LeCroy 9400A Manual page 26

FUNCTIONAL DESCRIPTION
FOURIER PROCESSING
Fourier processing is a mathematical technique
which permits a time-domain waveform to be
described in terms of frequency-domain magnitude
and phase, or real and imaginary spectra. In spec-
tral analysis, a waveform can be sampled and digi-
tized, then transformed by a discrete Fourier trans-
form (DFT). Fast Fourier Transforms are a set of
algorithms used to reduce the computation time
(by better than a factor of 100 for a 1000 point
FFT) needed to evaluate a DFT. The principal
advantage of the FFT is the rapidity with which it
can analyze large quantities of waveform samples.
In effect, using standard measurement techniques,
it converts a time-domain instrument into digital
spectrum analyzer.
The WP02 Fast Fourier Processing Package enhances
the outstanding features of the LeCroy 9400A Digital
Oscilloscope. It provides high resolution, wide-band
spectrum analysis capabilities along with sophisticat-
ed window functions and fast processing.
FFT AND THE LeCROY 9400A DIGITAL
OSCILLOSCOPE
In FFT mode, the 9400A provides measurement capa-
bilities superior to those of common swept spectrum
analyzers.
In particular, it is now possible to perform spectral
analysis on continuous and single events at an eco-
nomic price. And it enables users to obtain time and
frequency values simultaneously and to compare
phases of the various frequency components with
each other. Rather than the commonly used "power of
two" record lengths the routines used in the WP02
package feature decimal record lengths, which can be
selected in a 1-2-5 order. Resulting spectra are there-
fore also calibrated in convenient decimal Hertz
values.
SPECIFICATIONS
The FFT's digital nature ensures high accuracy, stabil-
ity and repeatability. These are strongly supported by
the 9400A's superb DC and dynamic accuracy speci-
fications, such as standard +2%, optional 4-1%, DC
accuracy, high effective-bit count and increased reso-
lution through signal averaging and dithering.
With the 9400A, signals may be acquired and proces-
sed simultaneously using Channels 1 and 2. This is
particularly useful when looking for common fre-
quency-domain characteristics in both signals or for
characterization of networks.
IMPROVED RESOLUTION
The Fast Fourier Transform calculates equally-spaced
frequency components from DC to the full 9400A
bandwidth. By lowering the sampling rate, it is pos-
sible to make measurements with 1 milli-Hertz resolu-
tion up to 12,5 Hz (Nyquist). By increasing the sam-
pling rate to 5 gigasamples/sec (200 psec/point)
Random Interleaved Sampling mode, the widest reso-
lution becomes 50 MHz and the Nyquist frequency
2.5 GHz... comfortably above the highest frequency
components recordable by the 9400A, thus virtually
eliminating aliasing effects.
VERSATILE WINDOW FUNCTIONS
The WP02-FFT software provides a selection of win-
dow functions, designed to minimize leakage and to
maximize spectral resolution of single and non-cyclic
events. These include the familiar rectangular or
unmodified window typically used for transient events,
the von Harm (Planning) and Hamming windows for
continuous signals, and, in addition, Flattop and
Blackman-Harris windows for more precise amplitude
(power) measurements or strong suppression of side
lobes respectively.
Furthermore, user-defined window functions may be
loaded onto the 9400A via the GPIB interface. Through
multiplication, they modify the acquired signal follow-
ed by FFT in an automated fashion.
VERTICAL ANALOG SECTION
Inputs: two; BNC connectors.
Sensitivity: 5 mV/div to 1 V/div at 50 £~ impedance
and 5 mV/div to 5 V/div at 1 M£) impedance; detents
at 1-2-5, variable 1:2.5.
DC accuracy: standard <_ _+ 2%; optional _< _+ 1°.
MEMORIES
Acquisition memory: 32K x 8 bits per channel
(CH1 and CH2).
Reference memory: 32K x 16 bits per reference
memory (C and D).
Function memory: 32K x 16 bits per function memory
Bandwidth (-3 dB):
@ 50 (5: DC - 175 MHz at 10 mV/div, up to
225 MHz at 1 V/div;
DC - 150 MHz at 5 mV/div.
@1 M~2 AC: 10 Hz-100 MHz typical
@1 M~ DC: DC-100 MHz typical
Bandwidth limiter: 30 MHz (-3 dB).
Input impedance: 1 M£~//50 pF and 50 £2
characteristic.
Maximum input: 250 V (DC + peak AC) at 1 M~
5 V DC (500 mW) or 4- 10 V peak AC at 50 ~2.
Offset: _+ 8 divisions in 0.04-division increments.
(E and F).
The content of the acquisition and function memories
can be stored in reference memories C and D.
Record length selection for FFT
Function memories E and F only: 50-25000 data
points in 9 steps in 1-2-5 sequence. Record lengths
are selected by decimation after signal acquisition.
This implies that the Nyquist criterion can be adjusted
and optimized after signal acquisition and prior to FFT
execution.