Agilent Technologies InfiniiVision 6000 Series User manual

Agilent Technologies
Agilent 6000 Series
Oscilloscope
User’s Guide

 Agilent Technologies, Inc. 2005-2006
Printed in Malaysia
Fourth Edition, June 2006
*54684-97011*
54684-97011
www.agilent.com
Agilent Technologies

Making Measurements 5
6000 Series Oscilloscope User’s Guide 169
FFT Measurement
FFT is used to compute the fast Fourier transform using analog
input channels or math functions 1 + 2, 1 – 2, or 1 * 2. FFT takes
the digitized time record of the specified source and transforms
it to the frequency domain. When the FFT function is selected,
the FFT spectrum is plotted on the oscilloscope display as
magnitude in dBV versus frequency. The readout for the
horizontal axis changes from time to frequency (Hertz) and the
vertical readout changes from volts to dB.
Use the FFT function to find crosstalk problems, to find
distortion problems in analog waveforms caused by amplifier
non-linearity, or for adjusting analog filters.
FFT Units
0 dBV is the amplitude of a 1 Vrms sinusoid. When the FFT
source is channel 1 or channel 2 (or channel 3 or 4 on 4-channel
models), FFT units will be displayed in dBV when channel units
is set to Volts and channel impedance is set to 1 MΩ.
FFT units will be displayed in dBm when channel units is set to
Volts and channel impedance is set to 50Ω.
FFT units will be displayed as dB for all other FFT sources or
when a source channel’s units has been set to Amps.
DC Value
The FFT computation produces a DC value that is incorrect. It
does not take the offset at center screen into account. The DC
value is not corrected in order to accurately represent
frequency components near DC.
Aliasing
When using FFTs, it is important to be aware of frequency
aliasing. This requires that the operator have some knowledge
as to what the frequency domain should contain, and also
consider the sampling rate, frequency span, and oscilloscope

170 6000 Series Oscilloscope User’s Guide
5 Making Measurements
vertical bandwidth when making FFT measurements. The FFT
sample rate is displayed directly above the softkeys when the
FFT menu is displayed.
Aliasing happens when there are frequency components in the
signal higher than half the sample rate. Since the FFT spectrum
is limited by this frequency, any higher components are
displayed at a lower (aliased) frequency.
The following figure illustrates aliasing. This is the spectrum of
a 990 Hz square wave, which has many harmonics. The FFT
sample rate is set to 100 kSa/s, and the oscilloscope displays the
spectrum. The displayed waveform shows the components of
the input signal above the Nyquist frequency to be mirrored
(aliased) on the display and reflected off the right edge.
Figure 26 Aliasing
Since the frequency span goes from ≈0 to the Nyquist
frequency, the best way to prevent aliasing is to make sure that
the frequency span is greater than the frequencies of significant
energy present in the input signal.
True frequency component
Aliased frequency component

Making Measurements 5
6000 Series Oscilloscope User’s Guide 171
Spectral Leakage
The FFT operation assumes that the time record repeats. Unless
there is an integral number of cycles of the sampled waveform
in the record, a discontinuity is created at the end of the record.
This is referred to as leakage. In order to minimize spectral
leakage, windows that approach zero smoothly at the beginning
and end of the signal are employed as filters to the FFT. The FFT
menu provides three windows: Hanning, flattop, and
rectangular. For more information on leakage, see Agilent
Application Note 243, “The Fundamentals of Signal Analysis” at
http://cp.literature.agilent.com/litweb/pdf/5952-8898E.pdf.
FFT Operation
1Press the Math key, press the FFT softkey, then press the
Settings softkey to display the FFT menu.
•Source — selects the source for the FFT. The source can be
any analog channel, or math functions 1 + 2, 1 – 2, and
1*2.
•Span — sets the overall width of the FFT spectrum that you
see on the display (left to right). Divide span by 10 to
calculate the number of Hertz per division. It is possible to
set Span above the maximum available frequency, in
which case the displayed spectrum will not take up the
whole screen. Press the Span softkey, then turn the Entry
knob to set the desired frequency span of the display.
Source
select
Frequency
Span
Return to
previous menu
Center
frequency
Preset Span
and Center
frequencies
Vertical and
Window FFT
controls
Sample
rate

