User`s guide

Measurements and Math Functions 6
InfiniiVision Oscilloscopes User’s Guide 239
• 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.
• Blackman Harris — window reduces time resolution compared to a
rectangular window, but improves the capacity to detect smaller impulses due
to lower secondary lobes.
• 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.
• 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.
• 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.
3 To make cursors measurements, press the [Cursors] key and set the Source softkey
to Math: f(t).
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).
4 To make other measurements, press the [Quick Meas] key and set the Source softkey
to Math: f(t).
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.