Fluke DDS Function Generator 271, User Manual, Page: 50 / 132

271
Users Manual
6-4
With the edit cursor in the
MODE
field, alternate presses of the
DIGIT
keys, or turning
the rotary control, will set the sweep direction to
BEG-END
(beginning to end) or
END-BEG
(end to beginning). There are no restrictions on the
BEG
in or
END
frequencies; this means that because the
BEG
in frequency can be higher than the
END
frequency, the
MODE
field simply provides an easy way to reverse the sweep direction.
With the edit cursor in the
LAW
field the sweep can be changed from
LIN
ear to
LOG
arithmic. With
LAW=LIN
set, the frequency changes linearly with time across the
sweep; with
LAW=LOG
set, the frequency changes exponentially with time across the
sweep. The term ‘log sweep’ is a convention; with the start frequency lower than the stop
frequency (the usual mode of operation) the mathematical relationship of frequency to
time is actually antilog.
The sweep rate is set with the cursor in the
RAMP TIME
field; ramp time can be set
with 3 digit resolution from 0.01 s (10 ms) to 999 s. The choice of ramp time affects the
number of discrete frequency steps in the sweep; faster sweeps will have fewer steps.
The trigger mode of the sweep is set with the cursor in the
TRIG SRC
(trigger source)
field; the options are
CONTINUOUS
,
EXT
ernal and
MAN/REMOTE
. In
CONTINUOUS
mode the sweep starts simultaneously with the high-to-low transition of the
TRIG/SWEEP OUT
signal; the sweep starts with the phase at 0 ° and at the output level
set by the dc offset. At the end of the sweep the signal returns to this dc offset level and
the
TRIG/SWEEP OUT
signal simultaneously goes high again. After a delay (long
enough for an oscilloscope retrace, for example) the cycle repeats.
In
EXT
ernal mode the trigger source is connected to the front panel
EXT TRIG
socket.
A sweep starts typically 200 to 800 µs after the rising edge of the trigger signal; the
sweep is completed before another trigger edge is recognized and a new sweep initiated.
The minimum trigger pulse width is 1 ms and the repetition rate should be greater than
110 % of the sweep time plus 5 ms.
In
MAN/REMOTE
mode a single sweep is initiated by each press of the
MAN/SYNC
key or by each remote command. If the
MAN/SYNC
key is pressed during a sweep
(continuous or single sweep) the sweep will be paused at the instantaneous sweep
frequency until
MAN/SYNC
is pressed again to allow the sweep to continue.
Frequency Stepping Resolution
The generator frequency is stepped, not continuously swept, between the
BEG
in and
END
frequencies. The number of discrete frequency steps in a sweep is determined by
the ramp time selected on the sweep edit menu; the size of each step, i.e. the frequency
stepping resolution, is determined by the number of steps and the sweep span.
For the fastest sweeps, in the range 10 ms to 200 ms, the frequency steps are pre-
calculated and output at 125 µs intervals; this means that there are 80 discrete steps in a
10 ms sweep, 160 in a 20 ms sweep, and so on up to 1600 steps in a 200 ms sweep.
For slower sweeps, up to 999s, each frequency step is calculated on-the-fly and output
every 5 ms; this means that there are 100 steps in a 500 ms sweep, 200 in a 1 s sweep,
and so on up to 199,800 steps in a 999 s sweep.
Note that at the fastest sweep rates, with fewest frequency steps (e.g. 10 ms sweep) two
effects can occur at extremes of frequency span which are not experienced with
conventional generators. Firstly, if the scan is very wide the frequency changes will be
quite large at each step; if the output is applied to a filter, for example, the response will
be a succession of step-change levels with (at higher frequencies) many cycles of the
same frequency at each step. Secondly, if the begin frequency is less than 800 Hz (the
ramp rate for fast sweeps), one or more of the low frequency steps will contain
incomplete cycles. These effects are only created because of the very wide sweeps that
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