National Instruments DAQArb 5411 User Manual

Also see for DAQArb 5411: Getting started guide

Contents

National Instruments DAQArb 5411 User Manual Manual pdf 35 page image

Chapter 4 Arb OperationDAQArb 5411 User Manual4-10 ©National Instruments CorporationFrequency Resolution and Lookup MemoryFor DDS-based waveform generation, you must first load one cycle ofthe desired waveform into the lookup memory. The size of the DDSlookup memory is 16,384 samples. Each sample is 16 bits wide.Note: One cycle of the waveform buffer loaded into the memory should be exactlyequal to the size of the DDS lookup memory.F c = update clock for the accumulatorSet the DAQArb 5411 at F c = 40 MHz.F a = desired frequency of the output signalN = accumulator size in bitsSet the DAQArb 5411 at N = 32.FCW = frequency control word to be loaded into the accumulatorto generate F a .This is calculated using the formula:FCW = (2 N * F a ) / F cThe frequency resolution is then given by:frequency resolution = F c / 2 N = (40 x 10 6 ) / 2 32 = 9.31322 mHzFor example, if you need to generate a frequency of 10 MHz, then theFCW is (2 32 * 10E6)/40E6, which equals 1,073,741,824. If you need togenerate a frequency of 1 Hz, then the FCW is (2 32 * 1)/40E6, whichequals 107.Note: On the DAQArb 5411, the maximum frequency of a sine wave you cangenerate reliably is limited to 16 MHz. Other waveforms like square ortriangular waves are limited to 1 MHz.You can also synthesize arbitrary waveforms using DDS. Generatingarbitrary waveforms this way will be very limited; you are restricted toa single buffer, and this buffer should be exactly equal to the size of thelookup memory.To update every next sample of an arbitrary waveform in lookupmemory at the maximum clock rate of 40 MHz, write an FCW value of2 (N-L) , where N is the size of the accumulator and L is the number ofaddress bits of lookup memory (L = 14 bits for the AT-5411 and thePCI-5411). Thus, the FCW value for the DAQArb 5411 equals 262,144.