Further Waveform Considerations - Metrix GX1010 Notice De Fonctionnement

values. As the RAM address is incremented the waveform values are output sequentially to
a Digital-to-Analogue Converter (DAC) which reconstructs the waveform as a series of
voltage steps. Sinewaves and triangles are subsequently filtered to smooth the steps in the
DAC output.
The frequency of the output waveform is determined by the rate at which the RAM
addresses are changed; in a DDS system the address changes are generated as follows.
The RAM contains the amplitude values of all the individual points of 1 cycle (360°) of the
waveform; each sequential address change corresponds to a phase increment of the
waveform of 360°/1024. Instead of using a counter to generate sequential RAM addresses,
a phase accumulator is used to increment the phase.
On each clock cycle the phase increment, which has been loaded into the phase
increment register by the CPU, is added to the current result in the phase accumulator;
the 10 most significant bits of the phase accumulator drive the RAM address lines. 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 but with phase continuity.
38 -4
The generator uses a 38-bit accumulator and a clock frequency which is 2 x 10
(~27·487MHz); this yields a frequency resolution (corresponding to the smallest phase
38
increment) of f /2 = 0·1mHz.
CLK
Only the 10 most significant bits of the phase accumulator are used to address the RAM.
At a waveform frequency of f /1024 (~26·84kHz), the 'natural' frequency, the RAM
CLK
address increments on 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
waveform 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.
The minimum number of points required to accurately reproduce a waveshape will
determine the maximum useful output frequency:
fmax = f /No. of points
CLK
For sinewaves the filter permits the waveform to be reproduced accurately up to the
Nyquist limit (f /2), although in this generator a practical limit of 10MHz is set.
CLK

6.5.2. Further Waveform Considerations

The various limitations on combinations of modes, mostly already mentioned in the
appropriate operational sections of the manual, are brought together here and explained
with reference to the simplified block diagram below.
GX1010 II - 57