?? direct digital synthesis (dds) mode (ni 5411-5431) - tutorial - development library - national instruments.htm
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precise hardware timing to switch between multiple waveforms, you
should use <B><U>Arb Mode</U></B> instead.
<P>
<P>The main features of DDS mode are:
<P>
<P>1. On-board memory for a single waveform with a length of exactly
16384 16-bit samples. (Analog output has a resolution of 12 bits,
taking the 12 most significant bits of each 16-bit sample.)
<P>
<P>2. Very fine waveform frequency resolution: 9.31 mHz.
<P>
<P>3. Two instructions per stage: waveform frequency, and the time
for which that frequency should be generated. Up to 512 stages may
be stored in DDS mode. (Click here for more information on
<B><U>stages and stage lists</U></B>).
<P>
<P>DDS mode can generate sine waves with frequencies up to 16 MHz on
the NI 5411 and 8 MHz on the NI 5431. The SYNC output may also be
used to produce TTL clock signals at these frequencies. DDS mode can
produce square, sawtooth, or triangle outputs at frequencies up to 1
MHz.
<P>
<P>An on-board FIFO stores up to 512 stages in DDS mode. Each stage
consists of two instructions: a frequency and time. When output
begins, the board begins to generate the waveform at the frequency
given in the first stage. It continues to do so for the length of
time specified in the first stage, and then changes the frequency to
that indicated in the second stage. Exactly how the output changes
from one stage to the next depends on which of four <B><U>triggering
modes</U></B> is being used. The figure below shows several stages
of a sine wave produced in DDS mode. Each stage is defined by a
frequency (f) and duration (<FONT face=Symbol>D</FONT>T). Also, note
that there are no phase discontinuities when the frequency changes.
<P>
<P><IMG height=122 alt=""
src="Direct Digital Synthesis (DDS) Mode (NI 5411-5431) - Tutorial - Development Library - National Instruments.files/0.gif"
width=504>
<P>
<P>NOTE: It is important to remember that the duration of each stage
is specified as a time (in seconds). The duration is not specified
as a number of waveform cycles or iterations. If you need to
generate a specific number of cycles, you will need to calculate how
long this will take based on the frequency for that stage.
<P>
<P>DDS mode can be used to sweep over a range of frequencies. One
application for a frequency sweep could be generating a Bode plot to
record the frequency response of an external system. You can
approximate a continuous sweep by using a large number of stages and
making small frequency changes from one stage to the next. The
minimum time for any stage is two microseconds, allowing the board
to change frequencies at rates up to 500 kHz. <BR></P><BR><FONT
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