National Instruments NI PXI-562X User Manual

Contents

National Instruments NI PXI-562X User Manual Manual pdf 18 page image

Chapter 2 Hardware OverviewNI PXI-562x User Manual 2-4 ni.comIncorporating the DDCOptionally, you can route the data through the DDC before storing it inonboard memory.The DDC is a digital signal processing (DSP) chip, the IntersilHSP50214B. The first stage uses a digital quadrature mixer that shifts asignal to baseband from any frequency within the range of the digitizer.The next stage decimates (reduces the sample rate) by an integer from4–16,384. A series of programmable digital lowpass filters prior to eachstage of decimation prevents aliasing when the sample rate is reduced. Youcan retrieve the decimated data as in-phase and quadrature, or as phase andmagnitude. A discriminator allows you to take the derivative of the phaseto demodulate an FM signal.By mixing, filtering, and decimating the sampled data, the DDC allows youto zoom in on a band of frequencies much narrower than the Nyquist bandof the ADC. The lower sample rate means that signals of longer durationcan be stored in the same amount of memory. For spectral analysis, you canuse 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 youcan use to program your NI 562x. For more information on using theonboard DDC with LabVIEW, refer to the online help included withNI-SCOPE and the Spectral Measurements Toolset software.Storing Data in MemorySamples are acquired into onboard memory on the NI 562x before beingtransferred to the host computer. The minimum size for a buffer isapproximately 256 samples although you can specify smaller buffers insoftware. When specifying a smaller buffer size, the minimum numberof points are still acquired into onboard memory, but only the specifiednumber of points are retrieved into the host computer memory.During the acquisition, samples are stored in a circular buffer that iscontinually rewritten until a trigger is received. After the trigger is received,the NI 562x continues to acquire posttrigger samples if you have specifieda posttrigger sample count. The acquired samples are placed into onboardmemory. The number of posttrigger or pretrigger samples is limited only bythe amount of onboard memory.