FFT Spectral Analysis 4
Keysight InfiniiVision 3000T X-Series Oscilloscopes User's Guide 91
• Bartlett — (triangular, with end points at zero) window is similar to the
Hanning window in that it is good for making accurate frequency
measurements, but its higher and wider secondary lobes make it not quite
as good for resolving frequencies that are close together.
• Vertical Units — lets you select Decibels or V RMS as the units for the FFT
vertical scale.
• FFT Gating — when the zoomed time base is displayed, press this softkey to
select:
• No Gating — the FFT is performed on the source waveform in the upper
Main time base window.
• Gate By Zoom — the FFT is performed on the source waveform in the lower
Zoom window.
• Auto Setup — 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 FFT sample rate, which is a function of the time per
division setting. The FFT resolution is the quotient of the sampling rate and
the number of FFT points (f
S
/N). The current FFT Resolution is displayed
above the softkeys.
3 To make cursor measurements, press the [Cursors] key and set the Source
softkey to Math N.
Use the X1 and X2 cursors to measure frequency values and difference between
two frequency values (ΔX). Use the Y1 and Y2 cursors to measure amplitude in
dB and difference in amplitude (ΔY).
4 To make other measurements, press the [Meas] key and set the Source softkey to
Math N.
Scale and offset considerations
If you do not manually change the FFT scale or offset settings, when you turn the horizontal
scale 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 horizontal scale knob will not change the
span or center frequency settings, allowing you see better detail around a specific frequency.
Pressing the FFT Auto Setup softkey will automatically rescale the waveform and span and
center will again automatically track the horizontal scale setting.
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