Agilent 33120A User's Guide

Chapter 7 TutorialSignal ImperfectionsThe maximum output frequency, with the condition that everywaveshape point in RAM is output every waveform cycle, is defined by:Fout = Fclk / PointsThe minimum number of points required to accurately reproduce awaveshape will determine the maximum useful output frequency usingthe same equation.The rule governing waveforms is referred to as the Nyquist SamplingTheorem, which states that you must include at least two points fromthe highest frequency component of the signal you are attemptingto reproduce.Signal ImperfectionsMost signal imperfections are easiest to observe in the frequencydomain using a spectrum analyzer. Sampling theory predicts thelocation and size of spurious signals resulting from the samplingprocesses used by DDS generators. In fact, since DDS generators use afixed sampling rate (40 MHz for the 33120A), spurious signals can beremoved with a fixed frequency “anti-alias” filter. A 17 MHz, ninth-orderelliptical filter providing a sharp cut-off (in excess of 60 dB attenuationfor signals greater than 19 MHz) is used for sine wave outputs. A 10MHz, seventh-order Bessel filter is used for non-sine wave outputs. TheBessel filter provides slower amplitude roll-off for anti-alias filtering,but maintains linear phase response to minimize shape distortion forcomplex waveshapes. The 33120A automatically selects the appropriatefilter when the output function is selected.All digital-to-analog converters, including those used in DDS generators,produce spurious signals resulting from non-ideal performance. Thesespurious signals are harmonically related to the desired output signal.At lower frequencies, the 33120A’s 12-bit waveform DAC producesspurious signals near the -74 dBc level (decibels below the carrier oroutput signal) as described by the equation on the following page.The 33120A uses the complete vertical resolution (N=1) of the DAC forall internal waveshapes, thus minimizing amplitude quantization error.276