Teledyne LeCroy WaveRunner МXi-A Getting Started Manual - Manual (Page 83)

Getting Started Manual
922138-00 Rev A 75
B
Touch the FFT tab in the right-hand dialog.
Choose whether to Truncate
2
or Zero-fill
3
the trace display.
Touch the Suppress DC checkbox if you
want to make the DC bin go to zero.
Otherwise, leave it unchecked.
Touch inside the Output type field, and
make a selection from the pop-up menu.
Touch inside the Window field and select a
window type.
Touch inside the Algorithm field and select
either Least Prime
4
or Power2
5
from the
pop-up menu.
2
When the FFT transform size does not match the record length, you can truncate the record and
perform an FFT on the shorter record. This increases the resolution bandwidth.
3
Zero-fill is useful when the source data for the FFT comes from a math operation that shortens the
record. This is commonly encountered in filtering operations like enhanced resolution. The missing
data points are replaced by data values, whose amplitudes are interpolated to fit between the last
data point and the first data point in the record. This guarantees that there is not a first-order
discontinuity in the filled data. Since the data at the end of the record is "filled" data, it is advisable
to select a weighting window other than rectangular to minimize the effect of the fill on the resulting
spectrum.
4
The default algorithm is a least primes algorithm that computes FFTs on transform sizes having
lengths that can be expressed as factors of 2
N
*5
K
. This is very compatible with the record lengths
encountered in the oscilloscope, which are often multiples of 1, 2, 4, 5, or 10. or Power of 2 The
other choice is a power of two algorithm where the record lengths are in the form of 2
N
. The power
of 2 algorithm generally runs faster than the least primes algorithm. The price that is paid is a
record length that is not the same as the acquired signal. The power-of-two FFT uses the first 2
N
points of the record. For example, if you acquire 500 points in your trace, the power-of-two FFT
would only use the first 256 points.