Fluke DDS Function Generator 271, Руководство пользователя

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Appendices
DDS Operation and Further Waveform Considerations
B
Further Waveform Considerations
The various limitations on combinations of modes, most of which have already been
mentioned in the appropriate operational sections of the manual, are brought together
here and explained with reference to the simplified block diagram below.
Simplified Generator Block Diagram
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The diagram shows simplified paths for the main and auxiliary outputs. LF and HF refer
to the low frequency and high frequency modes set for square wave and pulses and for
the auxiliary output in the
SQWAVE GEN=
and
AUX=
fields of the options menu, as
described in the
Waveform Generation Options
section of chapter 5,
Main Generator
Operation
. When these fields are set to
AUTO
the modes automatically change from LF
to HF above 30 kHz, but they can be overridden and setting to LF or HF will set that
generating mode whatever the generator frequency.
Similarly, when set to
AUTO
on the options menu, the filter will be switched in or out
depending on waveform as shown; again this can be overridden and setting the filter
ON
or
OFF
will override this so that all or none (respectively) of the waveforms will be
filtered.
Interaction of Various Option Settings
Important points to consider when setting the option menu fields to anything other than
AUTO
are as follows:
1. The comparator which generates HF square waves and pulses for the
MAIN OUT
socket is driven, by default, by a filtered sine wave. If the filter is set
OFF
, the
waveform driving the comparator will be degraded, leading to degradation of the HF
square wave, pulse etc.
2. The same comparator is used to derive the signals for the
AUX OUT
socket; the
waveform driving the comparator depends on the main waveform selection. For HF
square wave and pulse
MAIN OUT
signals the driving waveform is a filtered sine
wave as described above; for sine wave and triangle
MAIN OUT
signals the
comparator drive is the waveform itself (also filtered). The main waveform also
drives the comparator for ramp, staircase and arbitrary waveforms plus LF square
waves and pulses, all of which are unfiltered; this means that if the
MAIN OUT
waveform shows edge jitter as the frequency increases, so will the HF
AUX OUT
.
For this reason, the default (
AUTO
) setting for the auxiliary output is LF mode at all
frequencies for main waveforms of ramp, staircase, arbitrary and LF square waves
and pulses.
B-3
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