to each
stage of decimation prevents aliasing when the sample rate is reduced.
The decimated data may be retrieved 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 canbe storedin thesame amountof memory. For spectral analysis,
a smaller, faster FFT may be used 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 5620. If you installed the included
measurement software, there is also online help for LabVIEW 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 buffer that is
continually rewritten untila trigger isreceived. Afterthe trigger isreceived,
the NI 5620 continues to acquire posttrigger samples if you have specified
a posttrigger sample count. The acquired samples are placed into onboard
memory.The number ofposttrigger or pretrigger samples is limited only by
the amount of onboard memory.