Agilent Technologies InfiniiVision 5000 Series Guia do Utilizador descarregar pdf (Página 171)

Making Measurements 4

5000 Series Oscilloscope User’s Guide 171

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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Agilent Technologies InfiniiVision 5000 Series Guia do Utilizador Página 171