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SECTION 3
Examples:
An error occurs if you attempt to set the DC offset to 1OV
when the amplitude
is set to lZVp-p. This error occurs be-
cause the sum voltage is 12/2 + 10 = 16V, which is greater
than the maximum
15V output voltage.
When the unit is set to the AUTO output range mode, an
error occurs if you attempt
to set the amplitude
to
lOOmVp-p when the DC offset is 5V. This error occurs be-
cause the specified amplitude
is less than the minimum
AC amplitude
of 286mVp-p
for sum voltages greater
than or equal to 1.5V.
Again, assume that the output range mode is AUTO, the
current amplitude
is lOOmVp-p, the offset is lV, and
that
you want to use a 500mVp-p amplitude
and a 5V DC off-
set in combination
with one another. An error occurs if
you attempt to set the offset to 5V before changing the
amplitude
(the error occurs because the current 1OOmV
amplitude
is less than the minimum
286mVp-p for sun
voltages greater than or equal to 1.5V). To avoid an error
in this situation, first set the amplitude
to 500mVp-p, and
then set the DC offset to 5V.
Although
the hardware
determines the ultimate resolu-
tion, the Model 3930A displays the amplitude
and DC
offset to the predetermined
number of digits and resolu-
tion.Asaresult,programmingamplitudeoroffsetvalues
below the resolution
of the hardware
will result in dis-
play changes only; the actual output will not change.
These resolution
limits apply to AC only, DC only, and
AC + DC output waveforms. In particular, if the AC + DC
voltage ratio is large, the accuracy of small setting values
is decreased. See Table B-1 and Table B-3 (Appendix
B)
for details on these limitations.
3.5.8
Waveform
Selection,
Square-Wave
Duty Cycle, and Synchronous
output
Waveform
Selection
Pressing the FCTN key displays the current waveform
function
along with its corresponding
number and en-
ables waveform
selection. Available
waveforms include:
DC,SIN,/G,
n,
n,and\.
To select a waveform
with
the numeric
keys, simply
press the numeric key that corresponds
to the desired
waveform. For example, press 1 to select a sine wave. The
waveform
will
change immediately
when the corre-
sponding
key is pressed; it is not necessary to press a
units key.
When the Model 3930A displays the current waveform,
and function
parameter
selection is enabled, you can
change the waveform
by turning
the MODIFY
knob.
Turning
the knob clockwise increases waveform
nun
bers, while turningthe
knob counter-clockwise
decreases
waveform
numbers
(one number increment
per knob
detent). When the highest or lowest waveform
numbers
are reached, the number wilI wrap around to the lowest
or highest selection
(function
numbers
range from 0
through 5).
For all AC waveforms,
the p-p value of the amplitude
will remain unchanged when you change the waveform.
If ,the amplitude
is specified in units other than a p-p
value (AUTO output range mode only), the amplitude
will be converted
to p-p units when the waveform
is
changed.
Finally, the waveform
output is momentarily
turned off
when the waveform
function
is changed.
Square-Wave Duty Cycle
The duty cycle is the ratio of the time period of the
waveform
high-level
duration
to the time period of one
complete cycle of the waveform
expressed as a percent-
age. For example, assume a 1OkHz square wave has a
time period
of 100psec. If the high
portion
of the
waveform has a period of 30psec, the duty cycle is 30/100
x 100 = 30%.
Two square-waveform
duty-cycle
modes are available:
onewith
thedutycyclefixedat50%
and thesecondmode
with a variable duty cycle. The duty cycle applies only to
the square-wave
function
(waveform
3), although
the
duty cycle can be programmed
while other waveforms
are selected. When the Model 3930A is in the variable
duty-cycle
mode, the upper-frequency
limit is lOOkHz,
and the maximum
jitter is 150nsec or less.
To change the duty cycle, press the n
DUTY key, and
enterormodifythedutycycle(5.0%
to95.0%).TheModel
3930A stays in the variable duty cycle mode even if you
set the duty cycle to 50%. To select the fixed 50% duty cy-
cle mode, press SHIFT FXD50. In the duty cycle display
mode, the Model 3930A displays
"fixed"
for the 50%
fixed mode or "vu"
for the variable duty-cycle
mode.
3-23
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