Tektronix TDS3FFT User Manual

User Manual

TDS3FFT
FFT Application Module

071-0349-01

*P071034901*

071034901

Advanced Test Equipment Rentals

www.atecorp.com 800-404-ATEC (2832)

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Copyright © Tektronix, Inc. All rights reserved.

WARRANTY SUMMARY

Tektronix warrants that the products that it manufactures and
sells will be free from defects in materials and workmanship
for a period of one (1) year from the date of shipment from an
authorized Tektronix distributor. If a product proves defective
within the respective period, Tektronix will provide repair or
replacement as described in the complete warranty statement.

To arrange for service or obtain a copy of the complete
warranty statement, please contact your nearest Tektronix sales
and service office.

EXCEPT AS PROVIDED IN THIS SUMMARY OR THE
APPLICABLE WARRANTY STATEMENT, TEKTRONIX
MAKES NO WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING WITHOUT LIMITATION THE
IMPLIED WARRANTIES OF MERCHANTABILITY AND
FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT
SHALL TEKTRONIX BE LIABLE FOR INDIRECT,
SPECIAL OR CONSEQUENTIAL DAMAGES.

Tektronix products are covered by U.S. and foreign patents,
issued and pending. Information in this publication supercedes
that in all previously published material. Specifications and
price change privileges reserved.

Tektronix, Inc., P.O. Box 500, Beaverton, OR 97077
TEKTRONIX, TEK, TEKPROBE, and Tek Secure are

registered trademarks of Tektronix, Inc.
DPX, WaveAlert, and e*Scope are trademarks of

Tektronix, Inc.

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Contents

Safety Summary 2. . . . . . . . . . . . . . . . . . . . . . . . . . . .

Installing the TDS3FFT Application Module 5. . . . . . . .

Introduction 5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

FFT Features 6. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Displaying an FFT Waveform 7. . . . . . . . . . . . . . . . . . .

FFT Math Menu 8. . . . . . . . . . . . . . . . . . . . . . . . . . . . .

FFT Windows 12. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Aliasing 14. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

FFT Examples 16. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Contacting Tektronix

Product
Support

For questions about using Tektronix measurement
products, call toll free in North America:
1-800-833-9200
6:00 a.m. – 5:00 p.m. Pacific time

Or contact us by e-mail:
[e-mail]

Service
Support

Toll-free
Number

Postal
Address

Web Site

For product support outside of North America,
contact your local Tektronix distributor or sales
office.

T ektronix offers a range of services, including
Extended Warranty Repair and Calibration
services. Contact your local Tektronix distributor or
sales office for details.

For a listing of worldwide service centers, visit our
web site.

In North America:
1-800-833-9200
An operator can direct your call.

Tektronix, Inc.
Department or name (if known)
P.O. Box 500
Beaverton, OR 97077
USA

www.tektronix.com

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Handle Components Carefully. Do not slide sensitive

components over any surface. Do not touch exposed
connector pins. Handle sensitive components as little as
possible.

Transport and Store Carefully . Transport and store sensitive

components in a static-protected bag or container.

Manual Storage

The oscilloscope front cover has a convenient place to
store this manual.

Safety Summary

To avoid potential hazards, use this product only as
specified. While using this product, you may need to
access other parts of the system. Read the General Safety
Summary in other system manuals for warnings and
cautions related to operating the system.

Preventing Electrostatic Damage

CAUTION. Electrostatic discharge (ESD) can

damage components in the oscilloscope and its
accessories. To prevent ESD, observe these
precautions when directed to do so.

Use a Ground Strap. W ear a grounded antistatic wrist strap

to discharge the static voltage from your body while
installing or removing sensitive components.

Use a Safe Work Area. Do not use any devices capable of

generating or holding a static charge in the work area
where you install or remove sensitive components.
Avoid handling sensitive components in areas that have a
floor or benchtop surface capable of generating a static
charge.

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Installing the TDS3FFT Application Module

Refer to the TDS3000 & TDS3000B Series Application
Module Installation Instructions for instructions on

installing and testing the application module.

Introduction

The FFT application module adds FFT (Fast Fourier
Transform) measurement capabilities to your oscillo­scope. The FFT process mathematically converts the
standard time-domain signal (repetitive or single-shot
acquisition) into its frequency components, providing
spectrum analysis capabilities.

Being able to quickly look at a signal’s frequency
components and spectrum shape is a powerful research
and analysis tool. FFT is an excellent troubleshooting aid
for:

H Testing impulse response of filters and systems
H Measuring harmonic content and distortion in

systems

H Identifying and locating noise and interference

sources

H Analyzing vibration
H Analyzing harmonics in 50 and 60 Hz power lines

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Time Signals and FFT Waveforms Displayed Together

The time signals and FFT waveforms can be shown
together on the display. The time signal highlights the
problem; the FFT waveform helps you determine the
cause of the problem.

