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271
Users Manual
Introduction
This section is a general introduction to the features and organization of the function
generator, and is intended to be read before using the instrument for the first time.
Detailed operation is covered in later sections starting with chapter 5, Main Generator
Operation.
DDS Principles
Waveforms are generated by direct digital synthesis (DDS). One complete cycle of the
waveform is stored in RAM as 1024 10-bit amplitude values. As the RAM address is
incremented, the waveform values are output to a digital-to-analogue converter (DAC)
which reconstructs the waveform.
Sine waves and triangles are subsequently filtered to smooth the steps in the DAC output.
The frequency of the waveform is determined by the rate at which the RAM addresses
are changed. Further details of how this rate is varied, i.e. how the frequency is changed,
are given later in the DDS Operation section; it is normally sufficient to know that at low
frequencies the addresses are output sequentially but at higher frequencies the addresses
are sampled.
The major advantages of DDS over conventional analogue generation are:
In addition, being a digital technique, it is easier to make every parameter programmable
from the keyboard, or remotely via RS232 or GPIB interfaces.
The fundamental limitation of the DDS technique is that as the generator frequency is
increased, each waveform cycle is constituted from fewer samples. This is not a problem
with sine waves which, because they are filtered, can be produced with low distortion up
to the frequency limit of the generator.
However with DDS square waves and pulse waveforms the uncertainty of one clock edge
sets a practical limit to the upper frequency. On this instrument the generation technique
changes at 30kHz (this limit can be overridden by the user) to make use of a comparator
driven by the DDS sine wave to ensure jitter-free square waves and pulses up to the
frequency limit of the generator.
Ramp and staircase waveforms are, by default, unfiltered (although filtering can be
selected) and therefore become degraded above the upper limit frequencies given in the
specification; all waveforms are, however, available up to the maximum frequency of the
generator.
Switching On
The power switch is located at the bottom left of the front panel.
At power up the generator displays the installed software revision whilst loading its RAM
with waveforms; if an error is encountered the message "SYSTEM RAM ERROR,
4-2
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Frequency accuracy and stability are those of the crystal oscillator.
1.
2. Frequencies can be set with high resolution from mHz to MHz.
3. The method delivers low phase noise and low distortion.
4. Very wide frequency sweeps are possible.
5. Fast, phase continuous frequency switching.
6. Non-standard waveforms such as multi-level square waves are easily generated.
7. Basic arbitrary waveform capability in the same instrument.
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