Oscilloscope Reference Spectral math frequency domain controls
Spectral math frequency domain controls
The gated region of the source waveform is transformed into a spectral phase or magnitude waveform. The
horizontal units are always hertz. The vertical units depend on whether phase or magnitude is selected.
The frequenc
y domain controls for the spectral waveform are span, center, and resolution bandwidth. The
spectrum normally appears on the display fittoascreenwidthof10divisions.
Span
Span is the frequency at the end of the spectral waveform minus the frequency at the beginning of
the waveform. The span control maximum value is equal to the current sample rate divided by two.
Therefore, if you are unable to increase the span to the desired value and if you want to keep the same
source waveform duration, go to the timebasecontrolsandincreasethesample rate using the resolution
control.
Or, if you want to also decrease the source waveform duration, adjust the sample rate control. If
you decrease the sample rate, the span setting may decrease, if necessary, to keep the span less than the
sample rate divided by two.
Center
This is the frequency at the center of the spectral waveform. Center is equal to the start frequency plus
one half of the span. The adjustment range depends on the sample rate and the current span setting.
Remember that the s pan must always be in the interval of zero to one half of the sample rate. Therefore, if
the s
pan is set to full, the center frequency control will not have any adjustment range and will remain at
one half of the sample rate. It is necessary to decrease the s pan before the center will have an adjustment
range. If you are unable to increase the center to the desired value, increase the sample rat e using either
the sample rate or resolution controls. You may also change sample rate by using the HORIZONTAL
SCALE knob on the instrument front panel.
Re
solution bandwidth ( RBW)
This is the 3 dB down bandwidth of the spectral analyzer frequency response to a sine wave input. The
r
esolution bandwidth is affected by two parameters:
Different window functions produce different filter response shapes in the spectrum and result in
d
ifferent resolution bandwidths.
The gate width of the input data affects the RBW because the RBW is inversely proportional to the
gate width. So, adjusting the gate width changes the RBW. Gate width has units of seconds. The RBW
directly controls the gate width, but the numerical value is entered in units of hertz. Therefore, the
time domain gate markers move as you adjust the RBW control.
The next figure demonstrates the effects of adjusting the center frequency and span. Center frequency is a
horizontal position control for the spectrum. Span is a horizontal scale control. Resolution bandwidth
adjusts the bandwidth of the analyzer filters without affecting the span and center frequency.
DSA/DPO70000D, MSO/DPO/DSA70000C, DPO7000C, and MSO/DPO5000 Series 763
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