Agilent Technologies InfiniiVision 6000 Series Manual de usuario

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.

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