LeCroy WAVESURFER User Manual

WaveSurfer DSO
WS-OM-E Rev B 107
For a digital oscilloscope with a memory of 100 kB, the highest frequency that can be analyzed is:
Delta f x N/2 = 10 kHz x 100 kB/2 = 500 MHz
FFT Pitfalls to Avoid
Take care to ensure that signals are correctly acquired: improper waveform positioning within the
observation window produces a distorted spectrum. The most common distortions can be traced
to insufficient sampling, edge discontinuities, windowing or the "picket fence" effect.
Because the FFT acts like a bank of band-pass filters centered at multiples of the frequency
resolution, components that are not exact multiples of that frequency will fall within two
consecutive filters. This results in an attenuation of the true amplitude of these components.
Picket Fence and Scallop
The highest point in the spectrum can be 3.92 dB lower when the source frequency is halfway
between two discrete frequencies. This variation in spectrum magnitude is the picket fence effect.
The corresponding attenuation loss is referred to as scallop loss. LeCroy scopes automatically
correct for the scallop effect, ensuring that the magnitude of the spectra lines correspond to their
true values in the time domain.
If a signal contains a frequency component above Nyquist, the spectrum will be aliased, meaning
that the frequencies will be folded back and spurious. Spotting aliased frequencies is often
difficult, as the aliases may ride on top of real harmonics. A simple way of checking is to modify
the sample rate and observe whether the frequency distribution changes.
Leakage
FFT assumes that the signal contained within the time grid is replicated endlessly outside the
observation window. Therefore if the signal contains discontinuities at its edges, pseudo-
frequencies will appear in the spectral domain, distorting the real spectrum. When the start and
end phase of the signal differ, the signal frequency falls within two frequency cells, broadening the
spectrum.
The broadening of the base, stretching out in many neighboring bins, is termed leakage. Cures
for this are to ensure that an integral number of periods is contained within the display grid or that
no discontinuities appear at the edges. Another is to use a window function to smooth the edges
of the signal.
Choosing a Window
The choice of a spectral window is dictated by the signal’s characteristics. Weighting functions
control the filter response shape, and affect noise bandwidth as well as side lobe levels. Ideally,
the main lobe should be as narrow and flat as possible to effectively discriminate all spectral
components, while all side lobes should be infinitely attenuated. The window type defines the
bandwidth and shape of the equivalent filter to be used in the FFT processing.
In the same way as one would choose a particular camera lens for taking a picture, some
experimenting is generally necessary to determine which window is most suitable. However, the
following general guidelines should help.

LeCroy WAVESURFER Specifications

General

Input Coupling	AC, DC, GND
Interface	USB, LAN, GPIB (optional)
Channels	4
Display	10.4" LCD
Vertical Resolution	8 bits
Input Impedance	1 MΩ || 50 Ω
Max Input Voltage	400 Vpk (1 MΩ)
Trigger Modes	Edge, Pattern
Bandwidth	200 MHz to 1 GHz

Summary

HOW TO USE ON-LINE HELP

Instrument Help

Accessing help specific to instrument controls and features via dedicated buttons or menus.

Windows Help

Accessing help for the underlying Windows operating system features by minimizing the scope application.

WARRANTY

Warranty and Service

Details warranty terms, conditions, and service procedures.

SPECIFICATIONS

Vertical System

Specifications for vertical signal acquisition.

Horizontal System

Specifications for horizontal timebase.

Triggering System

Specifications for trigger functionality.

INSTALLATION

Hardware Installation

Instructions for physical setup and connections.

CONNECTING TO A SIGNAL

ProBus Interface

Details the ProBus probe connection system.

SAMPLING MODES

Single-shot sampling mode

Explanation of single-shot acquisition.

VERTICAL SETTINGS AND CHANNEL CONTROLS

Adjusting Sensitivity and Position

How to adjust vertical gain and offset.

Coupling

Options for signal coupling (AC, DC, GND).

TIMEBASE AND ACQUISITION SYSTEM

Timebase Setup and Control

How to configure timebase settings.

TRIGGERING

Simple Triggers

Covers basic edge triggering.

SMART Triggers

Details advanced trigger types like Glitch and Width.

DISPLAY FORMATS

Display Setup

Configuring grid, intensity, and line styles.

Persistence Setup

Using persistence to view waveform history.

SAVE AND RECALL

Saving and Recalling Scope Settings

Procedures for saving/loading instrument setups.

PRINTING AND FILE MANAGEMENT

Printing

Setting up and performing print operations.

WAVEFORM MEASUREMENTS

Measuring with Cursors

Using cursors for precise measurements.

Parameter Calculations

Explains various waveform parameters and their calculation.

WAVEFORM MATH

Introduction to Math Traces and Functions

Overview of applying mathematical operations to waveforms.

FFT

Details the Fast Fourier Transform analysis.

UTILITIES

Status

Accessing system status and configuration.

Remote communication

Setting up network communication for remote control.

Options

Preferences

Customizing instrument behavior and settings.