Fluke DDS Function Generator 271, User Manual

Page: 59 / 132

Amplitude Modulation
VCA (External)
8
8-3
output beyond this limit. The maximum generator output setting at which correct
operation is maintained reduces from 20 V p-p to 10 V p-p (open circuit) as the
modulation is increased from 0 % to 100 %.
Modulation Waveform
The default modulation waveform is a square wave because this permits the full
frequency range of the internal trigger generator to be used. Alternatively, a fixed 1 kHz
sine wave can be selected by moving the edit cursor to the
INT MOD
field in the last
line of the display; the
DIGIT
keys or rotary control can be used to toggle the setting
between
SQUARE
(at the frequency set on the internal trigger generator) and
SINE
.
Note that selecting
SINE
forces the
TGEN
field to display
1.00ms 1.000kHz
but the user setting is not lost and if
INT MOD= SQUARE
is reselected the
TGEN
setting returns to its original value.
VCA (External)
With the cursor in the
SOURCE
field of the AM edit menu, set the source to
EXT VCA
. Connect the modulating signal to the rear panel
VCA IN
socket (nominal
6 k Ω input impedance); a positive voltage increases the generator output and a negative
voltage decreases the output. Note that as with internal AM, clipping will occur if the
combination of generator setting and VCA signal attempts to drive the output above
20 V p-p (open circuit).
External AM is achieved by setting the generator to the required output level and
applying the modulation signal (which may be ac coupled if required) at the appropriate
level to obtain the modulation depth required. If the generator output level is changed the
amplitude of the modulating signal will have to be changed to maintain the same
modulation depth. As with internal AM, the maximum output setting of the generator at
which clipping is avoided is reduced from 20 V p-p to 10 V p-p (open circuit) as
modulation is increased from 0 % to 100 %. Modulation frequency range is dc to
100 kHz.
The generator’s amplitude control circuit has four quadrant operation, allowing the
generator output to be inverted if the external VCA voltage is taken sufficiently negative.
Suppressed carrier modulation (SCM) can be achieved by applying a modulating signal
with a negative offset between 0 V and -3 V (depending on output level setting) sufficient
to reduce the carrier output to zero.
It is also possible to modulate a dc level from the generator with a signal applied to
VCA IN
, as follows. Set the generator to 0 Hz sine wave on the main menu and +90 °
phase on the trigger menu. Select
EXT TRIG
(the default) and turn trigger mode on
with the
TRIG
key but do not apply a trigger signal. The
MAIN OUT
is now set at the
peak positive voltage defined by the amplitude setting on the main menu; setting -90 °
phase on the trigger menu will give the peak negative voltage. Select
EXT VCA
on the
AM edit menu and turn AM on; the dc level will now be modulated by the signal applied
to the
VCA IN
socket.
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