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ABC's of Waveform
AWG - How it works
In an arbitrary waveform generator, you define the waveform, using either the standard functions or custom profile data files to load into waveform memory. A set of
start and stop addresses that corresponds to a group of waveform memory locations is assigned a waveform number. The address generator sequentially presents data values
from each memory location to the digital-to-analog converter (DAC). The precision DAC converts the data into analog voltage values. This series of sequential voltage
levels describes the output waveform with the frequency determined by the sample clock rate divided by the number of samples in the waveform. Changing the sample clock
rate causes the address generator to change the speed at which the data is presented to the DAC, thereby changing the output frequency.
The waveforms you create are a series of data points consisting of X- and Y-axis values. For 16-bit generators, such as the Pragmatic 2711A, the Y values range between
+32767 and -32768. For 12-bit generators, such as the Pragmatic 2714A, the Y values range between +2047 and -2048. In describing the first point, '0' is given a Y
value. The next point has another Y value, and so on through the last address in your waveform. This series of points makes up the wave shape. With the large waveform
memories typical of Pragmatic generators at least 100 non-volatile waveforms may be stored.
AWG Block Diagram
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Sample Clock Generator
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Address Generator
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Waveform Memory (RAM)
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Digital-to-Analog Converter (DAC)
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Output Amplifier
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AWG - Waveform Cycle
All the data points in the specified waveform memory location make up one waveform cycle. The waveform generator will output all the points in the waveform at the
sample clock rate specified. The resulting frequency is equal to the sample clock rate divided by the number of data points in the waveform. If multiple cycles of the
waveform are entered into the same waveform memory location, the output frequency will increase proportional to the number of cycles in memory. For example, if you
create a triangle wave having three cycles using the same number of data points and the same sample clock as in the example above, the frequency will be three times
higher.
Waveform Frequency = (Fs / L) x (N)
| Fs: Sample Clock Rate (Hz) |
| L: Waveform Length (points) |
| N: Number of Cycles in One Waveform Frame |
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AWG - Sequence Generator
One of the more powerful options in an AWG is the sequence generator. A sequence is a stored program which sequentially selects (linking) stored waveforms and repeats
(looping) the waveform from 1 to over 1,000,000 times. The process begins by selecting a waveform, repeating this waveform a specified number of times. Then, the
process continues synchronously generating the next waveform a specified number of times. Each step in the sequence specifies a waveform and the number of repetitions.
This process continues until all steps have been completed. At this point the sequence generator may terminate or continue depending on the mode which may be single
cycle or continuous.
This additional dimension of the AWG offers virtual memory expansion up to 100,000,000 times. Pragmatic offers this option
in a majority of the AWG product line. Only traditional AWGs using the conventional, sequential address scheme can offer sequence generator capability. This feature is
not available in an AWG using direct digital synthesizer (DDS) based waveform memory addressing.
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