LeCroy 9400A Manual page 194

(3) With transient signals use the Rectangular window. You should adjust
the time base and the triggering conditions so that the transient is
completely contained in the time domain window (i.e. on the screen).
(4) For the best amplitude accuracy of isolated spectrum peaks, use the
Flat Top window.
(5) For the best reduction of leakage and good detection of small peaks
several bins away from a large peak, use the Blackman-Harris window.
(6) For moderate improvement of amplitude accuracy and of leakage
rejection, use the von Hann or Hamming window.
(7) If your time domain signal is repetitive but noisy, preventing you
from having a stable trigger, you can define Function E as the FFT
of the signal channel and Function F as the FFT Power Average of
Function E to obtain a stable spectrum of the input signal.
(8) You can display the FFT Power Average either on a linear scale
(Magnitude) or on a dBm scale (Power Spectrum or Power Density).
11.5.2 Relationships of 9400A FFT output vaveforms to the FFT computation steps
For comparison of the 9400A's FFT results with those obtained from other
FFT instruments the following information may be useful:
(I) In the 9400A's FFT computation, the first step is the sub-sampling
of the source waveform. Data points for the FFT calculation are
measured at regular intervals over the full length of the waveform
displayed on the screen.
You can select the maximum transform size in the FFT menu or via
remote control. The actual transform size, N, is selected to be
equal to or smaller than the Displayed Record Length (see
Table 5.1). Exception: you may get a 125 point transform when you
have specified 50 points.
The sub-sampling interval and the actual transform size selected
provide the frequency scale factor in a 1-2-5 sequence.
11-12
Fast Fourier Waveform Processing
Option (WP02, V 2.06FT)