Teledyne LeCroy WaveExpert 100H Operators Manual - Manual (Page 212)

Operator’s Manual
210
WE-OM-E Rev A
The table of Coefficients of Window Functions lists the coefficients a
m
. The window functions seen
in the time domain are symmetric around the point k = N/2.
Coefficients of Window Functions
Window Type a
0
a
1
a
2
Rectangular 1.0 0.0 0.0
Hanning (Von
Hann)
0.5 -0.5 0.0
Hamming 0.54 -0.46 0.0
Flattop 0.281 -0.521 0.198
Blackman–Ha
rris
0.423 -0.497 0.079
FFT Setup
To Set Up an FFT
1. In the menu bar touch Math, then Math Setup... in the drop-down menu.
2. Touch a Math function trace button: F1 through Fx; a pop-up menu appears. Select FFT
from the menu.
3. Touch the Single or Dual (function of a function) button if the FFT is to be
of the result of another math operation.
4. Touch inside the Source1 field and select a channel, memory, or math trace on which to
perform the FFT.
5. Touch inside the Operator1 field: Select FFT from the pop-up menu if you selected Single
function. Select another math function if you selected Dual function. Then touch inside the
Operator2 field and select FFT from the pop-up menu.
6. In the right-hand dialog, touch the FFT tab.
7. Choose whether to Truncate ‡ or Zero-fill § the trace display.
8. Touch the Suppress DC checkbox if you want to make the DC bin go to zero. Otherwise,
‡ 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 will increase the resolution bandwidth.
§ 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 th 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.