Agilent Technologies Signal Analyzer N9030a - Manual (Page 1194)

6 RLC Swept SA Measurement Front-Panel & SCPI Reference
Sweep/Control
However, in order to increase dynamic range, most X-series models provide a set of analog
prefilters that precede the ADC. Unlike swept measurements, which pass the signal through a
bandpass before the ADC, FFT measurements present the full signal bandwidth to the ADC,
making them more susceptible to overload, and requiring a lower signal level. The prefilters act to
alleviate this phenomenon - they allow the signal level at the ADC to be higher while still avoiding
an ADC overload, by eliminating signal power outside the bandwidth of interest, which in turn
improves dynamic range.
Although narrowing the segment width can allow higher dynamic ranges some cases, this comes at
the 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:
1194 Remote Language Compatibility Measurement Application Reference