172 6000 Series Oscilloscope User’s Guide
5 Making Measurements
•Center — sets the FFT spectrum frequency represented at
the center vertical grid line of the display. It is possible to
set the Center to values below half the span or above the
maximum available frequency, in which case the displayed
spectrum will not take up the whole screen. Press the
Center softkey, then turn the Entry knob to set the desired
center frequency of the display.
•Preset — sets the frequency Span and Center to values that
will cause the entire available spectrum to be displayed.
The maximum available frequency is half the effective FFT
sample rate, which is a function of the time per division
setting. The current FFT sample rate is displayed above
the softkeys.
2Press the More FFT softkey to display additional FFT
settings.
•Scale — lets you set your own vertical scale factors for FFT
expressed in dB/div (decibels/division). Press the Scale
softkey, then turn the Entry knob to rescale your math
function.
•Offset — lets you set your own offset for the FFT. The offset
value is in dB and is represented by the center horizontal
grid line of the display. Press the Offset softkey, then turn
the Entry knob to change the offset of your math function.
FFT Scale FFT Offset Return to
previous menu
Window
Scale and offset considerations
If you do not manually change the FFT scale or offset settings, when you turn the
horizontal sweep speed knob, the span and center frequency settings will
automatically change to allow optimum viewing of the full spectrum. If you do
manually set scale or offset, turning the sweep speed knob will not change the
span or center frequency settings, allowing you see better detail around a specific
frequency. Pressing the FFT Preset softkey will automatically rescale the
waveform and span and center will again automatically track the horizontal
sweep speed setting.

Making Measurements 5
6000 Series Oscilloscope User’s Guide 173
•Window — selects a window to apply to your FFT input
signal:
•Hanning — window for making accurate frequency
measurements or for resolving two frequencies that are
close together.
•Flat Top — window for making accurate amplitude
measurements of frequency peaks.
•Rectangular — good frequency resolution and amplitude
accuracy, but use only where there will be no leakage
effects. Use on self-windowing waveforms such as
pseudo-random noise, impulses, sine bursts, and decaying
sinusoids.
3To make cursors measurements, press the Cursors key and set
the Source softkey to Math.
Use the X1 and X2 cursors to measure frequency values and
difference between two frequency values (∆X). Use the Y1
and Y2 cursors to measure amplitude in dB and difference in
amplitude (∆Y).
4To make other measurements, press the Quick Meas key and
set the Source softkey to Math.
You can make peak-to-peak, maximum, minimum, and
average dB measurements on the FFT waveform. You can
also find the frequency value at the first occurrence of the
waveform maximum by using the X at Max measurement.

174 6000 Series Oscilloscope User’s Guide
5 Making Measurements
The following FFT spectrum was obtained by connecting the
front panel Probe Comp signal (~1.2 kHz) to channel 1. Set
sweep speed to 5 ms/div, vertical sensitivity to 500 mV/div,
Units/div to 10 dBV, Offset to -34.0 dBV, Center frequency to
5.00 kHz, frequency Span to 10.0 kHz, and window to Hanning.
Figure 27 FFT measurements
FFT Measurement Hints
The number of points acquired for the FFT record is 1000 and
when frequency span is at maximum, all points are displayed.
Once the FFT spectrum is displayed, the frequency span and
center frequency controls are used much like the controls of a
spectrum analyzer to examine the frequency of interest in
greater detail. Place the desired part of the waveform at the
center of the screen and decrease frequency span to increase
the display resolution. As frequency span is decreased, the
number of points shown is reduced, and the display is
magnified.
While the FFT spectrum is displayed, use the Math and Cursors
keys to switch between measurement functions and frequency
domain controls in FFT menu.

Making Measurements 5
6000 Series Oscilloscope User’s Guide 175
Decreasing the effective sampling rate by selecting a slower
sweep speed will increase the low frequency resolution of the
FFT display and also increase the chance that an alias will be
displayed. The resolution of the FFT is the effective sample rate
divided by the number of points in the FFT. The actual
resolution of the display will not be this fine as the shape of the
window will be the actual limiting factor in the FFTs ability to
resolve two closely space frequencies. A good way to test the
ability of the FFT to resolve two closely spaced frequencies is to
examine the sidebands of an amplitude modulated sine wave.
For the best vertical accuracy on peak measurements:
•Make sure the probe attenuation is set correctly. The probe
attenuation is set from the Channel menu if the operand is a
channel.
•Set the source sensitivity so that the input signal is near full
screen, but not clipped.
•Use the Flat Top window.
•Set the FFT sensitivity to a sensitive range, such as
2dB/division.
For best frequency accuracy on peaks:
•Use the Hanning window.
•Use Cursors to place an X cursor on the frequency of
interest.
•Adjust frequency span for better cursor placement.
•Return to the Cursors menu to fine tune the X cursor.
For more information on the use of FFTs please refer to Agilent
Application Note 243, ”The Fundamentals of Signal Analysis” at
http://cp.literature.agilent.com/litweb/pdf/5952-8898E.pdf.
Additional information can be obtained from Chapter 4 of the
book “Spectrum and Network Measurements” by Robert A.
Witte.