Chapter 2
NI 5450 Overview
©
National Instruments Corporation
2-23
NI PXIe-5450 User Manual
range from negative full scale (
−
1) to positive full scale (+1). Any time the
prefilter offset changes, the OSP block ignores all overflows for the next
I/Q samples. If an overflow occurs during these samples, the data is clipped
but an error is not returned.
Prefilter Offset Overflow
Any time the following condition is not true for the I or Q data stream, an
overflow occurs when prefilter offset is applied:
–
1
≤
(User Data × Pre-Filter Gain) + Pre-Filter Offset
≤
1
If an overflow occurs, the data is clipped and NI-FGEN returns an error. To
prevent data clipping, reduce the prefilter gain, attenuate the waveform
data, or reduce the prefilter offset.
Tip
To disable error reporting caused by OSP overflows, use the OSP Overflow Error
Reporting property or the
NIFGEN_ATTR_OSP_OVERFLOW_ERROR_REPORTING
attribute.
FIR Filter Types
There are a number of built-in low-pass pulse-shaping filters available in
NI-FGEN. Because the coefficients are scaled for unity-gain, the filters
may overflow if transients (such as step response) are presented at the input
of the filter. Use the Filter Type property or the
NIFGEN_ATTR_OSP_FIR_FILTER_TYPE
attribute to set the FIR filter
type. The following filters are currently available:
•
Flat
•
Raised Cosine
•
Root Raised Cosine
Note
Enabling filters in the OSP will add a constant group delay to your signal.
Digital Filter Overflow
FIR filters may overflow. If an overflow occurs, data is clipped and
NI-FGEN returns an error.
Digital filter overflows are caused by transients. Transients can occur as an
abrupt jump at the beginning of a waveform or they may be present in the
data you supply. NI-FGEN ignores overflows that are caused by transients
at the beginning of a waveform. You can choose to ignore overflow errors
caused by transients present in your data with the OSP Overflow Error
Reporting property or the
NIFGEN_ATTR_OSP_OVERFLOW_ERROR_REPORTING
attribute.
