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Turning the rotary control will increment or decrement the numeric value in steps determined by
the position of the edit cursor (flashing underline); the cursor is moved by the left and right
arrowed cursor keys. Because DC offset can have negative values, the rotary control can take
the value below zero; although the display may autorange to a higher resolution if a step takes
the value close to zero, the increment size is maintained correctly as the offset is stepped
negative. For example, if the display shows
program = +205· mVdc
with the cursor in the most significant digit, the rotary control will decrement the offset in 100mV
steps as follows:
program = +205· mVdc
program = +105· mVdc
program = +5·00 mVdc
program =
−
95·0 mVdc
program =
−
195· mVdc
The actual DC offset at the MAIN OUT socket is attenuated by the fixed
−
step output attenuator
when this is in use. Since it is not obvious when the signal is being attenuated the actual offset is
shown in brackets as a non
−
editable field below the programmed value.
For example, if the amplitude is set to 2·5Vpp the output is not attenuated by the fixed attenuator
and the actual DC offset (in brackets) is the same as that set. The
DC OFFSET
display shows:
DC OFFSET:
p1.50 Vdc
(1.50 Vdc)
load: hiZ
◊
If the amplitude is now reduced to 250mVpp which introduces the attenuator, the actual DC offset
changes by the appropriate factor:
DC OFFSET:
p1.50 Vdc
(151. mVdc)
load: hiZ
◊
The above display shows that the set DC offset is +1.50V but the actual offset is +151mV.
Note that 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 250mVpp exactly and takes account of the small error in the
fixed attenuator; the offset is 151.mV exactly, taking account of the effect of the known
attenuation (slightly less than the nominal) on the set offset of 1.50V.
Whenever the set DC offset is modified by a change in output level in this way a warning
message that this has happened will be displayed. Similarly, because the DC offset plus signal
peak is limited to ± 10V to avoid waveform clipping, a warning message will be displayed if this
condition is set. This is explained more fully in the Warnings and Error Messages section.
The output attenuation is controlled intelligently to minimise 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 150mV, for example, the amplitude can be reduced to
nominally 50mVpp before the fixed attenuator causes the actual offset to be different from the
programmed value.