Special Considerations In Scalar Mode; Sweep Time Limitations; Stepped Measurements - Agilent Technologies 89410A Operator's Manual

Fundamental Measurement Interactions

Special Considerations in Scalar Mode

Sweep time limitations

Sweep time is the time required for the analyzer to complete one full sweep on the
display. With the standard swept-tuned analyzer, sweep time is limited by analog
IF filter dynamics because the filter must have time to respond to the input signal
before accurately measuring a frequency.
In the 89400 series analyzers the digital IF and FFT act like a parallel filter bank so
the sweep speed is limited by data collection and digital processing time rather
than filters. For this reason, the 89400 series analyzers allow measurements for low
resolution bandwidths to run much faster than would be case with a swept
analyzer. The sweep time is adjusted automatically—the analyzer selects the
optimum sweep time, based on the frequency span, the resolution bandwidth, and
the maximum number of frequency points.

Stepped measurements

Traditional swept-tuned analyzers are subject to speed and frequency limits
imposed by the resolution bandwidth filters which determine the information
bandwidth. Traditional FFT analyzers have been subject to fixed resolution
bandwidth-to-span limitations. However, the information bandwidth of the 89400
series analyzers is determined by the span rather than the resolution bandwidth.
Therefore, the resolution bandwidth can be changed without affecting the span.
As discussed previously in the chapter "What Makes this Analyzer Different?", the
scalar measurement mode uses a stepped local oscillator to perform multiple FFTs
which are then combined into a single trace. This technique, combined with digital
signal processing, allows the analyzer to maximize the sweep time and overcome
the fixed relationship between resolution bandwidth and span. In effect the
minimum resolution bandwidth is limited only by the maximum number of
frequency points and the minimum span.
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