Displaying an FFT Waveform

1. Set the source signal Vertical SCALE so that the

signal peaks do not go off screen. Off-screen signal
peaks can result in FFT waveform errors.

2. Set the Horizontal SCALE control to show five or

more cycles of the source signal. Showing more
cycles means the FFT waveform shows more
frequency components, provides better frequency
resolution, and reduces aliasing.

If the signal is a single-shot (transient) signal, make
sure that the entire signal (transient event and ringing
or noise) is displayed and centered on the screen.

3. Push the Vertical MATH button to show the math

menu.

4. Push the FFT screen button to show the FFT side

menu.

5. Select a signal source. You can do an FFT on any

channel or any stored reference waveform.

6. Select the appropriate vertical scale and FFT window .

7. Use zoom controls and the cursors to magnify and

measure the FFT waveform.

FFT Features

The FFT application module provides the following
features:

FFT Windows

Four FFT windows (Rectangular, Hamming, Hanning,
and Blackman-Harris) let you match the optimum
window to the signal you are analyzing. The Rectangular
window is best for nonperiodic events such as transients,
pulses, and one-shot acquisitions. The Hamming,
Hanning, and Blackman-Harris windows are better for
periodic signals.

Analyze Repetitive, Single-Shot, and Stored Waveforms

You can display an FFT waveform on any actively-ac­quired signal (periodic or one-shot), the last acquired
signal, or any signal stored in reference memory.

dB or Linear RMS Scales

The FFT vertical graticule can be set to either dB or
Linear RMS. A dB scale is useful when the frequency
component magnitudes cover a wide dynamic range,
letting you show both lesser and greater- magnitude
frequency components on the same display. A Linear
scale is useful when the frequency component magni­tudes are all close in value, allowing direct comparison
of their magnitudes.

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H Signals that have a DC component or offset can cause

incorrect FFT waveform component magnitude
values. To minimize the DC component, choose AC
Coupling on the source signal.

H To reduce random noise and aliased components in

repetitive or single-shot events, set the oscilloscope
acquisition mode to average over 16 or more samples.
Average mode attenuates signals not synchronized
with the trigger.

H Do not use the Average acquisition mode if the

source signal contains frequencies of interest that are
not synchronized with the trigger rate.

H Do not use Peak Detect and Envelope modes with

FFT. Peak Detect and Envelope modes can add
significant distortion to the FFT results.

H For transient (impulse, one-shot) signals, set the

oscilloscope to trigger on the transient pulse in order
to center the pulse information in the waveform
record.

FFT Math Menu

Bottom Side Description

FFT

Set FFT
Source to

Set FFT Vert
Scale to

Set FFT
Window to

Sets the FFT signal source.
Valid input sources are Ch 1
and Ch 2 (2-channel instru­ments), Ch 1 through Ch 4
(4-channel instruments), and
Ref 1 through Ref 4 (all instru­ments).

Sets the display vertical scale
units. Available scales are
dBV RMS and Linear RMS.

Sets which window function
(Hanning, Hamming, Blackman­Harris, or Rectangular) to apply
to the source signal. Refer to
page 12 for more FFT window
information.

FFT Source Key Points

H Push the side menu button to select the source.
H Using FFT slows down the oscilloscope’s response

time in Normal acquisition mode (10k record length).

H A waveform acquired in Normal acquisition mode

has a lower noise floor and better frequency
resolution than a waveform acquired in Fast Trigger
mode.

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FFT Vertical Scale Key Points

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Zooming an FFT Display. Use the Zoom button ,

along with horizontal POSITION and SCALE controls,
to magnify FFT waveforms. When you change the zoom
factor, the FFT waveform is horizontally magnified
about the center vertical graticule, and vertically magni­fied about the math waveform marker. Zooming does not
affect the actual time base or trigger position settings.

NOTE. FFT waveforms are calculated using the

entire source waveform record. Zooming in on a
region of either the source or FFT waveform
will not recalculate the FFT waveform for that
region.

Measuring FFT Waveforms Using Cursors. You can use

cursors to take two measurements on FFT waveforms:
magnitude (in dB or signal source units) and frequency
(in Hz). dB magnitude is referenced to 0 dB, where 0 dB
equals 1 V

RMS

. Use horizontal cursors (H Bars) to
measure magnitude and vertical cursors (V Bars) to
measure frequency.

Magnitude cursors

Frequency cursors

H Push the side menu button to select a scale. A vailable

scales are dBV RMS and Linear RMS.

H Use the Vertical POSITION and SCALE knobs to

vertically move and rescale the FFT waveform.

H To display FFT waveforms with a large dynamic

range, use the dBV RMS scale. The dBV scale
displays component magnitudes using a log scale,
expressed in dB relative to 1 V
V

or in source waveform units (such as amps for

RMS,

current measurements).

, where 0 dB =1

RMS

H To display FFT waveforms with a small dynamic

range, use the Linear RMS scale. The Linear RMS
scale lets you display and directly compare
components with similar magnitude values.

