Fluke DDS Function Generator 271, Руководство пользователя

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271
Users Manual
7-4
required phase shift by -45 ° the desired condition can still be achieved. At the same time,
however, the reduction in the number of samples making up each cycle of the waveform
means that the start/stop point covers a region of uncertainty which is 1 clock wide.
Note that these effects apply even when the phase is set to 0 °; at frequencies approaching
10 MHz the phase shift can be 90 ° or more and the uncertainty band becomes wide.
Because this effect is seen at 0 ° phase it is also evident when the auxiliary output is in
HIGH FREQ
uency mode, i.e. when there is no phase control. In fact, because the
AUX OUT
signal is derived from the filtered DDS sine wave in this mode the filter adds
further phase delay, creating even longer phase shifts at a given frequency than are
evident with
AUX OUTPUT
in
LOW FREQ
uency mode.
In summary, phase errors and uncertainty will increase as the main frequency is increased
above 30 kHz, even with 0 ° phase set. However, stop/start phase control can be used,
with care, to much higher frequencies by ‘backing-off’ the phase to compensate for the
hardware delays.
Gated Mode
Gated mode is turned on and off with alternate presses of the
GATE
key; the lamp beside
the key lights when gate mode is on.
The selection of the gate source signal is made from the gate edit menu which is selected
by pressing the
EDIT
key followed by the
GATE
key. when gate edit is selected the
lamp beside the
GATE
key flashes to show edit mode regardless of whether gate
operation is currently selected to be on or off.
SOURCE=EXT
TGEN=1.00ms 1.000kHz
Gate Source
With the edit cursor in the
SOURCE
field of the gate edit menu, the digit keys or rotary
control can be used to select
EXT
ernal,
MAN/REMOTE
, or
TGEN
(Trigger GENerator)
as the gate source. In all cases, when the gate condition is true, the main generator signal
is gated through to the
MAIN OUT
socket. Since the main generator is free-running and
not synchronized with the gate source the start and stop phase of the waveform is entirely
arbitrary; there will be an instantaneous transition from the dc offset level to the current
waveform phase at the start of the gating period and an instantaneous transition back to
the dc offset level at the stop.
With the source set to
EXT
ernal, the generator waveform is gated on whilst the external
signal applied to the
EXT TRIG
input exceeds the gate threshold (1.5 V) (see External
trigger input above).
With the source set to
MAN/REMOTE
, the generator waveform is gated on and off with
alternate presses of the
MAN/SYNC
key or by the appropriate commands via the RS232
or GPIB interfaces.
With the source set to
TGEN
, the generator waveform is gated on as explained in the
Internal trigger generator section; the trigger generator is set exactly as described in the
Trigger source section.
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