1279
Sweep/Control
expense of losing some of the speed advantages of the FFT, because narrower segments require more
acquisitions and proportionately more processing overhead.
However, the advantages of narrow segments can be significant. For example, in pulsed-RF
measurements such as radar, it is often possible to make high dynamic range measurements with signal
levels approaching the compression threshold of the analyzer in swept spans (well over 0 dBm), while
resolving the spectral components to levels below the maximum IF drive level (about –8 dBm at the
input mixer). But FFT processing experiences overloads at the maximum IF drive level even if the RBW
is small enough that no single spectral component exceeds the maximum IF drive level. If you reduce the
width of an FFT, an analog filter is placed before the ADC that is about 1.3 times as wide as the FFT
segment width. This spreads out the pulsed RF in time and reduces the maximum signal level seen by the
ADC. Therefore, the input attenuation can be reduced and the dynamic range increased without
overloading the ADC.
Further improvement in dynamic range is possible by changing the FFT IF Gain (in the Meas Setup
menu of many measurements). If the segments are reduced in width, FFT IF Gain can be set to High,
improving dynamic range.
Depending on what IF Bandwidth option you have ordered, there can be up to three different IF paths
available in FFT sweeps, as seen in the diagram below:
The 10 MHz path is always used for Swept sweeps. It is always used for FFT sweeps as well, unless the
user specifies ~25 MHz in which case the 25 MHz path will be used for FFT sweeps, or ~40 MHz, in
which case the 40 MHz path will be used for FFT sweeps. Note that, although each of these keys picks
the specified path, the analyzer may choose an FFT width less than the full IF width, in order to optimize
speed, trading off acquisition time versus processing time.
Pause/Resume
Pauses a measurement after the current data acquisition is complete.
When Paused, the label on the key changes to Resume. Pressing Resume un-pauses the measurement.
10
MHz
25 MHz
40 MHz
ADC
ADC
~ 10MHz
~ 40MHz
~ 25MHz