Fluke DDS Function Generator 271, User Manual

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271
Users Manual
6-2
Introduction
DDS operation gives the significant advantage over conventional function generators of
phase-continuous sweeps over very wide frequency ranges, up to 10
10
:1. However it must
be remembered that the frequency is actually stepped, not continuously swept, and
thought needs to be given as to what the instrument is actually doing when using extreme
combinations of sweep time and frequency span. Refer to
Frequency Stepping Resolution
at the end of this chapter for additional information.
Sweep mode is turned on and off with alternate presses of the SWEEP key; the lamp
beside the key lights when sweep mode is on. The sweep parameters (begin, end and
marker frequencies, sweep direction, law, ramp time and source) are all set from the
sweep edit menu which is selected by pressing the
EDIT key followed by the SWEEP
key. When sweep edit is selected the lamp beside the SWEEP key flashes to show edit
mode, regardless of whether sweep operation is selected to be on or off. The sweep mode
parameters are set up on two pages of the display; the flashing edit cursor is moved
around each page, and between pages, by the
FIELD and DIGIT keys as described in the
Principles of Editing section in chapter 4, General Operation .
Return to the main menu from either page of the edit menu is achieved by pressing the
ESCAPE key.
See also Square Wave Generation in chapter 5, Main Generator Operation , for
information concerning the use of sweep with square waves.
Connections for Sweep Operation
Sweeps are usually used with an oscilloscope or hard-copy device to investigate the
frequency response of a circuit. The instrument's
MAIN OUT
is connected to the circuit
input and the circuit output is connected to an oscilloscope or, for slow sweeps, a
recorder.
To show the markers on the display instrument the rear panel
TRIG/SWEEP OUT
socket should be connected to a second channel; an oscilloscope should be triggered from
this channel (negative edge). Alternatively, if no marker display is required, the
TRIG/SWEEP OUT
can be connected directly to the external trigger of the
oscilloscope.
The
TRIG/SWEEP OUT
socket provides a 3-level waveform in sweep mode. The
output changes from high (4 V) to low (0 V) at the start of the sweep and goes high again
at the end of the sweep. If the display device is a chart recorder
TRIG/SWEEP OUT
can
therefore be used as a pen-lift signal (note that it may be necessary to invert the signal).
Additionally the
TRIG/SWEEP OUT
output provides narrow 1 V pulses at each marker
frequency, as described in the next section.
For externally triggered sweeps a trigger signal must be provided at the front panel
EXT TRIG
socket. A sweep is initiated by the rising edge of the trigger signal.
The generator does not provide a ramp output for use with X-Y displays or recorders.
Setting Sweep Span and Markers
Pressing the
EDIT
key followed by the
SWEEP
key displays the first page of the sweep
parameters with values set to factory defaults.
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