5
Traditional swept-spectrum analysis
1
, in effect, sweeps a narrowband filter
across a range of frequencies, sequentially measuring one frequency at a time.
Unfortunately, sweeping the input works well for stable or repetitive signals,
but will not accurately represent signals that change during the sweep. Also,
this technique only provides scalar (magnitude only) information, though some
other signal characteristics can be derived by further analysis of spectrum
measurements.
The VSA measurement process simulates a parallel bank of filters and over-
comes swept limitations by taking a “snapshot,” or time-record, of the signal;
then processing all frequencies simultaneously. For example, if the input is a
transient signal, the entire signal event is captured (meaning all aspects of
the signal at that moment in time are digitized and captured); then used by the
FFT to compute the “instantaneous” complex spectra versus frequency. This
process can be performed in real-time, that is, without missing any part of the
input signal. For these reasons, VSA is sometimes referred to as “dynamic signal
analysis” or “real-time signal analysis. The ability to track a fast-changing signal
with VSA, however, is not unlimited. It depends on the computation capability
available.
The VSA decreases measurement time
Parallel processing yields another potential advantage for high-resolution
(narrow resolution bandwidth) measurements: faster measurement time. If
you’ve used a swept-tuned spectrum analyzer before, you already know that
narrow resolution bandwidth (RBW) measurements of small frequency spans
can be very time-consuming. Swept-tuned analyzers sweep frequencies from
point to point slowly enough to allow the analog resolution bandwidth filters to
settle. By contrast, VSA measures across the entire frequency span at one time.
However, there is analogous VSA settling time due to the digital filters and DSP.
This means that the VSA sweep speed is limited by data collection and digital
processing time rather than analog filters. But this time is usually negligible
when compared to the settling time of analog filters. For certain narrow band-
width measurements, VSA can complete a measurement up to 1000 times faster
than conventional swept-tuned analysis.
In swept-tuned spectrum analysis, the physical bandwidth of the sweeping
filter limits the frequency resolution. VSA doesn’t have this limitation. VSA can
resolve signals that are spaced less than 100 μHz apart. Typically, VSA resolution
is limited by the signal and measurement front end’s frequency stability, as well
as by the amount of time you are willing to devote to the measurement. Increas-
ing the resolution also increases the time it takes to measure the signal (the
required time-record length).
Time-capture is a great tool for signal analysis and
troubleshooting
Another feature that is extremely useful is the time-capture capability. This
allows you to record actual signals in their entirety without gaps, and replay
them later for any type of data analysis. All measurement features can be ap-
plied to the captured signal. For example, you could capture a transmitted digital
communications signal and then perform both spectrum and vector-modulation
analysis to measure signal quality or identify signal impairments.
1. For more information on spectrum
analysis, see Keysight Application Note
150, Spectrum Analysis Basics, literature
number 5952-0292EN.
Vector analysis measures dynamic signals and produces
complex data results (continued)
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