Nyquist Frequency Key Point

H To determine the Nyquist frequency, push the

ACQUIRE menu button. This displays the current
sample rate on the bottom right area of the screen.
The Nyquist frequency is one-half of the sample rate.
For example, if the sample rate is 25.0 MS/s, then the
Nyquist frequency is 12.5 MHz.

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FFT Window

Characteristics Best for measuring

Blackman­Harris

Best magnitude,

worst at resolving

frequencies.

Predominantly single fre­quency waveforms to look
for higher order harmonics.

Hamming,
Hanning

Better frequency,

poorer magnitude

accuracy than

Rectangular.

Hamming has

slightly better fre-

quency resolution

than Hanning.

Sine, periodic, and narrow­band random noise.

Transients or bursts where
the signal levels before and
after the event are signifi­cantly different.

Rectangular

Best frequency,

worst magnitude

resolution. This is

essentially the

same as no win-

dow.

Transients or bursts where
the signal levels before and
after the event are nearly
equal.

Equal-amplitude sine waves
with frequencies that are
very close.

Broad-band random noise
with a relatively slow varying
spectrum.

FFT Windows

Applying a window function to the source waveform
record changes the waveform so that the start and stop
values are close to each other, reducing FFT waveform
discontinuities. This results in an FFT waveform that
more accurately represents the source signal frequency
components. The ’shape’ of the window determines how
well it resolves frequency or magnitude information.

Source

waveform

Waveform
data points

×

=

FFT

Point-by-point
multiply

Window
function
(Hanning)

Source waveform
after windowing

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With windowing

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If the increased number of frequency components
shown on the screen makes it difficult to measure
individual components, use the Zoom button to
magnify the FFT waveform.

H Use a filter on the source signal to bandwidth limit

the signal to frequencies below that of the Nyquist
frequency. If the components you are interested in are
below the built-in bandwidth settings (20 MHz
bandwidth for all oscilloscopes, 150 MHz bandwidth
for 300 MHz and 500 MHz oscilloscopes), set the
source signal bandwidth to the appropriate value.
Push the Vertical MENU button to access the source
channel bandwidth menu.

Aliasing

Problems occur when the oscilloscope acquires a signal
containing frequency components that are greater than
the Nyquist frequency (1/2 the sample rate). The fre­quency components that are above the Nyquist frequen­cy are undersampled and appear to “fold back” around
the right edge of the graticule, showing as lower frequen­cy components in the FFT waveform. These incorrect
components are called aliases.

0 Hz

Frequency

Amplitude

Aliased frequencies Actual frequencies

Nyquist frequency

(½ sample rate)

Use the following methods to eliminate aliases:
H Increase the sample rate by adjusting the Horizontal

SCALE to a faster frequency setting. Since you
increase the Nyquist frequency as you increase the
horizontal frequency, the aliased frequency
components should appear at their proper frequency.

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The first component at 20 MHz (figure label 1) is the
source signal fundamental frequency. The FFT wave­form also shows a second-order harmonic at 40 MHz (2)
and a fourth-order harmonic at 80 MHz (3). The pres­ence of components 2 and 3 indicate that the system is
distorting the signal. The even harmonics suggest a
possible difference in signal gain on half of the signal
cycle.

FFT Examples

FFT Example 1

A pure sine wave can be input into an amplifier to
measure distortion; any amplifier distortion will
introduce harmonics in the amplifier output. Viewing the
FFT of the output can determine if low-level distortion is
present.

You are using a 20 MHz signal as the amplifier test
signal. You would set the oscilloscope and FFT parame­ters as listed in the table:

FFT Example 1 Settings

Control Setting

CH 1 Coupling AC
Acquisition Mode Average 16
Horizontal Resolution Normal (10k points)
Horizontal SCALE 100 ns
FFT Source Ch 1
FFT Vert Scale dBV
FFT Window Blackman-Harris

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FFT Example 2

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Note the component at 31 MHz (figure label 1); this
coincides with a 31 MHz memory strobe signal in the
example system. There is also a frequency component at
62 MHz (figure label 2), which is the second harmonic
of the strobe signal.

Noise in mixed digital/analog circuits can be easily
observed with an oscilloscope. However, identifying the
sources of the observed noise can be difficult.

The FFT waveform displays the frequency content of the
noise; you may then be able to associate those frequen­cies with known system frequencies, such as system
clocks, oscillators, read/write strobes, display signals, or
switching power supplies.

The highest frequency on the example system is
40 MHz. To analyze the example signal you would set
the oscilloscope and FFT parameters as listed in the
following table:

FFT Example 2 Settings

Control Setting

CH 1 Coupling AC
Acquisition Mode Sample
Horizontal Resolution Normal (10k points)
Horizontal SCALE 4.00 ms
Bandwidth 150 MHz
FFT Source Ch 1
FFT Vert Scale dBV
FFT Window Hanning

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