Fluke DDS Function Generator 271, User Manual

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271
Users Manual
5-10
to a very low level. With the default condition of
FILTER=AUTO
set on the
OPT
ion
menu, the filter is switched in automatically for sine, triangle, high frequency square
wave and high frequency pulse waveforms (although the square and pulse waveforms
themselves are not passed through the filter); the filter is automatically switched out for
low frequency square and pulse waveforms, ramps, staircases and arbitrary waveforms
because of the degrading effect it has on fast transitions. However, for all these
waveforms the filter can be set to be always on (
FILTER=ON
) or always off
(
FILTER=OFF
); this has the advantage that, for example, an arbitrary waveform with an
essentially sinusoidal content can be output with the filter on.
When
NOISE
is selected (see chapter 10,
Special Waveforms
), this 7-stage filter is
always off, whatever the
FILTER=
setting, and a simple 700 kHz low pass RC filter is
switched in instead.
Auxiliary Output
When sine, triangle, square wave or pulse waveforms are selected and with
AUX=AUTO
the auxiliary output square wave generation switches automatically at 30 kHz from DDS
generation to a signal derived from a comparator driven by the DDS sine wave; the
advantages of this approach are the same as those explained earlier. However, as
explained above in the
Auxiliary Output Phase
section, the high frequency generation
mode has the disadvantage that a phase difference can no longer be set between
AUX OUT
and
MAIN OUT
. The automatic switchover at 30 kHz can therefore be
overridden by setting
AUX=LOFRQ
uency, to maintain it in true DDS mode, or
AUX=HIFRQ
uency to lock it in high frequency mode. With
AUX=AUTO
there is no
automatic mode changeover if ramp, staircase or arbitrary waveforms are selected; high
frequency mode can however be forced by setting
AUX=HIFRQ
.
Note that there is some second order interaction between the square wave generation,
filter and auxiliary output settings which demands a little thought before deviations from
the default conditions are applied. For example, if
SQWAVE GEN
and
AUX
options
are set to
AUTO
but
FILTER
is set to
OFF
the edges of both the
MAIN OUT
and
AUX OUT
square waves will exhibit some jitter at high frequencies (e.g. 1 MHz)
because the sine wave driving the comparator from which both are derived will itself be
subject to some jitter.
Frequency Stop
In the default mode of
FSTOP=OFF
there are no frequency limits on any waveform and
the frequency and waveform can be set as described in the
Main Generator
section;
waveform quality will however deteriorate progressively as the frequency increases for
certain waveforms, as discussed in the
DDS Principles
section.
With
FSTOP=ON
the maximum settable frequency for triangle, ramp, staircase and
arbitrary is limited to 100 kHz. An error message will be shown if an attempt is made to
enter a frequency above 100 kHz whilst one of these waveforms is selected, or if an
attempt is made to select one of these waveforms with the frequency already set above
100 kHz. This mode is useful in ensuring that frequencies are not accidentally set too
high for waveforms whose quality will deteriorate above 100 kHz, the frequency to
which their specifications apply.
Trigger/Sweep Output
With
SWEEP/TGEN=AUTO
the function of the rear panel
TRIG/SWEEP OUT
socket
changes automatically when the operating mode is changed between sweep, hop and any
other mode; the two functions of this output are described in chapter 3,
Connections
.
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.com