The maximum output frequency, with the condition that every
waveshape point in
RAM is output every waveform cycle, is defined by:
F
out = Fclk / Points
The minimum number of points required to accurately reproduce a
waveshape will determine the maximum useful output frequency using
the same equation.
The rule governing waveforms is referred to as the Nyquist Sampling
Theorem
, which states that you must include at least two points from
the highest frequency component of the signal you are attempting
to reproduce.
Signal Imperfections
Most signal imperfections are easiest to observe in the frequency
domain using a spectrum analyzer. Sampling theory predicts the
location and size of spurious signals resulting from the sampling
processes used by
DDS generators. In fact, since DDS generators use a
fixed sampling rate (40 MHz for the 33120A), spurious signals can be
removed with a fixed frequency “anti-alias” filter. A 17 MHz, ninth-order
elliptical filter providing a sharp cut-off (in excess of 60 dB attenuation
for signals greater than 19 MHz) is used for sine wave outputs. A 10
MHz, seventh-order Bessel filter is used for non-sine wave outputs. The
Bessel filter provides slower amplitude roll-off for anti-alias filtering,
but maintains linear phase response to minimize shape distortion for
complex waveshapes. The 33120A automatically selects the appropriate
filter 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. These
spurious signals are harmonically related to the desired output signal.
At lower frequencies, the 33120A’s 12-bit waveform
DAC produces
spurious signals near the -74 dBc level (decibels below the carrier or
output signal) as described by the equation on the following page.
The 33120A uses the complete vertical resolution (N=1) of the
DAC for
all internal waveshapes, thus minimizing amplitude quantization error.
Chapter 7 Tutorial
Signal Imperfections
276
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