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5
Digital Up-Conversion
Amplitude Scaling
70
For more details about markers, refer to chapter
The marker to sample output delay is different in the digital
NOTE
up-conversion mode compared to the corresponding value in direct
modes (For details, refer to the M8121A datasheet).
The amplitude multiplier allows to scale the amplitude of the DAC output
signal (see
The chain of interpolation filters and the IQ modulator could increase the
signal value, which then exceeds the valid DAC range. This leads to a
clipping of samples. This effect depends on the input data and therefore
cannot be handled automatically.
The instrument is able to detect clipping and signals it to the user by the
status register or a dialog box in the soft front panel. Clipping could be
prevented by using scale factor values of 1.0 or smaller. This scale factor
value reduces the output amplitude relative to the direct mode to approx.
37.6%.
It is possible to increase the scale factor up to 2.828, but depending on the
input IQ sample sequence this can result in signal clipping. The value can
be separated into two multiplication factors:
•
Factor 2.0 is related to the interpolation filters and is dependent from
the input sample sequence.
•
Factor 1.414 is related to the IQ modulators. The maximum I value (1.0)
and the maximum Q value (1.0) causes a resulting vector with a length
of sqrt (2)≈1.414.
So whenever the user can guarantee that the sum of I and Q doesn't
exceed the unit circle, it is safe to set the scale value to 1.414 and increase
the overall amplitude.
A scale factor of 1.0, which is a "safe" value in terms of clipping, reduces
the overall possible output amplitude to approx. 37.6 %. Clipping is highly
depending on IQ input data provided by the user. Therefore, this allows
scale factor values in the range from 0.0 to 2.828 and allows using as
much as possible from the available DAC range. The user is responsible to
select a proper scale value depending on his input data.
Figure 21
on page 68).
Keysight M8121A Arbitrary Waveform Generator User Guide
Markers
on page 63.
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