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The width of the gate affects the resolution bandwidth of the spectral analyzer. Gate width has
units of seconds. The resolution bandwidth directly controls the gate width, but the numerical
value is entered in units of Hz. Therefore, the time domain gate markers move as you adjust the
resolution bandwidth control.
The data contained in the gated region is transformed into the frequency domain.
The gate is identified on the display using dashed markers (similar to cursors). The default gate
width setting is equal to the duration of the source waveform.
Using the Frequency Domain Controls
The gated region of the source waveform is transformed into a spectral phase or magnitude
waveform. The horizontal units are always Hz. The vertical units depend on whether phase or
magnitude is selected. The frequency domain controls for the spectral waveform are span, center,
and resolution bandwidth. The spectrum normally appears on the display fit to a screen width of
10 divisions.
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 time base controls and increase the sample 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 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 span must always be in the interval of zero to one half of
the sample rate. Therefore, if the span 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
span before the center will have an adjustment range. If you are unable to increase the center to the
desired value, increase the sample rate 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.
Resolution Bandwidth (RBW) is the 3 dB down bandwidth of the spectral analyzer frequency
response to a sine wave input. The resolution bandwidth is affected by two parameters:
 Different window functions produce different filter response shapes in the spectrum and result
in different 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 Hz. Therefore, the time domain gate markers move as you adjust the RBW control.
RBW = Window Bin Width ÷ Gate Width
The Window Bin Width is the resolution bandwidth in units of bins, and depends on the window
function in use. The Gate Width is in units of seconds.
The following 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.
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