FLUKE 284-U User Manual for the Fluke 284-U Arbitrary Waveform Generator and Manager, 40 MS/s, 4 Channel

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281, 282, 284

Users Manual

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Figure 4-2. Clock Synthesis Mode

DDS Mode

In DDS mode all waveforms are stored in RAM as 4096 points. The frequency of the

output waveform is determined by the rate at which the RAM addresses are changed. The

address changes are generated as follows:

The RAM contains the amplitude values of all the individual points of one cycle (360 º)

of the waveform; each sequential address change corresponds to a phase increment of the

waveform of 360/4096 degrees. Instead of using a counter to generate sequential RAM

addresses, a phase accumulator is used to increment the phase.

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Figure 4-3. Direct Digital Synthesis Mode

On each clock cycle the phase increment, which has been loaded into the phase increment

significant bits of the phase accumulator drive the lower 12 RAM address lines, the upper

4 RAM address lines being held low. The output waveform frequency is now determined

by the size of the phase increment at each clock. If each increment is the same size then

the output frequency is constant; if it changes, the output frequency changes as in sweep

mode.

The generator uses a 38 bit accumulator and a clock frequency which is 2

x 10

-4

(approximately 27·4878 MHz); this yields a frequency resolution of 0·1 mHz.

Only the 12 most significant bits of the phase accumulator are used to address the RAM.

At a waveform frequency equal to the clock frequency divided by 4096, approximately

6·7 kHz, the natural frequency, the RAM address increments at every clock. At all

frequencies below this (i.e. at smaller phase increments) one or more addresses are output

for more than one clock period because the phase increment is not big enough to step the

address at every clock. Similarly at frequencies above the natural frequency the larger

phase increment causes some addresses to be skipped, giving the effect of the stored

waveform being sampled; different points will be sampled on successive cycles of the

waveform.

4-6

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FLUKE 284-U Arbitrary Waveform Generator and Manager, 40 MS/s, 4 Channel