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 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
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
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 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)
This 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:
â–
Different window functions produce different filter response shapes in the spectrum and result in different resolution
bandwidths.
â–
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.
Oscilloscope reference
720 DPO70000SX, MSO/DPO70000DX, MSO/DPO70000C, DPO7000C, and MSO/DPO5000B Series
To Next Page
Loading...
Need help?
General
