As a result, instrumentation with very good sensitivity is essential. To achieve the best sensitivity, instruments such as the Agilent PNA network analyzer use mixer-based receivers. These provide better sensitivity than sampler-based converters.
Main path response
Unwanted responses
Figure 49. RCS signals are small and returns caused by the measurement chamber can interfere with the measurement results.
Because the signals are tiny, small reflections caused by elements in the range itself can contribute a significant amount of reflected energy. To solve this problem, advanced network analyzers such as the PNA/PNA-X provide a time-gating feature that can remove the unwanted signals. This is achieved by computing an inverse fast Fourier transform (IFFT) on the measured frequency data, mathematically removing the unwanted signals, and then computing an FFT to restore the frequency result. Figure 49 illustrates this concept. An artifact of computing the IFFT on a finite sampling is that it will create repetitions of the fundamental signal in time (“aliases”). These artifacts or aliases can be worked around through a process of testing to create an alias-free measurement span. The width of this span will depend partly on the number of data points the analyzer is able to measure and process. A typical number of data points for a network analyzer may be 1601, which is suitable for an alias-free range required for many measurements. However, because more may be needed, the PNA/PNA-X network analyzer provides 20,001 points to ensure wide alias-free spans. More detail on the time gating process can be found in Agilent Application Note 1287-12, Time-domain Analysis with a Network Analyzer, publication number 5989-5723EN. [9]
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