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Standard Waveform Operation
Warning and Error Messages 5
DC OFFSET:
program +1.50 Vdc
(actual +1.50 Vdc)
load:hiZ
If the amplitude is now reduced to, say, 250 mV pp, this introduces the attenuator and the
actual dc offset changes by the appropriate factor:
DC OFFSET:
program +1.50 Vdc
(actual +151 mVdc)
load:hiZ
The above display shows that the set DC offset is +1.50V but the actual offset is
+151mV.
Note
The actual offset value also takes into account the true attenuation provided
by the fixed attenuator, using the values determined during the calibration
procedure. In the example displayed the output signal is 250 mV p-p exactly
and takes account of the small error in the fixed attenuator; the offset is 151
mV (to three significant figures) and takes account of the effect of the
calibrated attenuation error on the set offset of 1.50 V.
Whenever the set dc offset is modified by a subsequent change in output level the display
shows a warning message. Similarly, settings which would result in peak offset+signal
levels outside the range ±10 V (and therefore clipping) generate a similar warning
message. There is additional information on these messages in the Warnings and Error
Messages section below.
The output attenuation is controlled intelligently to minimize the difference between the
programmed and actual offset when the combination of programmed amplitude and
offset allows this. Thus when the offset is set to 150 mV, for example, the amplitude can
be reduced to nominally 50 mV pp before the fixed attenuator causes the actual offset to
be different from the programmed value.
Warning and Error Messages
Two classes of message are displayed on the screen when an illegal combination of
parameters is attempted.
WARNING messages are shown when the entered setting causes some change which the
user might not necessarily expect, as in the following two examples:
1. Changing the amplitude from, for example, 2·5 V p-p to 25 mV p-p brings in the step
attenuator; if a non-zero offset has been set then this will also be attenuated. The message
DC OFFSET CHANGED BY AMPLITUDE will be shown temporarily on the screen
but the setting will be accepted; in this case the actual attenuated offset will be shown in
brackets below the set value.
2. With the output level set to 10 V p-p, increasing the dc offset beyond ± 5 V will
cause the message OFFSET + SUM + LEVEL MAY CAUSE CLIPPING. The
offset change will be accepted (producing a clipped waveform) and the user may then
5-5
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