Functional Description; General; Dds Principles; Switching On - Metrix GX1010 Notice De Fonctionnement

4.

FUNCTIONAL DESCRIPTION

This section is a general introduction to the features and organisation of the function
generator intended to be read before using the instrument for the first time. Detailed
operation is covered in later sections starting with Main Generator Operation.
4.1.

General

4.1.1. DDS Principles

In this instrument 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. Sinewaves 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 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:
•
Frequency accuracy and stability is that of the crystal oscillator.
• Frequencies can be set with high resolution from mHz to MHz.
• Low phase noise and distortion.
• Very wide frequency sweeps are possible.
• Fast phase continuous frequency switching.
• Non-standard waveforms such as multi-level squarewaves are easily generated.
• Basic arbitrary waveform capability in the same instrument.
In addition, being a digital technique, it is easier to make every parameter programmable
from the keyboard, or remotely via RS-232 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 sinewaves which, because they are filtered, can be produced with low distortion up to
the frequency limit of the generator. With DDS squarewaves and pulse waveforms the 1
clock edge uncertainty sets a practical limit to the upper frequency. However, on this
instrument the generation technique changes at 30kHz (but is overridable by the user) to
use a comparator driven by the DDS sinewave; this ensures jitter-free squarewaves 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 frequencies indicated in the Specification; all waveforms are, however, available
up to the maximum frequency of the generator.

4.1.2. 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,
BATTERY FLAT?" will be displayed, see the Warnings and Error Messages section.
Loading takes a few seconds, after which the Main menu is displayed, showing the
generator parameters set to their default values, with the MAIN OUT set off. Refer to the
System Menu section for how to change the power up settings to either those at power
down or to any one of the stored settings.
GX1010 II - 9