Fluke DDS Function Generator 271, User Manual, Page: 118 / 132

271
Users Manual
B-4
3. By default, ramp, staircase, arbitrary and all waveforms whose symmetry is set to
other than 50% are unfiltered. It may be desirable to force the filter
ON
under some
circumstances to improve waveform quality, e.g. for higher frequency sine waves
that are only slightly asymmetric.
4. Similarly asymmetric HF square waves, pulses and auxiliary outputs generated from
the comparator will be improved if the filter is forced
ON
to filter the signal driving
the comparator.
5. When staircase, arbitrary or LF mode square waves or pulses are selected the
comparator is driven by the unfiltered main waveform. With all except square waves
it is possible to have a wave shape which never crosses the comparator threshold,
thus the HF auxiliary output may be permanently high or low. To avoid this situation,
the default (
AUTO
) AUX setting is LF mode; however, in this mode edge jitter will
become increasingly significant at higher frequencies.
6. Phase shift between the
MAIN OUT
and
AUX OUT
sockets at higher frequencies
(only possible by setting AUX to LF mode) will be different for those signals which
are unfiltered compared to those which, by default, have the filter in the signal path.
For example, HF square waves and pulses from the comparator will be further phase
shifted compared to LF mode square waves of the same frequency because the sine
wave driving the comparator is significantly delayed by the filter.
7. Setting square waves or pulses to LF mode at higher frequencies will also introduce
an uncertainty of one clock edge at the
AUX OUT
socket, even if this is still set to
AUTO
or
HF
, because the comparator is now being driven by an LF mode waveform
instead of the filtered sine wave.
Frequency Modes for Sweep and FSK
For sweep and FSK operation the
MAIN OUT
and
AUX OUT
waveform modes are
fixed
HF
or
LF
even if the setting on the option menu is
AUTO
. The setting under
these circumstances is that of the main generator before either sweep or FSK was turned
on. For example, if the two FSK frequencies are 25 kHz and 50 kHz and the main
generator frequency before FSK was turned on was 25 kHz, FSK waveforms will be
generated in LF mode. In both cases however, the automatic choice can be overridden by
selecting
HF
or
LF
instead of
AUTO
in the
SQWAVE GEN=
and
AUX=
fields of
the options menu.
Phase-shifted Asymmetric Waveforms
The interaction of symmetry adjustment and start/stop phase of triggered bursts gives
waveforms which are difficult to anticipate. In principle, adjusting the symmetry moves
the 180 ° phase point from the 50:50 position of 50 % symmetry to, for example, the
40:60 point of 40 % symmetry. The 0 ° to 180 ° points are now mathematically scaled
down to fit into the first 40 % of the cycle and the 180 ° to 360 ° points are scaled up to
fit into the remaining 60% of the cycle. Start/stop phase still works with the true phase
settings but they are not necessarily at the expected point on the waveform, particularly
for more complex wave shapes.
1.888.475.5235 info@Fluke-Direct.com
Fluke-Direct
.com