Tektronix RSA6100A Series, Help Manual

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General Signal Viewing > DPX Spectrum DPX Primer
Figure 15. During swept DPX operation, the Dwell time control adjusts the observation time of each frequency
segment used to construct the composite DPX spectrum display.
Guaranteed Capture of Fast Events
The main reason that swept-tuned and step-tuned spectrum analyzers can’t provide 100% Probability of
Intercept, POI, for a signal that isn’t continuously present is that they spend only a short period of time
tuned to each segment of their frequency span during each sweep. If something happens in any part of
the span other than where it is tuned at that instant, that event will not be detected or displayed. There
is also a period of time between sweeps, retrace time, during which the analyzer is not paying attention
to the input signal. FFT-based analyzers, including vector signal analyzers, also miss signals during
the time between acquisitions. Their POI depends on a combination of factors including span, number
of FFT points, acquisition time, memory read/write time, and signal processing speed. Vector analyzers
process information sequentially, so when read/write from data and processing is occurring, data is not
being acquired.
RSAs, on the other hand, capture data across all frequencies within their real-time span during every
acquisition. With Tektronix’ exclusive Frequency Mask trigger and DPX Density trigger, POI increases to
100%, insuring capture of any spectral event matching the trigger de
fi nition. When operating in free run as
a simple spectrum analyzer, the RSA has a POI similar to other FFT-based analyzers, with gaps between
each acquisition. Processing is done concurrent with the acquisitions.
Guaranteed Capture in DPX Real-Time Spans
The DPX Spectrum display captures any signal that is at least 10.3 microseconds long (for instruments
with Option 200) and within the real-time bandwidth. This performance is possible because the RSA
computes up to 292,000 spectrum transforms per second. The faster the spectrum updates, the shorter the
time between acquisitions and the greater the probability that any signal will be detected.
Table 1 shows the speci fi ed minimum signal duration (MSD) for 100% probability of intercept under
various combinations of Span and RBW in DPX for a representative RSA model. As you can see, MSD is
affected by multiple factors.
Table 2: Minimum signal duration speci fi cations for RSA6100A series spectrum analyzers 1
Span
(MHz)
RBW
(kHz) Span/RBW
Ratio
FFT
Length Spectrum/sec
MSD for
100% POI ( μ s)
110 1000 110 1024 292,969 10.3
110 300 367 2048 146,484 20.5
RSA6100A Series Real-Time Spectrum Analyzers Help
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